Principles and Practices of Traceability and Calibration
Welcome!PRINCIPLES AND PRACTICESOF TRACEABILITY ANDCALIBRATION1By:Jasmin NUHIC
OBJECTIVE2• To learn and understand different types ofmeasurements units, measurementconstants, calibration and measurementstandards as well as principles andpractices of treaceability.
AGENDA3• Introduction• Base SI Units• Derived SI Units• SI Multipliers and Conversions• Fundamental Constants• Common Measurements• Principles and Practices of Traceability• Types of Measurement Standards• Substitution of Calibration Standards• Sample Questions• Q & A Session
BASE SI UNITS4Characteristic FundamentalUnitDescriptionLength Meter (m) Path of light traveling in vacuum during1/299,792,458 of a secondTime Second (s) Duration of 9,192,631,770 periods ofradiation corresponding to thetransition between two hyperfine levelsof ground state of the cesium atomMass Kilogram (kg) Equal to international prototypeplatinum-iridium alloy cylinderElectric Current Ampere (A) Constant flow that produces 2X10^-7Newtons per each meter of lengthbetween two straight conductors
BASE SI UNITS (cont’d)6Characteristic Fundamental Unit DescriptionTemperature Kelvin (K) Fraction of 1/273.16 of thethermodynamic temperature of thetriple point of water (0.01 C)NOTE: know how to convert fromKelvin to Celsius and vice-versaLight Candela (cd) Luminous intensity of a source thatemits monochromatic radiation offrequency 540x10^12 hertz and hasradiant intensity in the same directionof 1/683 wattAmount of Substance Mole (mol) Amount of substance of a systemwhich contains many elementaryentities as there are atoms in 0.012kilogram of carbon 12
DERIVED SI UNITS7Characteristic Fundamental Unit DescriptionArea m^2 Length multiplied by lengthVolume m^3 Length multiplied by length multipliedby lengthFrequency Hz Time invertedDensity kg / m^3 Mass divided by volumeVelocity m / s Length divided by timeAcceleration m / s^2 Length divided by squared timeForce N Mass multiplied by acceleration
DERIVED SI UNITS (cont’d)8Characteristic Fundamental Unit DescriptionPressure Pa Newton divided by volumeKinematic Viscosity m^2 / s Squared length divided by timeWork (energy) J Newton multiplied by lengthPower W Power divided by timeElectric Charge C Amperes multiplied by timeVoltage(electromotive force)V Power divided by amperesElectric Resistant Ω Voltage divided by amperes
DERIVED SI UNITS (cont’d)9Characteristic Fundamental Unit DescriptionElectric Capacitance F Amperes multiplied by time divided byvoltageMagnetic Flux Wb Voltage multiplied by timeInductance H Voltage multiplied by time divided byamperesMagnetic Flux Density T Magnetic flux divided by areaMagnetic FieldStrengthA/m Amperes divided by lengthMagnetomotive Force A AmperesLuminance cd / m^2 Candela divided by area
DERIVED SI UNITS (cont’d)10Characteristic Fundamental Unit DescriptionLuminance flux Im Candela multipliedIlluminations Lx Luminance flux divided by area
FUNDAMENTAL CONSTANTS12 Speed of light in vacuum;unchanging in space or time Not dependent on time orplace; gravitational attractionof matter Varies by place; within the US,it varies 0.2% (scales should becalibrated at point of use) Relationship betweenpressure, volume andtemperature in an ideal gas Relationship between amount ofsubstance and number of molecules inthat amount Back body used in calibration; tempof black body measured by its color
COMMON MEASUREMENTS13• Inspection, Measurement, and TestEquipment (IM&TE)• To calibrate any equipment, it is necessary togenerate a known amount of the variable to bemeasured and apply it to the unit under test.• Variable can be generated by using known generator(i.e. gage block) or unknown generator (in the case itmust be measured simultaneously with calibrateddevice).• Where IM&TE is also a generator then the outputmust be known.
COMMON MEASUREMENTS (cont’d)14• Laboratory Measurement of Temperature:– Liquid-in-glass thermometers must be immersed in thecalibration bath to a predefined depth.– Resistance-Temperature-Devices work on the basis oftemperature versus resistance characteristics.– Thermocouples work the basis of temperature versusvoltage characteristics.– Optical Pyrometer is used to measure temperatures above200 C by measuring the color of the object from thedistance.
COMMON MEASUREMENTS (cont’d)15• Laboratory Measurement of Humidity:– Humidity is best measured using a chilled mirrorhydrometer.– Psychrometer measures humidity by comparingthe temperature near a dry bulb with that of a wetbulb (the lower the humidity the greater thecooling)
COMMON MEASUREMENTS (cont’d)16• Laboratory Measurement of Pressure:– The most accurate way to measure pressure is togenerate it (weight divided by the area).– Low pressures can be measured using manometer(column of liquid responds to positive andnegative pressures).– The Bourdon gage measures pressure bymechanical means of elasticity (elastic elementused).– The Quartz Bourdon gage measures pressure bymeans of electronic transducer.
