Introduction to Linear Metrology
Metrology is the back bone of quality control and assurance. In this modern era of globalisation and standardization of products, a finished product should conform to the international standard for the universal acceptance. Step by step inspection of the product right from the raw material stage till its reduction to scrap is required to ensure total quality. Metrology and calibration plays the vital role in quality control. Precision instruments used for measurements, gauges, templates and other jigs and fixtures; all are contributing to the quality of a finished product, its periodic calibration and validation is inevitable for any manufacturing industry. Hence the services rendered by calibration laboratories within the country and overseas are second to none for the delivery of quality products and services and thus the industrial developments of any country. For maintaining a global acceptance and inter changeability of spares ; units and measurements are to be traceable to a common accepted standard or a natural constant, Keeping this in view ISO has formulated the slandered IS/ISO/IEC 17025:2005 for testing and calibration laboratories.
Few notes about the basics of linear metrology and calibration are put together here for the benefits of the beginners in calibration and metrology. These chapters are only a guidelines and a welcome notes to the students, which should not be taken as a reference.
Divakaran M Ali Otupara
Isotech Metrology Solutions Isotech Metrology Solutions
DEFINITIONS OF IMPORTANT TERMS IN METROLOGY
- METROLOGY AND CALIBRATION
- ACCOMODATION AND ENVIRONMENT
- HOUSE KEEPING AND 5”S”
- SLIP GAUGES
- CALIPER CHECKER
- MICRIHITE 2D
MEANINGS OF TERMS USED IN METROLOGY
Metrology and calibration can be traceable back many millennia. Some of the early examples are mentioned in the texts of Manusmrithi in India and in the ancient Egyptian literatures. Metrology has undergone several industrial revolutions leading to the complexities of modern day microprocessor-controlled measurements. Today’s technological evolution has made it possible to measure parameters deemed impossible only a few years ago. Improvements in accuracy, tighter control, and waste reduction have also been achieved.
This book is specifically written as an introduction to modern day metrology and calibration for the benefit of technical, vocational, or degree students, and as a reference manual for managers, engineers, and technicians working in the field of calibration. It is anticipated that the prospective student will have a basic understanding of mathematics, and physics. This course should adequately prepare a prospective technician, or serve as an introduction for a prospective engineer wishing to get a solid basic understanding of metrology and calibration. Metrology involves a wide range of technologies and sciences, and they are used in an unprecedented number of applications.
Metrology is defined by the International Bureau of Weights and Measures (BIPM) as “the science of measurement, including both experimental and theoretical determinations at any level of uncertainty in any field of science and technology. Core concept in metrology is traceability, defined as “the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons, all having stated uncertainties.” The level of traceability establishes the level of comparability of the measurement: whether the result of a measurement can be compared to the previous one, a measurement result a year ago, or to the result of a measurement performed anywhere else in the world.
Traceability is most often obtained by calibration, establishing the relation between the indication of a measuring instrument and the value of a measurement standard. These standards are usually coordinated by national metrological institutes: National Physical Laboratory (NPL) New Delhi, etc. Traceability, accuracy, precision, evaluation of measurement uncertainty are critical features of a calibration management system.
DEFINITIONS OF IMPORTANT TERMS IN METROLOGY
International System of Units, SI
The system of units adopted and recommended by the General Conference on Weights and
The SI is based at present on the following seven base units:
Quantity SI base unit Symbol
length metre m
mass kilogram kg
time second s
electric current ampere A
temperature kelvin K
amount of substance mole mol
luminous intensity candela cd
The science of measurement is called Metrology
Metrology includes all aspects both theoretical and practical with Reference to measurements, whatever their uncertainty, and in whatever fields of science or technology they occur.
The totality of features and characteristics of a product or service that bear on its ability to satisfy stated or implied needs.
Set of operations that establish, under specified conditions, the relationships between values of quantities indicated by a measuring instrument or measuring system and the corresponding values realised by standards.
1. The result of a calibration permits either the assignment of values of measurands to the indications or the determination of corrections with respect to indications.
2. A calibration may also determine other metrological properties such as the effect of influence quantities.
3.The result of a calibration may be recorded in a document, sometimes called a calibration certificate or calibration report.
Is the smallest difference in dimensions that the instrument can detect.
The ability of a measurement to match the actual value of the quantity being measured.
the degree to which the instrument gives repeated Measurements of the same standard.
High accuracy = Small systematic error. High precision = Small random error.
The deviation between the results of measured value to the actual value.
True absolute error: It is the algebraic difference between the
result of measurement and the conventional true value of the quantity
Alignment Error (Cosine Error):
This error is based on Abbes principle of alignment which states that the line of measurement of the measuring component should coincide with the measuring scale or axis of the measuring instrument. These errors are caused due to non- alignment of measuring scale to the true line of dimension being measured. Cosine errors will be developed generally while measurement of a given job is carried out using dial gauge or using steel rule.
The axis or line of measurement of the measured portion should exactly coincide with the measuring scale or the axis of measuring instrument, when the above thing does not happen then cosine error will occur. To measure the actual size of the job L, using steel rule it is necessary that the steel rule axis or line of measurement should be normal to the axis of the job as shown in Figure. But sometimes due to non-alignment of steel rule axis with the job axis, the size of job 1 measured is different than the actual size of job L, as shown in Figure.
Parallax Error (Reading Error):
The position of the observer at the time of taking a reading (on scale) can create errors in measurement. For this two positions of the observers are shown (X and Y), which will be the defect generating positions. Position Z shows the correct position of the observer i.e. he should take readings by viewing eye position exactly perpendicular to the scale.
rings as show in Figure whose thickness is to be measured. Number of times, the
contact of jaws with work piece plays an
important role while measure in laboratory or work shops. The following
example shows the contact error. If the jaws of the instrument are placed
as shown in Figure the error ‘e’ is developed, which is because of poor
The numerical value which should be added to the measured value to get the correct result.
Property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.
1 The concept is often expressed by the adjective traceable.
2 The unbroken chain of comparisons is called a traceability chain.
Closeness of the agreement between the results of successive measurements of the measurand carried out under the same
conditions of measurement. Conditions include: same procedure, observer,instrument,conditions,location; and carried out over a short period of time.
Closeness of the agreement between the results of measurements of the measurand under same conditions by two or more operators.
Parameter, associated with the result of a measurement, that characterizes the dispersion of values that could reasonably be attributed to the measurand.
*The degree of doubt about a measurement!
* Parameter, associated with the result of a measurement, that characterises the dispersion of the values that could reasonably be attributed to the measurand.
Length : Meter
The metre (or meter), symbol m, is the base unit of length in the International System of Units (SI). Originally intended to be one ten-millionth of the distance from the Earth’s equator to the North Pole (at sea level),
The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.
Symbol = m
It used to be one ten millionth of the distance of
the meridian through Paris.
Multification factors of Length
Formal recognition that a laboratory is competent to carry out specific tests or calibrations.
Notes: 1. Accreditation is normally awarded following successful laboratory assessment and is followed by appropriate surveillance.
2. The competence of the laboratory is stated in the accreditation decision, covering: best measurement capability, physical quantity, calibration method and measurement range.
