Service Detail from Norbar Torque Tools, Inc.
As a UKAS accredited calibration Laboratory No. 0256, Norbar is required to calibrate torque measuring devices that are within the Laboratory's scope, in accordance with BS 7882:2008. . The sections below summarize the main features of this document, but purchase and careful study of the standard is advised for those who wish more detailed information.
- The "device" is defined as all parts of a system, e.g. Display, Transducer Cable, and Transducer. Transducer cables will therefore be serial numbered if they are separate items.
- It is preferable to calibrate all parts of a system together. If a transducer is sent for calibration without its normal display unit, an equivalent calibrated display held in the laboratory will be used. The normal display must also be in a calibrated state or the certification for the transducer is invalidated.
- Norbar is currently the only laboratory accredited for UKAS for the calibration of Electrical Torque Measuring Indicators.
- The device is calibrated with at least five approximately equal steps from 20% to 100% of resolution.
- Two series of readings are taken, and the device is then disturbed, generally by being disconnected from the calibration fixture and rotated through 90º. A third series of readings is then taken.
- Should readings be required in both directions, the three series of readings are repeated in the opposite direction.
- The calibration data is then analysed to establish the following parameters.
The variation between the indicated readings from series 1 and 2, expressed as a percentage of the average indicated reading.
The variation between series 1, 2 and 3, expressed as for Repeatability.
Error of Interpolation
Where the results are expressed in volts or units other than torque units, a 2nd order polynomial (best fit line) is established and the variation of reading from the calculated value is expressed as a percentage of the calculated value.
Error of Indication
Where the results are expressed in units of torque, the errors of indications are the variation between each reading and the average reading of all three series at that torque.
Error of Zero Torque
The zero readings taken from after each loading series are given a mean value, which is expressed as a percentage of the mean maximum indicated reading.
- The five parameters are each compared with a table to establish the device's classification. Class 0.1 is the highest performance, and class 5 is the lowest defined by the standard. The overall class reported will be that of the lowest performing parameter. For example reproducibility may be a class 1 when all other parameters meet class 0.5. The device will be classified as 1.
- Additionally the uncertainty of measurement of the applied torque must be five times better than the overall class reported. Norbars uncertainty of measurement (typically 0.02%) therefore allows classification of the highest class devices.
Different classes may be quoted for ranges below 20% of maximum capacity.