BS7882:2017 Method for calibration and classification of torque measuring devices
The latest edition of BS7882 was published on the 31st August 2017. Current best practice has been incorporated into this revision which provides a practical solution for the calibration of torque measuring devices including the technical requirements needed for high accuracy classes without making the calibration too time consuming to perform and hence commercially unacceptable. There are minor changes to the calibration procedure compared to the 2008 edition. The minimum certificate content has also been increased to reflect the enhanced technical requirements of the standard.
What has changed?
The major change in BS7882:2017 is the introduction of the “relative measurement uncertainty interval”, the range in which the expected error is likely to lie, calculated as the sum of the absolute relative deviation and the expanded uncertainty. Effectively the uncertainty interval encompasses all of a transducers reported errors and uncertainty of calibration, providing the user with a maximum error value of the calibrated device.
Accredited calibrations performed to BS7882:2017 also meet the requirements of BS EN ISO6789-2:2017 clause 4.3 and annex C 7.3 and BS EN ISO6789-1:2017 clause 6.1.
Other changes are as follows: -
The minimum preload wait time is reduced from 60 seconds, to 30 seconds. ref clause 4.4.
The allowable variation in temperature throughout a measurement series has changed from ± 1°C to 1°C. ref clause 4.2.3.
Clause B.1.8, the calculation of error of interpolation uncertainty u7 has changed to
Clause B.1.9 the calculation of reversibility uncertainty u8 has changed to
Certificate content has been revised and it’s now mandatory to include at least the following information, ref clause 8
- the date of the issue of the certificate;
- a unique identifiable reference;
- the date of calibration;
- the serial numbers of the torque measuring device and, where appropriate, any mechanical fittings and electrical cables;
- a brief description of the calibration method and the type of calibration equipment used, including the uncertainty of measurement of the applied calibration torque;
- a reference to this British Standard as a basis of test, i.e. BS 7882:2017;
- the classification(s) for the range and loading direction of torques over which the torque measuring device conforms to Table 2;
- reversibility where determined;
- a table of the applied torques and corresponding deflections;
- the uncertainty of measurement for the calibration values given and the relative measurement uncertainty interval (see Annex B).
- where the deflection is expressed in units other than those of torque, the degree of the equation determined and the coefficients obtained.
- the maximum and minimum temperatures measured during the calibration.
- where the calibration laboratory supplies the indicating devices, the uncertainty of measurement of the conditioning devices (including the display, energizing device and cables).
- a statement that the results are an indication of the device performance on the date of calibration only.
- a statement that subsequent overloading of, or damage to, the device might invalidate the classification.
- a recommendation that the torque measuring device be recalibrated at periods not exceeding 24 months and whenever it suffers any damage or has been subject to any repair.
BS7882:2017 gives clear worked examples of calibrations in torque units (N·m) & mV/V, for both incremental only and incremental/decremental series of torques, and these can be used by users to validate their spreadsheets and software.
If you have any questions please make contact with us here: BS7882@norbar.com
Head of Laboratory
Member of BSI Panel ISE/101/01/02 Torque measurement and testing