METROLOGY AND CALIBRATION
WHAT IS THE TIME NOW? A very common question
It is a very simple question but the answer is not so simple. What time is it exactly and how do you know it? Most people are used to calibrating and adjusting their watches whenever necessary. Working standards (i.e. clocks) are visible almost everywhere and making a comparison calibration is easy and almost an unconscious act. If the watch is slow or fast, we adjust it according to the reference that may be a wall clock or friends watch. An official time is often available by television or Internet. Calibration and adjustment against official time provides us the traceability for time. Imagine how the world would operate if we did not have a common source for time? Everybody would have his or her own interpretation of time.It would create an utter confusion.
The above narration is only about the parameter of Time. These calibrations are required for all other parameters.
HISTORY OF MEASUREMENT OF LENGHT
Thousands of years ago the Egyptians and Babylonians used a unit of length called cubit. Originally the cubit was defined as the length of a man’s arm from the elbow to the end of the middle finger(actual length of the cubit varied from place to place and time to time) Local name for cubit during the construction of pyramids was called ”meh” and divided into units called ”sheps” means palms there are seven sheps in a cubit and sheps inturn divided into four parts called “zebos”
The ancient Greek cubit was about 20.7 inches but another unit of measurement the foot was more widely used.
Standards of Measurements
The different types of standards of length are
- Material Standards
(a)Line Standard – When length is measured as the distance between centers of two engraved lines.
(b)End Standard – When length is measured as the distance between to flat parallel faces.
International Prototype Meter
International Prototype meter is defined as the straight line distance, at 0’c between the engraved lines of a platinum iridium alloy of 1020 mm of total length.
Line and End Standards and differentiate between them.
When length is measured as the distance between centers of two engraved lines, it is called Line Standards. Both material Standards, yard and metre are line standards
E.g. Scale, Rulers, Imperial Standard Yard.
Characteristics of Line Standards :
(i) Scale can be accurately marked, but the engraved lines posses thickness and it is not possible to accurately measure
(ii) Scale is used over a wide range
(iii) Scale markings are subjected to wear. However the ends are subjected to wear and this leads to undersize measurements
(iv) Scale does not possess built in datum. Therefore it is not possible to align the scale with the axis of measurement
(v) Scales are subjected to parallax errors
(vi) Assistance of magnifying glass or microscope is required.
When length is expressed as the distance between centers of two flat parallel faces, it is called End Standards. Slip Gauges, End Bars, Ends of micrometer Anvils.
Characteristics of End Standards
(i) Highly accurate and used for measurement of closed tolerances in precision engineering as well as standard laboratories, tool rooms, inspection departments.
(ii) They require more time for measurement and measure only
(iii) They wear at their measuring faces
(iv) They are not subjected to parallax error.
The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
It is very much essential to calibrate the instrument so as to maintain its accuracy. Calibration is usually carried out by making adjustment such that when the instrument is having zero measured input then it should read out zero and when the instrument is measuring some dimension it should read it to its closest accurate value. It is very much important that calibration of any measuring system should be performed under the environmental conditions that are much closer to that under which the actual measurements are usually to be taken.
Calibration is the process of checking the dimension and tolerances of a gauge, or the accuracy of a measurement instrument by comparing it to the instrument/gauge that has been certified as a standard of known accuracy. Calibration of an instrument is done over a period of time, which is decided depending upon the usage of the instrument or on the materials of the parts from which it is made. The dimensions and the tolerances of the instrument/gauge are checked so that we can come to whether the instrument can be used again by calibrating it or is it wear out or deteriorated above the limit value. If it is so then it is to be taken for work or it is scrapped.
If the gauge or the instrument is frequently used, then it will require more maintenance and frequent calibration. Calibration of instrument is done prior to its use and afterwards to verify that it is within the tolerance limit or not. Certification is given by making comparison between the instrument/gauge with the reference standard whose calibration is traceable to accepted National standard.
Dimensional and Geometric Tolerance:
In the design and manufacture of engineering products a great deal of attention has to be paid to the mating, assembly and fitting of various components. In the early days of mechanical engineering during the nineteenth century, the majority of such components were actually mated together, their dimensions being adjusted until the required type of fit was obtained. These methods demanded craftsmanship of a high order and a great deal of very fine work was produced. Present day standards of quantity production, interchange ability, and continuous assembly of many complex compounds, could not exist under such a system, neither could many of the exacting design requirements of modern machines be fulfilled without the knowledge that certain dimensions can be reproduced with precision on any number of components.
Geometric Dimensioning and Tolerance (GD&T) is a universal language of symbols, much like the international system of road signs that advise drivers how to navigate the roads. GD&T symbols allow a Design Engineer to precisely and logically describe part features in a way they can be accurately manufactured and inspected. GD&T is expressed in the feature control frame. The feature control frame is like a basic sentence that can be read from left to right. For example, the feature control frame illustrated would read: The 5 mm square shape (1) is controlled with an all-around (2) profile tolerance (3) of 0.05 mm (4), in relationship to primary datum A (5) and secondary datum B (6). The shape and tolerance determine the limits of production variability.
|There are seven shapes, called geometric elements, used to define a part and its features. The shapes are: point, line, plane, circle, cylinder, cone and sphere.There are also certain geometric characteristics that determine the condition of parts and the relationship of features. These geometric symbols are similar to the symbols used on maps to indicate features, such as two and four lane highways, bridges, and airports. They are like the new international road signs seen more frequently on highways. The purpose of these symbols is to form a common language that everyone can understand. Geometric Characteristic Symbols Straightness — A condition where all points are in a straight line, the tolerance specified by a zone formed by two parallel lines. Flatness — All the points on a surface are in one plane, the tolerance specified by a zone formed by two parallel planes. Roundness or Circularity — All the points on a surface are in a circle. The tolerance is specified by a zone bounded by two concentric circles. Cylindricity — All the points of a surface of revolution are equidistant from a common axis. A cylindricity tolerance specifies a tolerance zone bounded by two concentric cylinders within which the surface must lie. Profile — A Tolerancing method of controlling irregular surfaces, lines, arcs, or normal planes. Profiles can be applied to individual line elements or the entire surface of a part. The profile tolerance specifies a uniform boundary along the true profile within which the elements of the surface must lie. Angularity — The condition of a surface or axis at a specified angle (other than 90°) from a datum plane or axis. The tolerance zone is defined by two parallel planes at the specified basic angle from a datum plane or axis. Perpendicularity — The condition of a surface or axis at a right angle to a datum plane or axis. Perpendicularity tolerance specifies one of the following: a zone defined by two planes perpendicular to a datum plane or axis, or a zone defined by two parallel planes perpendicular to the datum axis. Parallelism — The condition of a surface or axis equidistant at all points from a datum plane or axis. Parallelism tolerance specifies one of the following: a zone defined by two planes or lines parallel to a datum plane or axis, or a cylindrical tolerance zone whose axis is parallel to a datum axis. Concentricity — The axes of all cross sectional elements of a surface of revolution are common to the axis of the datum feature. Concentricity tolerance specifies a cylindrical tolerance zone whose axis coincides with the datum axis. Position — A positional tolerance defines a zone in which the center axis or center plane is permitted to vary from true (theoretically exact) position. Basic dimensions establish the true position from datum features and between interrelated features. A positional tolerance is the total permissible variation in location of a feature about its exact location. For cylindrical features such as holes and outside diameters, the positional tolerance is generally the diameter of the tolerance zone in which the axis of the feature must lie. For features that are not round, such as slots and tabs, the positional tolerance is the total width of the tolerance zone in which the centre plane of the feature must lie. Circular Run out — Provides control of circular elements of a surface. The tolerance is applied independently at any circular measuring position as the part is rotated 360 degrees. A circular run out tolerance applied to surfaces constructed around a datum axis controls cumulative variations of circularity and co axiality. When applied to surfaces constructed at right angles to the datum axis, it controls circular elements of a plane surface. Total Run out — Provides composite control of all surface elements. The tolerance applied simultaneously to circular and longitudinal elements as the part is rotated 360 degrees. Total runout controls cumulative variation of circularity, cylindricity, straightness, co axiality, angularity, taper, and profile when it is applied to surfaces constructed around a datum axis. When it is applied to surfaces constructed at right angles to a datum axis, it controls cumulative variations of perpendicularity and flatness.|
National Accreditation Board for Testing and Calibration Laboratories (NABL) is an autonomous body under the aegis of Department of Science & Technology, Government of India, and is registered under the Societies Act. NABL has been established with the objective to provide Government, Industry and Society in general with a scheme for third-party assessment of the quality and technical competence of testing and calibration laboratories. Government of India has authorized NABL as the sole accreditation body for Testing and Calibration laboratories. In order to achieve this objective, NABL provides laboratory accreditation services to laboratories that are performing tests / calibrations in accordance with NABL criteria based on internationally accepted standard for laboratory accreditation IS/ISO/IEC 17025:2005. These services are offered in a non-discriminatory manner and are accessible to all testing and calibration laboratories in India and abroad, regardless of their ownership, legal status, size and degree of independence.
