Calibration Process And Classification Criteria for Torque Measuring Devices (Transducer) As per The Standard BS 7882:2008

A.1 Environmental conditions required for calibration:

Calibration shall be performed at a stable temperature. This temperature shall be within 18 °C to 28 °C and within ±1 °C variation. Sufficient time shall be allowed for the force-proving instrument to attain a stable temperature. 

 

 

 

  1. Number of operations to be done and number of readings to be recorded for estimation of the uncertainty of a Torque Measuring Device is as given below. 

1. If the shaft is round, then rotate the Torque sensor to 120° & 240° from the first position

2. If the shaft is square, then rotate the Torque sensor to 90°, 180° & 270° from the first position

3. If, the design of the UUT does not permit for rotation then remove the sensor completely from the test rig and refit it before taking the next set of readings

To determine

Fitting position

Preload 

Initial Zero

Ascending

Descending

Zero reading after removal of force

After 30 s

Repeatability wb

0 degree

3

1

2

1

Reproducibility wb`

90 degree

1

1

1

1

120 /180 degree

1

1

1

1

240/270 degree

1

1

1

1

1

For devices which cannot be rotated

First fitted

3

1

2

2

1

Refitted

3

1

1

1

1

 

B Classification of the Torque Measuring Device:

   The table-2 as per the Standard BS 7882:2008, gives the maximum allowable values in % of these parameters for each class of Torque measuring device and the uncertainty of the calibration Torque. The range of classification shall cover at least cover 20 % to 100 % of the calibrated range

Relative Error component

Symbol

Class 0.05

Class 0.1

Class 0.2

Class   0.5

Class     1

Class   2

Class    5

Reproducibility

b

0.025

0.05

0.1

0.25

0.5

1.00

2.5

Repeatability

b`

0.05

0.1

0.20

0.50

1.00

2.00

5.00

Interpolation ±

fc

0.025

0.05

0.10

0.25

0.50

1.00

2.50

Residual Zero ±

f0

0.0125

0.025

0.04

0.10

0.20

0.40

1.00

Reversibility

h

0.063

0.125

0.250

0.625

1.250

2.50

6.250

Indication ±

R

0.025

0.05

0.10

0.25

0.50

1.00

2.50

Applied Torque uncertainty ± (in 95% confidence level)

utcm

0.01

0.02

0.04

0.10

0.20

0.40

1.00

Resolution criteria

r

4000*r

2000*r

1000*r

400*r

200*r

100*r

40*r

Minimum number of counts required to display 10% to 100% of the range

4000 

To

 40000

2000

To 

20000

1000

To

10000

400 

To

4000

200 

To

2000

100 

To

1000

40

To

400

Expanded uncertainty ‘U’ with k=2 (With reversibility)

0.06

0.12

0.24

0.61

1.22

2.44

6.10

Expanded uncertainty ‘U’ with k=2 (Without reversibility)

0.05

0.10

0.20

0.5

1.0

2.0

5.0

B.2 Components for determination of measurement uncertainty of the Torque measuring devices 

The relative standard uncertainty associated with applied calibration Torque;

wtfc

The relative standard uncertainty associated with reproducibility of calibration results;

wb

The relative standard uncertainty associated with repeatability of calibration results;

wb’

The relative standard uncertainty associated with resolution of indicator;

wr

The relative standard uncertainty associated with the drift in the zero output;

wf0

The relative standard uncertainty associated with interpolation.

wfc

The relative standard uncertainty associated with reversibility

wh

Combined uncertainty, [with interpolation]

wc =wtfc2+wb2+wb’2+wr2+ wf02+wfc2

Combined uncertainty, [when readings are in units]

wc =wtfc2+wb2+wb’2+wr2+ wf02+wfc2

Combined uncertainty, [readings with incremental and decremental torque]

wc =wtfc2+wb2+wb’2+wr2+ wf02+wfc2 +wh2

Relative expanded uncertainty of measurement, W=k x wc, where k =2 coverage factor

Overall accuracy of the device

Overall accuracy of the device can be obtained by combining classification accuracy 

with the expanded uncertainty in quadrature. 


Oa = [√(Ei/√3) 2 + (U/2) 2] 2


Then, overall accuracy Oa is multiplied by coverage factor k=2 to give UOa 

The classification accuracy is obtained through Ei (error of Indication) column in the table 2 of BS 7882. 


Note: This applies to the calibration device having deflection in Torque units like Nm, cNm etc.

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