Measurement & CalibrationMeasurement & Calibration

ByByRamesh DhamRamesh DhamAM-InstrumentationAM-InstrumentationVenue: Training HallVenue: Training HallDate: 25-Sep-2008Date: 25-Sep-2008Time 4:30 PMTime 4:30 PM

MeasurementMeasurement

Measurement is the first step that leads to control and eventually to improvement- If you can’t measure something, you can’t understand it. If you can’t understand it, you can’t control it. If you cant control it, you can’t improve it.

What gets measured gets done.

One accurate measurement is worth a thousand expert opinions.

UNITS OF MEASUREMENTUNITS OF MEASUREMENT

The measurement of quantity is done by comparing it with some standard called “unit”. A unit, therefore, is any division of quantity, which is accepted as one unit of that quantity. A quantity (Q) is expressed as the product of a number (number of times in comparison to the standard) and the name given to the unit or standard.

• Q=nXname of unit • Q=nu

System of units System of units

• Our earlier units have been human based (in the context of what we use in our daily life) and, therefore, varied from country to country and even from society to society. We had measure of length (foot) in terms of the length of a foot step as unit.

• Relating units to immediate physical world is not wrong; rather it is desirable. What is wrong is that there are many units for the same quantity with no relative merit over each other. We are led to a situation, where we have different units for the same quantity, based on experiences in different parts of the world. These different units of the same quantity do not bear any logical relation amongst themselves. We, therefore, need to have uniform unit system across the world.

System of units System of units

Classify quantities in two groups :

• Basic or fundamental quantities • Derived quantities

Basic or fundamental units are a set of units for physical quantities from which other units can be derived. We are limited to study few of basic units; others (derived) are derived from them.

Features of fundamental units

• They are not deducible from each other. • They are invariant in time and place (in classical context). • They can be accurately reproduced. • They describe human physical world.

International System of UnitsInternational System of Units• The International System of Units (abbreviated SI from systeme

internationale , the French version of the name) is a scientific method of expressing the magnitudes or quantities of important natural phenomena. There are seven base units in the system, from which other units are derived. This system was formerly called the meter-kilogram-second (MKS) system.

• All SI units can be expressed in terms of standard multiple or fractional quantities, as well as directly. Multiple and fractional SI units are defined by prefix multipliers according to powers of 10 ranging from 10 -24 to 10 24 .

There are seven basic quantities included in SI system of measurement : • Length • Mass • Time • Current • Temperature • Amount of substance (mole) • Luminous intensity

Definitions of the SI base unitsDefinitions of the SI base units • Unit of length  meter 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.  • Unit of mass kilogram  The kilogram is the unit of mass; it is equal to the mass of the

international prototype of the kilogram.  • Unit of time second The second is the duration of 9 192 631 770 periods of the

radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.  

• Unit of electric current  ampere The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length.  

• Unit of thermodynamic temperature kelvin The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.  

• Unit of amount of substancemole1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is "mol." 2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.  

• Unit of luminous intensity candela The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian

Conversion of unitsConversion of units

Despite endeavor on world level for adoption of SI unit, there are, as a matter of fact, wide spread variation in the selection of unit system. Engineering world is full of inconsistencies with respect to the use of unit system. We often need to have skill to convert one unit into another. We take a simple example here to illustrate how it is done for the case of basic quantity like mass.

Let us consider a mass of 10 kg, which is required to be converted into gram - the mass unit in cgs unit (Gaussian system). Let the measurements in two systems are “ n1u1 ” and “ n2u2 ” respectively. But, the quantity, “Q”, is “10 kg” and is same irrespective of the system of units employed. As such,

Q=n1u1=n2u2 n2=104 Q=n2u2=104gm The process of conversion with respect to basic quantities is straight

forward. The conversion of derived quantities, however, would involve dimensions of the derived quantities. We shall discuss conversion of derived quantities in a separate module.

Imperial measuring SystemImperial measuring System Imperial System There is an older set of units which some people still use or refer to and some

materials are still ‘named in this measurement. It is called Imperial measuring and uses feet, inches, yards and miles.