The concept of Laboratory Accreditation was developed to provide a Means for third-party certification of the competence of laboratories to perform specific type of testing and calibration. Laboratory accreditation provides formal recognition of competent laboratories, thus providing a ready means for customers to find reliable testing and calibration services in order to meet their demands. Laboratory Accreditation enhances customer confidence in accepting testing / calibration reports issued by accredited laboratories. The globalization of Indian economy and the liberalization policies initiated by the Government in reducing trade barriers and providing greater thrust to exports makes it imperative for Accredited Laboratories to be at international level of competence. Laboratory Accreditation provides formal recognition of competent laboratories, thus providing a ready means for customers to find reliable testing and calibration services in order to meet their demands. Laboratory Accreditation enhances customer confidence in accepting testing / calibration reports issued by accredited laboratories.
Potential increase in business due to enhanced customer confidence and satisfaction. Savings in terms of time and money due to reduction or elimination of the need for re-testing of products. Better control of laboratory operations and feedback to laboratories as to whether they have sound Quality Assurance System and are technically competent. Increase of confidence in Testing / Calibration data and personnel performing work. Customers can search and identify the laboratories accredited by NABL for their specific requirements from the directory of Accredited Laboratories. Users of accredited laboratories will enjoy greater access for their products, in both domestic and international markets, when tested by accredited laboratories.
The laboratories should be legally identifiable & appropriately registered. They can be a part of a big organization or an independent entity. NABL can provide accreditation to: Laboratories undertaking any sort of testing or calibration in the specified fields. Private or government laboratories. Small operations to large multi-field laboratories. Site facilities, temporary field operations and mobile laboratories.
A screw pitch gauge also known as a micrometer is a precision instrument. A micrometer is used for measuring diameter of circular objects mostly wires, with an accuracy of 0.001mm. It consists of a hollow cylinder mounted on a U frame. The hollow cylinder leads to a ratchet which is meant for fine adjustment. The U frame consists of a flat end known as stud and a screw on the other side. This screw can be moved inside the nut by fitted in the U frame by rotating the hollow cylinder called the thimble. This is called the main scale.
Every micrometer prior to its use should be thoroughly checked for backlash error or zero error.
- Backlash error: Sometimes due to wear and tear of the screw threads, it is observed that reversing the direction of rotation of the thimble, the tip of the screw does not start moving in the opposite direction immediately, but remains stationary for a part of rotation. This is called back lash error.
- Zero error: If on bringing the flat end of the screw(Spindle) in contact with the stud(Anvil), the zero mark of the circular scale coincides with the zero mark on base line of the main scale, the instrument is said to be free from zero error. Otherwise an error is said to be there. This can be both positive and negative zero error.
What is least Count of Micrometer
Least count of any precision instrument is defined as the least distance travelled by it. For a micrometer it is measured in the following manner.
Least count (L.C) of a screw gauge = Pitch/ Number of circular scale division.
Pitch and Number of circular scale divisions are the two factors determining the Least count of Micrometer
Calculation of least Count of Micrometer
Pitch = Every screw advances through a constant distance when it gives one full rotation. This distance is called its pitch. Pitch is the distance between two consecutive threads of a screw. This is usually measured in millimeters. The pitch of a screw gauge can be 0.5 mm or 1 mm.
No. of circular scale division – The thimble of a micrometer is calibrated with reading. Each of such reading coincides with the main scale and hence the final observation is made. Corresponding to the pitch of the screw gauge which may be 0.5 mm or 1 mm the number of circular division can be accordingly 50 divisions or 100 divisions.Hence, least count is calculated by the formula above:
Description of Vernier Calipers: A Vernier callipers(150mm) consists of mainly two parts i) A 2cm wide 15cm long rectangular metal strip .The left end bottom side of this strip consists of a fixed jaw and at the same end jaw at the top of this strip. On the strip a scale is graduated in Inches along the upper edge and another scale is graduated inCentimeters along the lower dge. This is called Main Scale ‘S’
Theory : Principle of vernier calipers – N divisions on the vernier scale is equal to (N-1) divisions on the main scale.
N (V.S.D) = (N-1) M.S.D
1 V.S.D = M.S.D
Least count (L.C) of vernier calipers : Minimum length or thickness measurable with the vernier calipers is called its least count.
Least count (L.C) = 1 M.S.D – 1 V.S.D
L.C = 1 M.S.D – M.S.D
L.C = 1 M.S.D [ 1- ]
L.C = =
Where S is the value of one Main scale division and N is the number of equal divisions on the vernier scale.
Least count = 0.5/50 or 1/100 = 0.01 mm or 0.001 cm
Dial indicators, also known as dial gauges and probe indicators, are instruments used to accurately measure small linear distances, and are frequently used in industrial and mechanical processes. They are named so because the measurement results are displayed in a magnified way by means of a dial.
A special variety of the dial indicator is the dial test indicator (DTI) which is primarily used in machine setups. The DTI measures displacement at an angle of a lever or plunger perpendicular to the axis of the indicator. A regular dial indicator measures linear displacement along that axis.
Dial indicators may be used to check the variation in tolerance during the inspection process of a machined part, measure the deflection of a beam or ring under laboratory conditions, as well as many other situations where a small measurement needs to be registered or indicated. Dial indicators typically measure ranges from 0 to 25 mm to 300 mm (0.015 in to 12.0 in), with graduations of 0.001 mm to 0.01 mm (metric) or 0.00005 in to 0.001 in (imperial)
A height gauge is a measuring device used either for determining the height of something, or for repetitious marking of items to be worked on. The former type of height gauge is often used in doctor’s surgeries to find the height of people.
