A.1 Environmental conditions required for calibration:
Calibration of torque measuring devices 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.
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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. | ||||||||||||