Distance• Inches " approximately equal to 2.54 cm or 25.4 mm• Feet ' 12 inches = 1 foot Approx. equal to 30 cm and 300 mm• Yard yd 36 inches = 3 feet Approx. equal to 91 cm and 910 mm• Chain chn 66 feet Approx. equal to 1980 cm and 19800 mm?• Mile m 5280 feet or 1760 yards Approx. equal to 158400 cm and 1584000 mm? • Weight• Ounces oz 28 grams

Pound lb 0.45 kg or 450 grams

• Volume• Pint pt 568 ml or 0.568 l

Gallon g 4.54 litres

CalibrationCalibration

The set of operations which establish under specific conditions, the relationship between values indicated by measuring instrument or measuring system or value represented by a material measure or a reference material, and the corresponding value of a quantity realized by a reference standard.

NPL India NPL, UK NIST, USA

ERTL-E ERTL-W ERTL-N ERTL-S OTHER

ETDCStates

OTHERLAB

ETDCStates

ETDCStates

ETDCStates

Calibration LabsCalibration Labs

NPL – National Physical Laboratory.

ERTL – Electronics Regional Test Laboratory.

ETDC – Electronics Test and Development Center

Need of CalibrationNeed of Calibration

The main objectives of calibration services are:

To maintain quality control and quality assurance in production.

To comply with requirements of global trade.

To meet the requirement of ISO guides.

To promote international recognition.

For tracking back measurement results to national standards.

Benefits of CalibrationBenefits of Calibration It fulfils the requirements of traceability to national /

international standards like ISO 9000, ISO 14000 etc. As a proof that the instrument is working. Confidence in using the instruments. Traceability to national measurement standard. Interchangeability. Reduced rejection, failure rate thus higher return. Improved product and service quality leading to satisfied

customers. Power saving. Cost Saving. Safety.

The Basic Requirements for CalibrationThe Basic Requirements for Calibration

Reference / Calibration Standards & other instruments / equipments

Controlled Environment Conditions.Competence of Calibration Lab personnel.Traceability of Reference / Calibration

standards.Documentation.

Reference / Calibration StandardReference / Calibration Standard

Generally calibration standards are categorised as Passive and Active standards.

Passive Standards: A passive standard is one that either (a) does not require or rely on

operating power from external source or (b) does not contain elements within its circuitry, that are responsible for amplification of the signal.

e.g. Decade Resistance, Current Shunt, RTD, Thermocouple etc.Active Standard: An active instrument on the other hand does rely on powered circuitry which

processes applied signal in some manner or other.e.g. A digital multimeter, A multifunctional calibrator etc.

Further Classification of Standards are made Primary Level standard Secondary level standard Tertiary or working Level Standard

Control On Environmental ConditionsControl On Environmental Conditions

In order to derive best accuracy and meaningful calibration result it is important to control environmental conditions in which measurements are made. Control and monitoring of following factors should be maintained, as recommended by manufacturer of standard / instrument.

Temperature ( e.g. 25 +/-4.0 deg.C) Relative Humidity (e.g. </=70% RH) Illumination level ( e.g. minimum 450 Lx.) Acoustic Level (e.g. max. 60 dB) Shock and Vibration should be adequately. Power supply Regulation (e.g. +/- 1%) Temperature gradient (e.g. 1.5 deg.C / hour) Proper earth etc..

DocumentationDocumentation In any quality system, documentation has very important role and

therefore, proper care shall be taken for documenting. Calibration Procedure / methods. Calibration Results( Recording of data) Calibration Report and Calibration Certificate

A full explanation of documentation one may find in guidelines of ISO 9000 ( ISO 10012-1) documents.

However, calibration report must be address answer for Who? ( Name / identification of Cal Lab Tech./Approving authority) What? ( Cal. Result / Data) When? ( Calibration Date) Where?( Location / address, where calibration done) How? ( Calibration Procedure / Method) Limitations / decision shall also be reflected.

UNDERSTANDING UNDERSTANDING INSTRUMENT’S SPECIFICATIONINSTRUMENT’S SPECIFICATION

The term “accuracy” in measurement refer to the typically closeness of a measurement result to the true value. The term “accuracy” is frequently used to denote a small difference from true value, really the “inaccuracy” of measurement. In common language we use the accuracy is +/- 1.0% of full scale, here, what is intended that the “ limits of inaccuracy are +/-1.0% of full scale”.