These measuring tools are used in metal working or metrology to either set or measure vertical distances; the pointer is sharpened to allow it to act as a scriber and assist in marking out work pieces.
They may also be used to measure the height of an object by using the underside of the scriber as the datum. The datum may be permanently fixed or the height gauge may have provision to adjust the scale, this is done by sliding the scale vertically along the body of the height gauge by turning a fine feed screw at the top of the gauge; then with the scriber set to the same level as the base, the scale can be matched to it. This adjustment allows different scribers or probes to be used, as well as adjusting for any errors in a damaged or resharpened probe.
There are two types of height gauges: Vernier height gauges and electronic height gauges. The Vernier height gauge has the additional refinement of a Vernier scale for greater accuracy in reading or setting the tool. The electronic height gauge has a digital readout that gives the height
A feeler gauge is an instrument used to measure gap widths. Feeler gauges are mostly used in engineering to measure the clearance between two parts.
They consist of a number of small lengths of steel of different thicknesses with measurements marked on each piece. They are flexible enough that, even if they are all on the same hinge, several can be stacked together to gauge intermediate values. It is common to have two sets for imperial units (typically measured in thousandths of an inch) and metric (typically measured in hundredths of a millimeter) measurements.
The lengths of steel are sometimes called blades, although they have no sharp edge.
VERNIER DEPTH GAUGES
A Vernier depth gauge is used to measure the depth of holes, recesses and distance from a plane surface to a projection. In Figure shown a Vernier depth gauge in use. The Vernier scales, fixed to the main body of depth gauge, and is read in the same way as vernier calipers. Running through the depth gauge body is the main scale the end of which provides the datum surface from which the measurements are taken. The depth gauge is carefully made so that the beam is perpendicular to the base in both directions. The end of the beam is square and flat, like the end of a steel rule and the base is flat and true, free from curves or waviness.
It is also known as ‘micrometer depth gauge’. Figure illustrates a depth micrometer. The measurement is made between the end face of a measuring rod and a measuring face. Because the measurement increases as the measuring rod extends from the face, the readings on the barrel reversed from the normal; the start at a maximum(when the measuring rod is fully extended from the measuring face) and finish at zero (when end of the measuring rod is flush with the face).
INTERNAL AND HIEGHT MICROMETERS
Figure shows a height micrometer. The same idea as discussed under depth micrometer is applied to the height micrometers.
These micrometers are used for measuring internal dimensions. The micrometer can be a rod provided with spherical anvils as show in Figure(a).The measuring range of this micrometer is from 25 to 37.5 mm
i.e.12.5 mm. By means of exchangeable anvil rods, the measuring capacity can increased in steps of 12.5 mm up to 1000 mm. Another type of internal micrometer is that shown in Figure (b),in which the measuring anvils are inverted . The measuring range of this micrometer is from 5 to 30 mm i.e.25 mm
A circle can be divided into 360 equal angles. Each
angle is called degree. So a circle is 360 degrees. For calculation a degree is
divided into 60 parts called minutes and a minute is sub-divided into 60 parts
The bevel protractor is used to establish and test angles to very close tolerances. It reads to 5 minutes or 1/20 and can be used completely through 360o.The bevel protractor consists of a beam, graduated dial and blade which is connected to swivel plate (with Vernier scale) by thumb nut and clamp.
When the edges of the beam and blade are parallel, a small line on the swivel plate coincides with the zero line on the graduated dial, and when any measurement of an angle between the beam and the blade of 90 degrees or under is desired, the reading may be obtained direct from the position of the line on the swivel plate with regard to the graduation numbers on the dial. But remember this: To obtain the measurement of the angle between the beam and the blade of over 90 degrees subtract the number of degrees as indicated on the dial from 180 degrees. This is because, the dial is graduated from opposite zero marks to 90 degrees each way
Since the spaces, both on the main scale and the vernier scale, are numbered both to the right and to the left from zero, any angle can be measured. The readings can be taken either to the right or to the left, according to the direction in which the zero on the main scale is moved.
bevel protractor vernier scale indicates every five minutes or 1/20 of a
degree. Each space on the vernier scale is 5 minutes less than two spaces on
the main scale. Twenty four spaces on the vernier scale equal in extreme length
twenty three double degrees. Thus, the difference between the space occupied by
two degrees on a main scale and the space of the vernier scale is equal to one
twenty fourth of two degrees or one twelfth of one degree(5minutes)
Read off directly from the main scale the number of whole degrees between 0 on this scale and the 0 of the vernier scale. Then count, in the same direction, the number of spaces from the zero on the vernier scale to a line that coincides with a line on the main scale; multiply this number by 5 and the product will be the number of minutes to be added to the whole number of degrees.
For example: Zero on the vernier scale has moved 28 whole degrees to the right of the 0 on the main scale and the 3th line on the vernier scale coincides with a line upon the main scale as indicated. Multiplying 3 by 5, the product, 15, is the number of minutes to be added to the whole number of degrees, thus indicating a setting of 28 degrees and 15 minutes.
ACCOMODATION AND ENVIRONMENT
Factors affecting Environment:
– temperature, humidity etc.,
– clean surrounding and minimum vibration enhance precision,
– adequate illumination,
– temperature equalization between standard, work piece, and
– thermal expansion effects due to heat radiation from lights,
– heating elements, sunlight and people,
– manual handling may also introduce thermal expansion.
The calibration area shall be adequately Free from vibrations generated by central air conditioning plants,vehicular traffics and other sources to ensure consistant and uniform operational conditions.The laboratory shall take all special precautions like mounting of special apparatus on vibration free tables and pillars etc.
Acoustic noice level in the laboratory shall be maintained to facilitate proper performance of calibration work.A threshold noise level of 60 dBA is recommended.
The calibration area shall have adequate level of illumination.Where permissible fluorescent lighting is preferred to avoid localized heating and temperature drift. The recommended level of illumination is 450-700Lux .
Environmental Conditions and monitoring
The environmental conditions for the activity of the laboratory shall be such as not to affect the required accuracy of measurement. Facility should be provided whenever necessary for recording temperature and humidity values prevailing during calibration.
Entry to the calibration area
As far as possible, only the staff engaged in the calibration activity shall be permitted entry inside the calibration area.
HOUSE KEEPING AND 5S
5S is the name of a workplace organization methodology that uses a list of five Japanese words which are seiri, seiton, seiso, seiketsu and shitsuke. translated into English, they all start with the letter “S”. The list describes how to organize a work space for efficiency and effectiveness by identifying and storing the items used, maintaining the area and items, and sustaining the new order. The decision-making process usually comes from a dialogue about standardization which builds a clear understanding among employees of how work should be done. It also instills ownership of the process in each employee.
Phases of 5S
There are 5 primary phases of 5S: sorting, straightening, systematic cleaning, standardizing, and sustaining. Additionally, there are three other phases sometimes included; safety, security, and satisfaction.
Eliminate all unnecessary tools, parts, and instructions. Go through all tools, materials, and so forth in the plant and work area. Keep only essential items and eliminate what is not required, prioritizing things as per requirements and keeping them in easily-accessible places. Everything else is stored or discarded.
