1. Accuracy Significance in Measurement.
Arumugham.V
Calibration Cell
BMTWing, SCTIMST

2. Index Accuracy – Concepts. Definition. Terms related to Accuracy.
Significance of Accuracy.
How Accuracy specified ?
Expressing Accuracy.

3. Accuracy- Concepts
Accuracy of a measuring instrument: is a qualitative indication of the ability of a measuring instrument to give responses close to the true value of the measurand (parameter being measured.) [VIM, 5.18] This accuracy is a design specification and it is what is verified during calibration.

4. Accuracy- Concepts
Accuracy of a measurement: is a qualitative indication of how closely the result of a measurement agrees with the true value of the measurand. [VIM, 3.5] Because the true value is always unknown, accuracy of a measurement is always an estimate. An accuracy statement by itself has no meaning other than as an indicator of quality. It has quantitative value only when accompanied by information about the uncertainty of the measuring system.

5. Accuracy- Concepts
Measured Value

6. Definition of Accuracy
ISO[5725-1] “closeness of agreement between a test result and the accepted reference value”
ISO[3534-2] “closeness of agreement between a test result or measurement result and the true value”
[VIM 3] “closeness of agreement between a measured quantity value and a true quantity value of a measurand”

7. Terms related to Accuracy
Precision
Resolution
Error
Bias
Tolerance
Uncertainty

8. Precision
The degree to which an instrument will repeat the same measurement over a period of time under the same conditions.

9. Resolution Resolution is 0.01 Volt Resolution is 1 Volt
The smallest change in a measured value that the instrument can detect. Resolution is also known as sensitivity.
Resolution is 0.01 Volt
Resolution is 1 Volt

10. Error
Difference between measured value and accepted reference value in an instance is called as 'ERROR'.
E = 5.0 – 4.6
= 0.4 Volts

11. Bias
The difference between the mean of repeat measurement results and the accepted reference value or true value is called the bias of the measurement process.

12. Tolerance
The unwanted but acceptable deviation from a desired dimension. It is the probability of largest error in a measurement. It can be estimated as half of the largest measurement and smallest measurement.
T = 1/2 ( largest measurement - smallest measurement)

13. Uncertainty
Uncertainty of measurement [Sec3.9, VIM]: Parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand
Calibration determines the measurement value’s deviation from a standard with a known value in order to assess and apply a systematic correction to those measurements. The associated uncertainty of every calibration result is also calculated. The calibration certificate compiles measurement results with their associated expanded uncertainties regardless of the instrument specifications.

14. Uncertainty Sources Environment Reference Equipment
Metrologist / Operator
Measurement Procedure

15. Accuracy in measurements
Measurements with adequate accuracy improves credibility of reporting numerical results and their application in research / testing.
But how to ensure that they are adequate ?
By using equipments with stated accuracy
Reproducing results with other standard methods

16. Significance of Accuracy
By knowing the accuracy in a measurement, one will be always aware of the deviation in the measurement results.
Hence improves the confidence in measurement results or performance of equipments.
Note: In short accuracy is one of the contributing element in uncertainty estimations.

17. Applicable to Equipments Methods
Ideally a measuring device should be both accurate and precise, with all measurements close to and tightly clustered around the reference value.

18. How accuracy can be specified
According to equipment literature / manuals
According to the requirement
As per reference/standard literature
As per norms specified by standards (TAR/TUR)

19. Test Accuracy Ratio
The comparison between the accuracy of the Unit Under Test/Calibration (UUT/UUC) and the accuracy of the standard is known as a Test Accuracy Ratio (TAR). However, this ratio does not consider other potential sources of error in the calibration process.

20. Test Uncertainty Ratio
The comparison between the accuracy of the UUT and the estimated calibration uncertainty is known as a Test Uncertainty Ratio (TUR). Based on ANSI/NCSL Z , “Collective uncertainty of the measurement standards shall not exceed 25% of the acceptable tolerance (e.g. Manufacturer specifications)”. This 25% represents a TUR of 4:1.

21. Equipment Literature Scenario
We want to extract plasma from blood using a centrifuge with following conditions.
Speed : 3500 rpm ± 10 rpm
Temperature : 22 to 25 degrees
Time : 20 to 60 minutes
A centrifuge with it’s specification is shown in next slide. Can it be used for the specific purpouse ?

22. Equipment Literature Cont.d
Specifications from Manual
Capacity L (4 x 400mL)
Control Microprocessor
Display Digital
Max. Speed 15,200rpm
Speed Accuracy ± 1% of Reading
Temperature 25C ± 5C
Timer 9 hr., 99 min.
Noise Level <61dBA

23. Equipment Literature Cont.d
Specifications from Manual of Given Centrifuge
Capacity L (4 x 400mL)
Control Microprocessor
Display Digital
Max. Speed 15,200rpm
Speed Accuracy ± 1% of Reading
Temperature 25C ± 5C
Timer 9 hr., 99 min.
Noise Level <61dBA
± 10 rpm
22 to 25 degrees
20 to 60 minutes
Conclusion : The specific centrifuge does meet the requirements

24. Reference / Standard Literature
From Work Procedures.
From National / International Standards
Scenario
We want to measure the temperature of a water bath. The work procedure describes the the incubation should be done in the water in 37C ± 1C. Maximum set temperature for Bath is 100C
Two thermometers are shown in next slide, which one to use ?

25. Reference / Standard Literature Cont.d
Thermometer 1
Temperature Range : 0C to 100C
Resolution : 0.1 C
Accuracy : ± 0.1 C
Thermometer 2
Temperature Range : 0C to 100C
Resolution : 0.02 C
Accuracy : ± 0.05 C

26. Reference / Standard Literature Cont.d
Thermometer 1
Temperature Range : 0C to 100C
Resolution : 0.1 C
Accuracy : ± 0.1 C
Both Thermometer 1 and Thermometer 2 will satisfy the requirement,
37C ± 1C.
Thermometer 2
Temperature Range : 0C to 100C
Resolution : 0.02 C
Accuracy : ± 0.05 C
Conclusion : Thermometer 1 is cost effective, so it can be used.

27. Expressing Accuracy as a tolerance limit as a % of Nominal Capacity
as a % of measurement reading
as a % of operating range
as a combination of any of the above.

28. Tolerance Limit

29. % of Nominal Capacity

30. % of Measurement Reading
Voltage DC Specification
Maximum Voltage: V
Accuracy:
Fluke 83 V: ± (0.1% of Reading)
Fluke 87 V: ± (0.05% of Reading)
Resolution:
Fluke 83 V: 100 µV
Fluke 87 V: 10 µV

31. % of Operating Range Temperature Probe Operating Range -20°C to +135°C
Accuracy
<0.1% upto 100.0°C
Response time
0.8 sec.
Electrode Material
Stainless Steel
Electrode Dimension
1.2 mm

32. NABL 141 Guidelines for estimation of uncertainty in measurement
FOR MORE INFO...
NABL 141 Guidelines for estimation of uncertainty in measurement
A Guide for ISO – 9000 : CALIBRATION
ISO Standard
Fluke :
NATA :
NIST Hand Book :

33. Thank You.....