COMMON MEASUREMENTS (cont’d)17• LaboratoryMeasurement of Torque:– Torque is difficult togenerate and measure.– Greatest uncertainty,when it comes tomeasuring torque, is thedistance from the centerof the mass to the centerof the ratating lever arm.
COMMON MEASUREMENTS (cont’d)18• Laboratory Measurementof Force:– Force is generate by hangingcalibrated weights on theunit under test (requirescorrection to local gravity).
COMMON MEASUREMENTS (cont’d)19• Laboratory Measurement ofMass:– Masses are calibrated bycomparison to known andtraceable reference standards.– Gravity correctionrequired?????• No, if the materials of thestandard are the same as of theunit under test.• Yes, where there is difference inmaterials.
COMMON MEASUREMENTS (cont’d)20• Laboratory Measurement of ElectricalQuantity:– Electronic Calibrators, Capacitors andInductors, Digital Multimeters, NullIndicators, Bridges and Transfer Standards.– Number of digits on the display does NOT meanthat the same level of accuracy has beenachieved.– In case where DC is used, special attention shouldbe paid to high and low voltage (potential resultsdistortion)
COMMON MEASUREMENTS (cont’d)21• Laboratory Measurement of ElectricalCalculations:– Calibration technician is expected to performsimple calculations when it comes to electronicsand their properties.– Electric Current is measured in amperes– Electronic Potential or electromotive force ismeasured in volts– Electrical resistance is measured in ohms nextslide– Electrical Power is measured in Watts
COMMON MEASUREMENTS (cont’d)22• Ohm’s Law:E = iR or i = E/R or R = E/IR – resistancei – currentE – voltage
COMMON MEASUREMENTS (cont’d)23• Laboratory Measurement of Time andFrequency:– GPS (Global Positioning System) signal isconsidered traceable to national standards andhas output of about 10MHz (at full capacity).When it comes to length measurements, the mostimportant fact to remember is that the temperature fordimensional measurements shall be 20 C!
PRINCIPLES AND PRACTICES OF TRACEABILITY24• Traceability is defined as ability to link the results ofthe calibration and measurement to related standardand/or reference (preferably national or internationalstandard) through an unbroken chain ofcomparisons.• Calibration is typically performed by measuring a testunit against a known standard or reference.• Master standard (i.e. gages) are kept by NationalMeasurement Institute (NMI) of each country.
PRINCIPLES AND PRACTICES OF TRACEABILITY (cont’d)25• National Institute of Standards and Technology (NIST)provides internal tracking numbers, which are oftenused as evidence of traceability.• WARNING! NIST does not certify or guarantee thatcalibration and measurements are correct, nor doesit provide any kind of certification of accuracy andcalibration and the internal number does mean thatthe test unit calibrated is indeed valid. NIST onlyprovides certifications for the work performed bythem.
TYPES OF MEASUREMENT STANDARDS (cont’d)27• International Standard– Highest level of reference standards agreed by multiplecountries for the common purpose (kept at Bureau ofWeights and Measures in Sevres, France).• Intrinsic Standard– If properly maintained they provide standards based onlaws of physics, fundamentals of nature, invariantproperties of materials.• National Standard– In US, it is maintained by NIST, and it is a standard formedby one or many groups within one country (or only fewcountries = adapted).
TYPES OF MEASUREMENT STANDARDS (cont’d)28• Reference Standard– Item of highest metrological quality located at a site wherecalibration is being conducted.• Master Standard– Lower level of Reference Standard and used for calibrationof lower level calibration requirements measuring devices.• Working Standard (working master)– Should be compared to Master Standard or ReferenceStandard on regular basis; Used for daily checks /comparisons of the calibrated devices.
TYPES OF MEASUREMENT STANDARDS (cont’d)29• Derived Standard– Combination of two or more standards for the sake of fulfillingtraceability requirements.• Consensus Standard– Example of such standard is Rockwell Hardness; This standard is usedwhen no traceability to a known standard can be established, butrather an agreement of all parties is considered the standard.• Transfer Standard– This standard is actually an artifact designed to be calibrated at onelocation and transferred to another location without its impact tovalidity of calibration (deviation ranges due to transportationsacceptable).– NOTE: Sometimes Transfer Standard is used to describe transferringvalues from a NIST standard to a local standard.
SUBSTITUTE OF CALIBRATION STANDARDS30• When no valid standard is available at point of use, atechnician can:– Postpone the calibration until the standard becomesavailable, or– Identify suitable substitute standard• If substitute standard is to be used then:– Procedure must allow it– Substitute standard must be available at point of use– Substitute standard must be of equal or betterspecifications– The uncertainty of standard must be equal or better thanrequired to calibrated the test unit
SUBSTITUTE OF CALIBRATION STANDARDS (cont’d)31• ISO standard for calibration laboratories is:– ISO 17025– This standard is NOT procedure heavy• ANSI standard for calibration is:– ANSI Z540-1Remember: Not all procedures and practices allow substitutions of standardsand sometimes they might be test unit specificRemember: Substitution is a “judgment call” made by a technician (where nodocumented procedure and/or practice exists)