Straightening or setting in order / stabilize (Seiton)
There should be a place for everything and everything should be in its place. The place for each item should be clearly labeled or demarcated. Items should be arranged in a manner that promotes efficient work flow, with equipment used most often being the most easily accessible. Workers should not have to bend repetitively to access materials. Each tool, part, supply, or piece of equipment should be kept close to where it will be used – in other words, straightening the flow path. Seiton is one of the features that distinguishes 5S from “standardized cleanup”. This phase can also be referred to as Simplifying
Shining or cleanliness / systematic cleaning (Seiso)
Clean the workspace and all equipment, and keep it clean, tidy and organized. At the end of each shift, clean the work area and be sure everything is restored to its place. This makes it easy to know what goes where and ensures that everything is where it belongs. Spills, leaks, and other messes also then become a visual signal for equipment or process steps that need attention. A key point is that maintaining cleanliness should be part of the daily work – not an occasional activity initiated when things get too messy.
Work practices should be consistent and standardized. All work stations for a particular job should be identical. All employees doing the same job should be able to work in any station with the same tools that are in the same location in every station. Everyone should know exactly what his or her responsibilities are for adhering to the first 3 S’s.
Sustaining the discipline or self-discipline (Shitsuke)
Maintain and review standards. Once the previous 4 S’s have been established, they become the new way to operate. Maintain focus on this new way and do not allow a gradual decline back to the old ways. While thinking about the new way, also be thinking about yet better ways. When an issue arises such as a suggested improvement, a new way of working, a new tool or a new output requirement, review the first 4 S’s and make changes as appropriate.
The LMM 300 T – Length measuring machine is a digital metrological instrument for direct measurements. Its high accuracy is not only obtained by the exact adoption of the Abbe comparator principle, but also by using a high Precision measuring principle. A rugged cat iron construction and precise guide-ways also make for the high accuracy of the machine. The bed is designed of high extension capability for numerous measurements. The LMM 300 T is mainly used in gauge and manufacturing industries, in stand rooms of engineering, automobile industries, calibration laboratories. Main field of application is measurements of gauges, which will retain their importance in
production and measurement control.
Internal measurements of
• Plain ring gauges
• Thread Ring Gauges – Effective Diameter
• Taper plain ring gauges
• Taper Thread ring gauges – Effective Diameter
External measurements of
• Plain Plug gauges
• Thread Plug Gauges – Effective Diameter
• Taper Thread Plug Gauges – Effective Diameter
• Gauge Blocks, Pin Gauges etc
The LMM 300 T – Length measuring machine meets the increased demands made
on accuracy, efficiency and ease of operation.
Measuring unit : Abbe measuring head
Resolution : Horizontal axis : 0.0001mm
Vertical axis : 0.001mm
Measuring range : Horizontal axis : 100mm Vertical axis : 50mm
Measuring Capacity : Horizontal axis : 300mm
Measurement Uncertainty : Horizontal axis : (0.5 + L/300)μm where L is in mm.
Vertical axis : 2.0μm over 50mm
Measuring pressure : 20 gm
Length : 900mm
Width : 300mm
Height : 450mm
Weight : 75kg
In order to ensure the high accuracy of the Length Measuring Machine, the principles laid down by Ernst Abbe in 1980 have rigorously been complied with. These principles are ……..
- Measurement, line of sight setting must always be based on the use of a graduate scale, with which the distance to be measured can directly be compared.
- The measuring instrument must be arranged that the distance to be measured is a straight line continuation of the measuring scale. With the length measuring machine lengths up to 100mm are directly measured by comparing the part with a precision steel scale which is sensed by the sensing head in Tran illumination. The scale is rigidly connected with the hosing. The ruling is located in the extended measuring axis, thus following the Abbe measuring principle. The sensing head is rigidly connected with the measuring spindle so that the sensing head automatically performs the axial displacements of the spindle. These displacements are indicated by a display unit and are read off as measuring values by the operator.
Coordinate measuring machine
A coordinate measuring machine (CMM) is a device for measuring the physical geometrical characteristics of an object. This machine may be manually controlled by an operator or it may be computer controlled. Measurements are defined by a probe attached to the third moving axis of this machine. Probes may be mechanical, optical, laser, or white light, amongst others.
The typical “bridge” CMM is composed of three axes, an X, Y and Z. These axes are orthogonal to each other in a typical three dimensional coordinate system. Each axis has a scale system that indicates the location of that axis. The machine will read the input from the touch probe, as directed by the operator or programmer. The machine then uses the X,Y,Z coordinates of each of these points to determine size and position with micrometre precision typically.
A coordinate measuring machine (CMM) is also a device used in manufacturing and assembly processes to test a part or assembly against the design intent. By precisely recording the X, Y, and Z coordinates of the target, points are generated which can then be analyzed via regression algorithms for the construction of features. These points are collected by using a probe that is positioned manually by an operator or automatically via Direct Computer Control (DCC). DCC CMMs can be programmed to repeatedly measure identical parts, thus a CMM is a specialized form of industrial robot.
Coordinate-measuring machines include three main components:
- The main structure which include three axes of motion
- Probing system
- Data collection and reduction system – typically includes a machine controller, desktop computer and application software.
The machines are available in a wide range of sizes and designs with a variety of different probe technologies. They can be operated manually or automatically through Direct Computer Control (DCC). They are offered in various configurations such as benchtop, free-standing, handheld and portable.
Portable Coordinate Measuring Machines
Portable CMMs are different from “traditional CMMs” in that they most commonly take the form of an articulated arm. These arms have six or seven rotary axes with rotary encoders, instead of linear axes. Portable arms are lightweight (typically less than 20 pounds) and can be carried and used nearly anywhere. The inherent trade-offs of a portable CMM are manual operation (always requires a human to use it), and overall accuracy is somewhat to much less accurate than a bridge type CMM. Certain non-repetitive applications such as reverse engineering, rapid prototyping, and large-scale inspection of low-volume parts are ideally suited for portable CMMs.
A gauge block (also known as a gage block, Johansson gauge, slip gauge, or Jo block) is a precision ground and lapped length measuring standard. Invented in 1896 by Swedish machinist Carl E. Johansson, they are used as a reference for the calibration of measuring equipment used in machine shops, such as micrometers, sine bars, calipers, and dial indicators (when used in an inspection role). Gauge blocks are the main means of length standardization used by industry.
Each gauge block consists of a block of metal or ceramic with two opposing faces ground precisely flat and parallel, a precise distance apart. Standard grade blocks are made of a hardened steel alloy, while calibration grade blocks are often made of tungsten carbide or chromium carbide because it is harder and wears less. Gauge blocks come in sets of blocks of various lengths, along with two wear blocks, to allow a wide variety of standard lengths to be made up by stacking them. The length of each block is actually slightly shorter than the nominal length stamped on it, because the stamped length includes the length of one wring film, a film of lubricant which separates adjacent block faces in normal use.
In use, the blocks are removed from the set, cleaned of their protective coating (petroleum jelly or oil) and wrung together to form a stack of the required dimension, with the minimum number of blocks. Gauge blocks are calibrated to be accurate at 68 °F (20 °C) and should be kept at this temperature when taking measurements. This mitigates the effects of thermal expansion. The wear blocks, made of a harder substance like tungsten carbide, are included at each end of the stack, whenever possible, to protect the gauge blocks from being damaged in use.
36 Johansson gauge blocks wrung together easily support their own weight.
Wringing is the process of sliding two blocks together so that their faces lightly bond. Because of their ultra flat surfaces, when wrung, gauge blocks adhere to each other tightly. Properly wrung blocks may withstand a 75 lbf (330 N) pull. While the exact mechanism that causes wringing is unknown.