In order to select appropriate test / measuring instrument it is imperative to have full understanding of instrument specifications.

Analog and Digital Meter AccuracyAnalog and Digital Meter Accuracy

The analog meters are usually given as a percentage of full scale, such as +/-1.0% of FSD. Analog instruments may also be classified according to their accuracy class such as 0.2, 0.5 etc, which means limits of error in percentage of full scale i.e. +/-0.2% of FS, 0.5% of FS so on.

The major consideration here is that accuracy of the reading (absolute accuracy) decreases as the reading become less than full scale.

Digital instrument manufacturers specify the performance specification in many ways. These normally includes input specification, range specification and some time a floor value. However in majority of cases accuracy is usually specified as : +/- % of reading +/- % of range +/- counts.

In this case % error indicated gives % of reading or input. The count errors depends on the resolution of the display.

Determination of Absolute AccuracyDetermination of Absolute AccuracyTypical Specification

X% of input +/- Y5 of range +/- Z counte.g. If accuracy of a meter is given as+/-(0.002% of input + 0.001% of range + 10 uV), Find out absolute accuracy on meter range 0.1 V for a voltage level of 25mV.

Solution:Step:1 +/- 0.002% of 25 mV = 0.002 x 25 = 0.0005 mV = +/- 0.5 uV 100Step:2 +/- 0.001% of 0.1 V = 0.001 x 0.1 = 0.000001 mV = +/- 1.0 uV 100Step: 3 = +/- 10 uVStep: 4 = +/- Add( Step1,2 &3) = 11.5 uV = Absolute accuracyStep: 5 = Divide 11.5 by 25000uV ( 25mV) x 100 = +/- 0.044% of reading (i.e. absolute accuracy in % of reading)

Accuracy RatioAccuracy Ratio( Reference Standard : Unit Under Calibration)( Reference Standard : Unit Under Calibration)

Preferred => 1 : 10As per ANSI / NCSL Z540-1-1994 Standard =>1 : 4As per ISO 10012-Part-1-1992 Standard => 1 :3

Calibration Standard

( e.g. Accuracy : +/- 0.33 deg.C

Process Instrument

( e.g. Accuracy : +/- 1.0 deg.C

Process Tolerance

( e.g. Temp. Required : 250 +/- 10 deg.C

Why Accuracy Ratio Why Accuracy Ratio Should Be 1 : 3Should Be 1 : 3

If S = UUC and U = Reference Standard Specification.The Resultant Specification, R is Given By R = Sqrt( S^2 + U^2)With U :S = 1 : 3 R = Sqrt( 3^2 + 1^2) = 3.162The resultant Specification Expands by [{(R-S) / S} X 100]

i.e. by 5.4%If U :S = 1: 10, The Specification Expands by 0.5%If U :S = 1: 4, The Specification Expands by 3.1%If U :S = 1: 3, The Specification Expands by 5.4%If U :S = 1: 2, The Specification Expands by 11.5%If U :S = 1: 1, The Specification Expands by 41.4%

ISO 9001:2000 – Calibration RequirementISO 9001:2000 – Calibration Requirement

Clause 7.6 of ISO9001:2000 Control of Monitoring & Measuring Devices.

Calibration may effect the following other clauses of ISO 9001:2000

7.3.5 :Design & Development Verification7.3.6 :Design & Development Validation7.4.3 :Verification of Purchased Product7.5.1 :Control of Production & Service Provision7.5.2 :Validation of Process for Production & Service Provision.6.3 :Infrastructure8.2.4 :Monitoring and Measurement of Product.

A Close Look to ISO9001:2000, A Close Look to ISO9001:2000, Clauses give rise to the points Like:Clauses give rise to the points Like:

Do you have procedure for inspecting on receipt at the factory to check for conformity of purchase products with specification?

Do the procedure cover the verification of test certification and other data received from the vendor along with purchased product?

Is product status identified with respect to measuring and monitoring requirement.

Are monitoring, measuring, analysis and improvement processes planned and implemented as needed to demonstrate conformity of the product?

Can it be demonstrated that the measurement and monitoring of the product is carried out at various stages of the product realization process in accordance with planned arrangements?