The process of wringing involves four steps.
- Wiping a clean gauge block across an oiled pad .
- Wiping any extra oil off the gauge block using a dry pad
- The block is then slid perpendicularly across the other block while applying moderate pressure until they form a cruciform.
- Finally, the block is rotated until it is inline with the other block.
Various types of Gauge blocks
Steel gauge blocks have proven their reliability for more than hundred years. This raw material remains the most commonly accepted for length standards. Steel gauge blocks provide high resistance to wear associated with a good property to adhere to other gauge blocks. However, steel must be protected against corrosion. Provided gauge blocks made from this material are properly handled, they will remain reliable for many years.
steel gauge blocks have the following key features:
• Highly alloyed steel
• Hardness guaranteed to 800 HV
• Artificially aged for optimum form and dimensional stability
• Coefficient of thermal expansion: (11,5 ± 1,0) x 10-6 K-1
Gauge blocks in tungsten carbide are 10 times as much resistant as steel gauges. They are intended for frequent use, also where superior wringing quality is required. Tungsten carbide gauge blocks provide:
• Hardness guaranteed to 1400 HV
• Coefficient of thermal expansion: (4,23 ± 0,1) x 10-6 K-1
Ceramic gauge blocks are extremely resistant to wear and scratches. Due to the properties of this material, any minor damage is unlikely to affect the wringability of their meas uring faces. Being corrosion resistant, these gauge blocks are insensitive to sweaty hands, among others. Manufactured from stabilised zirconia, ceramic gauge blocks have the following key features:
• Hardness guaranteed to 1400 HV
• Coefficient of thermal expansion: (9,7 ± 0,8) x 10-6 K-1
Grades of slip gauges
These gauge blocks are commonly used as «Working Standards» in inspection rooms within the
production to set and calibrate measuring instruments and other equipment as well as to inspect
tools, fixtures and machines.
Gauge blocks of this class are mainly used as «Working Standards» to set and calibrate plug
gauges and measuring instruments in measuring rooms or inspection areas within the production.
These gauge blocks are designated for use as «Company Standards» in calibration laboratories or
environmentally controlled inspection room to set and calibrate plug gauges as well as measuring
Calibration grade K
Gauge blocks of this tolerance class are intended for use as «Reference Standards» in metrology
oriented laboratories of National Institutes, precision measuring rooms and other laboratories of
National Calibration Services, whether officially accredited or not. They should be used as masters
to calibrate gauge blocks, length standards of same accuracy and measuring instruments as well.
|Used for the calibration of Venire Calipers and Height Gauges, Caliper Checker is designed to check accuracy of Main Scale at regular intervals through out the range, which meets the requirements of IS Standards.|
The instrument is sufficiently rigid and
consists of stepped slip gauges permanently fixed in inner casing .The Caliper
Checker is ideal for in house periodic calibration of measuring instruments
i.e. Venire Caliper and Height Gauge.
|Range(mm) Height Measurement External Measurement Internal Measurement 0-300 370 300 300 0-600 670 600 600 0-1000 1070 1000 1000|
Made to measure any size in in the form of internal, external, height, depth, step and distance dimensions of geometric part features having either a flat, parallel or cylindrical surface. Automatic capture of the culmination point on bores or shafts
- Measuring Range : 0-615 mm
- Application Range
With standard Accessory: 0-770mm
With probe inset holder No.00760057: 0-825mm
With probe inset holder No.S07001622: 0-995mm
- Accuracy : 2+1.5L μm (L in m)
- Repeatability: 0.5μm on flat surfaces and 1μm into bores
- Least Count: 0.1 μm
- Frontal, Mechanical : 7 μm
- Rugged nickel plate gauge base having 3 resting points, finely lapped.
- Built-in air bearing for easy move of the column over the surface plate.
- Incremental glass scale with datum point, 20 grating division .Opto-electric data acquisition
- Special Feature: _Measuring head mounted on a ball-bearing.
_Motorized head displacement at a varying speed from 7- 40mm/s
_Manual displacement: ≤600mm/s
MEANINGS OF TERMS USED IN METROLOGY
Absolute accuracy The accuracy stated as a definite amount, i.e., not as a percentage.
Absolute position measurement Position measured from a fixed point.
Absolute pressure Pressure measured with reference to a perfect vacuum.
Accelerometer A sensor for measuring acceleration or the rate of change of velocity.
Accuracy A measure of the difference between the indicated value and the true value.
Actuator A device that performs an action on one of the input variables of a process according to a signal received from the controller.
ADC An analog-to-digital converter that converts an analog voltage or current into a digital signal.
Alarm A warning that a variable has exceeded set limits.
Alternating current Current that flows in one direction during one half of a regular time period and the opposite direction during the other half.
Ammeter An instrument for measuring electrical current or electron flow.
Ampere The unit of current or electron flow.
Amplifier An electrical circuit that increases the magnitude of a signal.
Analog A continuously varying signal.
Aneroid barometer Abarometer which uses an evacuated capsule as a sensing element.
Anticipatory action See Derivative action.
Aqueous solution A solution containing water.
Atmospheric pressure The pressure acting on objects on the earth’s surface caused by the weight of the air in the earth’s atmosphere, normally measured at sea level.
Barometer An instrument used for measuring atmospheric pressure.
Bellows A pressure sensor that converts pressure into linear displacement.
Bernoulli equation A flow equation based on the conservation of energy which includes velocity, pressure, and elevation terms.
Beta ratio The ratio of the diameter of a restriction to the diameter of the pipe containing the restriction.
Bimetallic A thermometer with a sensing element made of two dissimilar metals with different thermal coefficients of expansion.
Binary Two values, or a numbering system using the base 2.
Bit A binary digit.
Bourdon tube A pressure sensor that converts pressure to movement. The device is a coiled metallic tube that straightens when pressure is applied.
Bridge A network of passive components arranged so that small changes in one of the components can be easily measured.
British thermal unit A measure of heat energy, i.e., the amount of heat required to raise 1 lb of water 1°F at 68°F and atmospheric pressure.
Buffer amplifier Acircuit for matching the output impedance of one circuit to the input impedance of another.
Buoyancy The upward force on an object floating or immersed in a fluid caused by the difference in pressure above and below the object.
Byte Eight bits of binary information.
Calorie A measure of heat energy, i.e., the amount of heat required to raise the temperature of 1 g of water by 1°C.
Capacitance A measure of a device’s ability to store electrical charge.
Capacitance probe An instrument using the capacitance between two metal plates for measuring fluid level.
Capacitor A device that can store electrical charge.
Cell Asimple power source that provides emf, usually by means of a chemical reaction.
Celsius One of the commonly used temperature scales.
Coefficient of heat transfer Aterm used in the calculation of heat transfer by convection.
Coefficient of thermal expansion A term used to determine the amount of linear expansion due to heating or cooling.
Comparator A device which compares two signals and outputs the difference.
Concentric plate A plate with a hole located at its center (orifice plate) used to measure flow by measuring the differential pressures on either side of the plate.
Conduction The movement of heat energy in a material by the transfer of energy from one molecule to another.
Conductivity probe An instrument using two electrodes to measure fluid level.
Continuity equation A flow equation which states that, if the overall flow rate is not changing with time, the flow rate past any section of the system must be constant.
Continuous level measurement A level measurement that is continuously updated.
Controlled variable The variable measured to indicate the condition of the process output.
Controller The element in a process control loop that evaluates any error of the measured variable and initiates corrective action by changing the manipulated variable.
Convection The movement of heat by the motion of warm or hot material
Converter A device that changes the format of a signal but not the type of energy used as the signal carrier, i.e., voltage to current.
Correction signal The signal to the manipulated variable.
DAC A device that converts a digital signal into an analog voltage or current.
Dead weight tester A device for calibrating pressure-measuring devices which uses weights to provide the forces.
Decibel (dB) A unit used to compare amplitude or power levels.
Density The amount of mass in a unit volume.
Derivative action Action that is proportional to the rate at which the measured variable is changing.
Dew point The temperature at which the water vapor in a mixture of water vapor and gas becomes saturated and condensation starts.
Dielectric constant The factor by which the capacitance between two plates changes when a material fills the space between the plates.
Differential amplifier An amplifier that amplifies the difference between two inputs.
Digital Signals having two discrete levels.
Dry-bulb temperature The temperature indicated by a thermometer whose sensing element is dry.
Dynamic pressure That part of the total pressure in a moving fluid caused by the fluid motion.
Dynamometer An instrument used for measuring torque or power.
Eccentric plate An orifice plate with a hole located below its center to allow for the passage of suspended solids.
Effective value The dc voltage or dc current that would produce the same power in a load as the ac voltage or ac current being measured.
Electromagnetic flow meter Aflow-measuring device which senses the change in a magnetic field between two electrodes as a fluid flows between them.
Electromagnetism The relationship between magnetic fields and electric current.
Electromotive force (emf) The force that causes electrons to move, and is measured in volts.
Error signal The difference in value between a measured signal and a set point.
Fahrenheit One of the commonly used temperature scales.
Farad The unit of capacitance.
Feedback (1) The voltage fed from the output of an amplifier to the input in order to control the characteristics of the amplifier. (2) The measured variable signal fed to the controller in a closed-loop system, so that the controller can adjust the manipulated variable to keep the measured variable within set limits.
Fiber optics The transmission of information through optical cables using light signals.
Flow nozzle Adevice placed in a flow line to provide a pressure drop that can be related to flow rate.
Flow rate The amount of fluid passing a given point in a given interval of time.
Flume An open-channel flow-measuring device.
Form drag The force acting on an object due to the impact of fluid.
Foundation fieldbus Process control bus used in the United States.
Free convection Movement of heat as a result of density differences.
Free surface The surface of the liquid in an open-channel flow that is in contact with the atmosphere.
Frequency The number of cycles completed in 1 s.
Gauge pressure The measured pressure above atmospheric pressure.
Gas thermometer Atemperature sensor that converts temperature to pressure in a constant volume system.
Hall-effect sensor A transducer that converts a changing magnetic field into a proportional voltage.
Head Sometimes used to indicate pressure, i.e., 1 ft of “head” for water is the pressure under a column of water 1 ft high.
Heat A form of energy related to the motion of atoms or molecules.
Heat transfer The study of heat energy movement.
Henry (H) The unit of inductance.
Hertz (Hz) A measure of frequency in cycles/second.
Hot-wire anemometry A velocity-measuring device for gas or liquid flow that senses temperature changes, due to the cooling effect of gas or liquid moving over a hot element.
Humidity A term to indicate the amount of water vapor present in the air or a gas.
Humidity ratio The mass of water vapor in a gas divided by the mass of dry gas in the mixture.
Hydrometer An instrument for measuring liquid density.
Hydrostatic paradox The fact that pressure varies with depth in a static fluid, but is the same throughout the liquid at any given depth.
Hydrostatic pressure The pressure caused by the weight of static fluid.
Hygrometer A relative humidity-measuring device.
Hygroscopic Amaterial that absorbs water and whose conductivity changes with moisture content.
Hysteresis The non reproducibility in an instrument caused by approaching a measurement from opposite directions, i.e., going from low up to the value, or high down to the value.
Impact pressure The sum of the static and dynamic pressure in a moving fluid.
Impedance An opposition to ac current or electron flow caused by inductance and/or capacitance.
Incremental position measurement An incremental position measurement from one point to another, absolute position is not recorded, and position is lost if the power fails.
Indirect level-measuring device A device that extrapolates the level from the measurement of another variable, i.e., liquid level from a pressure measurement.
Inductance An electrical component that opposes a change in current or electron flow.
Inductor A device that exhibits inductance.
Instrument A device used to measure a physical variable.
Integral action The action designed to correct for long-term loads.
Kelvin The absolute temperature scale associated with the Celsius scale.
Kirchoff’s current law The sum of the currents flowing at a node is zero.
Kirchoff’s voltage law The algebraic sum of voltages around a closed path is zero.
Ladder logic The programmable logic used in PLCs to control automated industrial processes.
Lag time The time required for a control system to return a measured variable to its set point.
Laminar flow A smooth flow in which the fluid tends to move in layers.
LED Light emitting diode
Linearity A measure of the direct proportionality between actual value of the variable being measured and the value of the output of the instrument to a straight line.
Load The process load is a term used to denote the nominal values of all variables in a process that affect the controlled variable.
Load cell A device for measuring force.
Loudness A subjective quantity used to measure relative sound strength.
LVDT A linear variable transformer that measures displacement by conversion to a linearly proportional voltage.
Magnetorestrictive element (MRE) A magnetic field sensor that converts a changing magnetic field into a proportional resistance.
Manipulated variable The variable controlled by an actuator to correct for changes in the measured variable.
Measured variable The variable measured to indicate the condition of the process output.
Meniscus The convex or concave surface of a column of liquid in a tube.
Moment The effect of a force acting at a given perpendicular distance from a point.
Natural convection The movement of heat as a result of density differences.
Newtonian fluid A fluid in which the velocity varies linearly across the flow section between parallel plates.
Node A junction of three or more conductors.
Noise The term usually used to indicate unwanted or undesirable sounds.
Nutating disk meter A flow-measuring device using a disk that rotates and wobbles in response to the flow.
Offset The nonzero output of a circuit when the input is zero.
Ohmmeter An instrument used to measure resistance.
ON/OFF control A system in which a process actuator has only two positions, i.e., on and off.
Open-channel flow The flow in an open conduit (e.g., as in a ditch).
Operational amplifier A circuit used to amplify electronic signals.
Orifice plate A plate containing a hole which when placed in a pipe causes a pressure drop which can be related to flow rate.
Over pressure The term used to describe the maximum amount of pressure a gauge can withstand without damage or loss of accuracy.
Overshoot The overcorrection of the measured variable in a control loop.
Parabolic velocity distribution Occurs in laminar flow when the velocity across the cross-section takes on the shape of a parabola.
Parallel transmission Simultaneous transmission of a number of binary bits.
Pascal Pressure reading units (SI), i.e., newtons per square meter
Pascal’s law The pressure applied to an enclosed fluid is transmitted to every part of the fluid.
Percent of reading The accuracy given in terms of the percentage of the reading.
Percentage full-scale accuracy The accuracy determined by dividing the accuracy of an instrument by its full-scale output taken as a percentage.