Study of above points ultimately lead to requirement for establishing monitoring Study of above points ultimately lead to requirement for establishing monitoring and measurement facility with the sole purpose that there exists to be a system and measurement facility with the sole purpose that there exists to be a system to demonstrate one’s capability to provide reliable measurements result. For to demonstrate one’s capability to provide reliable measurements result. For establishing an acceptable system there is need to introduce element of establishing an acceptable system there is need to introduce element of ISO100012 part-1 content of which could be briefly outlined as follows:ISO100012 part-1 content of which could be briefly outlined as follows:

Selection and identification of measuring equipment. Procuring measuring Equipment. Calibrating measuring equipment Frequency of calibration. Calibration Status Calibration Record Safeguard & care of measuring equipments Handling and controlling equipments Action when equipment is out of calibration Establishing traceability Cumulative effects of uncertainty Controlling environment conditions.

Interval / Frequency of CalibrationInterval / Frequency of Calibration

The Frequency of calibration is one of the most argued point in calibration control program and considered to be gray area however, we need to assign frequency for each piece of equipment. Any prudent exercise for fixing interval or frequency of calibration shall include:

Type of equipment Frequency of use Manufacturer’s recommendations Environmental conditions of use Maintenance and service Accuracy of measurement sought Frequency of cross-check Loss due to an incorrect data getting accepted because of measuring

equipment has become faulty.

While setting the guideline for frequency of calibration, cost of calibration can’t be ignored and hence one make balance between cost of calibration and cost due to incorrect data getting accepted due to measuring equipment is faulty.

For initial choice of Interval factor to be taken into account are:a) The equipment manufacturer’s recommendations.b) The extent and severity of usec) The influence of environmentd) The accuracy of measurement sought

A range of methods is available for reviewing the confirmation intervals. These differ according to whether:

Items of equipment are treated individually or as a groups( e.g. by make or by type) Item fail to comply with their specifications due to drift with the lapse of time, or by use Data are available and importance is attached to the history of calibration of the

equipment.

No single method is ideally suited for the full range of equipment encountered.

There are five different methods for thata. Automatic or “Staircase “ adjustment methodb. Control Chart Methodc. Calendar time methodd. “In-use” time methode. In-service or “black-box testing” method

Method of Finding intervalsMethod of Finding intervals

Method of Finding intervalsMethod of Finding intervals

a Automatic or “Staircase “ adjustment method : Each time an item or equipment is confirmed on a

routine basis, the subsequent interval is extended if it is found to be with in tolerance.

b. Control Chart Method: The same calibration points are chosen from every

confirmation and the results are plotted against time. From these plots, scattered and drift are calculated, the drift being either the mean drift over one confirmation interval or, in case of very stable equipment, the drift over several intervals. From these figures the effective drift may be calculated.

Method of Finding intervalsMethod of Finding intervals

c. Calendar time method: Item of measuring equipment are initially arranged into groups on

the bases of their similarity of construction and of their expected similar reliability and stability. A confirmation interval is assign to the group, initially on the bases of engineering intuition. In each group, the quantity of items which return at their assigned confirmation interval and are found to have excessive error or to be otherwise non-confirming is determined and expressed as a proportion of the total quantity of item in that group which are confirmed during a given period. If the proportion of nonconforming items of equipment is excessively high, the confirmation interval should be reduced. If it appears that a particular sub-group of items( such as a particular make or type) does not behave like the other member of the group, this sub-group should be removed to a different group with a different confirming level. If the proportion of nonconforming items of equipment in a given group proves to be very low, it may be economically justifiable to increase the confirmation level.

Method of Finding intervalsMethod of Finding intervalsd. “In-use” time method This is a variation on the foregoing methods. The basic method

remain unchanged but the confirmation level is expressed in hour of use rather than in calendar months of elapsed time.

e. In-service or “black-box testing” method This method is complementary to a full confirmation. It can provide

useful interim information on characteristics of measuring equipment between full confirmation and can give guidance on the appropriateness of the confirmation programme. This method is suitable for complex instruments. Critical parameters are checked frequently ( once per day or even more often) by portable calibration gear or preferably, by a “black-box” made up specifically to check the selected parameters. If the equipment is found to be nonconforming by using “black-box”, it is returned for a full confirmation.

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