Period A fixed amount of time during which alternating current is completing one full cycle and is the inverse of the frequency in Hertz.
pH A term used to indicate the activity of the hydrogen ions in a solution, it helps to describe the acidity or alkalinity of the solution.
Phase A term used to describe the state of matter, i.e., solid, liquid, or gas.
Phons A unit for describing the difference in loudness levels.
Photodiode A sensor used to measure light intensity by measuring the leakage across a pn junction.
PlD Proportional control with derivative and integral action.
P&ID Stands for piping and instrument diagrams.
Piezoelectric effect The electrical voltage developed across certain crystalline materials when a force or pressure is applied to the material.
Pitot-static tube A device used to measure the flow rate using the difference between dynamic and static pressures.
PLC Programmable logic controller.
Pneumatic System that employs gas for control or signal transmission.
Poise The measurement unit of dynamic or absolute viscosity.
Potentiometer (Pot) An adjustable resistance device.
Precision The smallest division that can be read on an instrument.
Pressure The magnitude of a force divided by the area over which it acts, i.e., psi or Pa.
Pressure differential The difference in pressure amplitudes at two locations.
Process A sequence of operations carried out to achieve a desired end result.
Process control The automatic control of certain process variables to hold them within given limits.
Processor A digital electronic computing system that can be used as a control system.
Profibus Process control bus used in Europe
Proportional action A controller action in which the controller output is directly proportional to the measured variable error.
Psychrometric chart A chart dealing with moisture content in the atmosphere.
Pyrometer An instrument for measuring temperature by sensing the radiant energy from a hot body.
Radiation The emission of energy from a body in the form of electromagnetic waves.
Range The lowest to the highest readings that can be made by a sensing device.
Rankine The absolute temperature scale associated with the Fahrenheit scale.
Rate action See Derivative action.
Reactance The opposition to an ac current or electron flow caused by a capacitor or an inductor.
Relative humidity The amount of water vapor present in a given volume of a gas, expressed as a percentage of the amount that would be present in the same volume of gas under saturated conditions at the same pressure and temperature.
Reluctance The opposition in a material to carrying magnetic flux, it is the magnetic equivalence to resistance.
Repeatability A measure of the closeness between several consecutive readings of a value.
Reproducibility The ability of an instrument to produce the same reading of a variable with repeated readings.
Reset action See Integral action.
Resistance A measure of the opposition to electron or current flow in a material.
Resistance thermometer (RTD) A temperature sensor that provides temperature readings by measuring the resistance of a metal wire (usually platinum).
Resistivity Atemperature-dependent “constant” that reflects a material’s resistance to electron flow.
Resistor A component that exhibits resistance.
Resolution The minimum detectable change of a variable in a measurement.
Reynolds Number A dimensionless number indicating whether the flow is laminar or turbulent.
Rotameter A flow-measuring device in which a float moves in a vertical tapered tube.
Saturated The condition when the maximum amount of a material is dissolved in another material at the given pressure and temperature conditions, i.e., water vapor in a gas.
Sealing fluid An inert fluid used in a manometer to separate the fluid whose pressure is being measured from the manometer fluid.
Segmented plate An orifice plate with a hole located so as to allow suspended solids to pass through.
Sensitivity The ratio of the change in output to input magnitudes.
Sensor A device that can convert a physical variable into a measurable quantity.
Serial transmission A sequential transmission of digital bits.
Set point The reference value for a controlled variable in a process control loop.
Signal conditioning The conversion of a signal to a format that can be used for transmission.
Single-point level measurement Indicates when a particular level has been reached.
Sling psychrometer A device for measuring relative humidity.
Smart sensor Integration of a processor directly into the sensor assembly to give direct control of the actuator and digital communication to a central controller.
Sone A unit for measuring loudness.
Sound pressure level The difference between the maximum air pressure at a point and the average air pressure at that point.
Span The difference between the lowest and highest reading for an instrument.
Specific gravity The ratio of the specific weight of a solid or liquid material and the specific weight of water, or for a gas, the ratio of the specific weight of the gas and the specific weight of air under the same conditions.
Specific heat The amount of heat required to raise a definite amount of a substance by one degree, i.e., 1 lb 1°F or 1 g 1°C.
Specific humidity The mass of water vapor in a mixture divided by the mass of dry air or gas in the mixture.
Specific weight The weight of a unit volume of a material.
Static pressure The part of the total pressure in a moving fluid not caused by the fluid motion.
Stoke The measurement unit of kinematic viscosity.
Strain gauge Asensor that converts information about the deformation of solid objects when they are acted upon by a force into a change of resistance.
Sublimation Passing directly from solid to vapor or vapor to solid.
Telemetry The electrical transmission of information over long distances usually by radio frequencies.
Temperature The term used to describe the hotness or coldness of an object.
Thermal conductivity A measure of the ability of a material to conduct heat.
Thermal expansion The expansion of a material as a result of its being heated.
Thermal time constant The time required for a body to heat or cool by 63.2 percent of the difference between the initial temperature and the aiming temperature.
Thermistor A temperature sensing element made from a metal oxide that usually has a negative temperature coefficient.
Thermocouple Atemperature sensing device that uses dissimilar metal junctions to generate a voltage proportional to the differential temperature between the metal junctions.
Thermometer An instrument used to measure temperature.
Thermopile A number of thermocouples connected in series.
Time constant (electrical) The amount of time needed for a capacitance C, to discharge or charge through a resistance R, by 62.3 percent of the difference between the initial voltage and the aiming voltage; the product of RC gives the time constant in seconds.
Torque The name given to a force moment that tends to create a twisting action.
Torr The pressure caused by the weight of a column of mercury 1 mm high.
Total flow The amount of flow past a given point over some length of time.
Total pressure The sum of the static and dynamic pressures in a moving fluid.
Transducer A device that changes energy from one form to another.
Transfer function An equation that describes the relationship between the input and output of the function.
Transmission The transferring of information from one point to another.
Transmitter A device that conditions the signal received from a transducer so that it is suitable for sending to another location with minimal loss of information.
Turbine flow meter A flow-measuring device using a turbine wheel.
Turbulent flow An agitated flow in which there are random velocity fluctuations on top of the average flow.
U-tube manometer A glass tube in the shape of the letter U that is used to measure pressure or pressure differences.
Ultrasonic probe An instrument using high-frequency sound waves to measure fluid levels.
Vacuum (pressure) The amount that the measured pressure is below atmospheric pressure.
Velocity Ameasure of speed, and in a flow is the average speed across the flow and the direction of movement of a liquid.
Vena contracta The narrowing down of the fluid flow stream as it passes through an obstruction.
Venturi tube Aspecially shaped restriction in a section of pipe that provides a pressure drop which can be related to flow rate.
Viscometer (viscosimeter) An instrument for measuring viscosity.
Viscosity The term describing the resistance to flow of a fluid.
Volt The unit of electromotive force.
Voltage An electromotive force that causes electrons or a current to flow.
Voltage drop The difference in voltage between two points.
Vortex Swirling or rotating fluid motion.
Wavelength The time for an alternating source to complete a full cycle.
Weir An open-channel flow-measuring device.
Wet-bulb temperature The temperature indicated by a thermometer whose sensing element is kept moist.
Wheatstone bridge The most common electrical bridge circuit used to measure small changes in the value of an element.