21.06.2014
The reliable measurement of static torques plays a major role in many fields of work. The reason for this is that – besides the measurement of reaction torques – it is still the basis for torque measurement in the rotating and often also in the dynamic application. The unit is disseminated from PTB via accredited laboratories to the end user. Good results of comparison measurements between the accredited laboratories and PTB are of great importance for a successful dissemination of the unit. Within the scope of the German Calibration Service (Deutscher Kalibrierdienst – DKD), PTB organized, carried out and completed such a comparison for static torque up to 200 N·m.
All 13 laboratories which are accredited in Germany for pure torque (this does not comprise torque wrenches, among other things) participated in this round robin. PTB's Working Group "Realization of Torque" was responsible for the organization, preparation and evaluation of the round robin. Every participant had to calibrate two torque transducers as travelling standards in the measuring range between 20 N·m and 200 N·m according to DIN 51309 [1] with right-hand and left-hand torque. The results were evaluated and discussed in the Technical Committee "Torque" of the DKD [2]. Among other things, the En value was determined for each participant and each of the 30 measurement points per sensor, i.e. altogether 780 measurement points. An additional 120 values were provided by two voluntary participants. Thereby, the En value represents a standardized value which puts the measured deviations into a ratio with a combination of the absolute measurement uncertainties which have been assigned according to the measurement results. The amount of this value should be smaller than 1, which was fulfilled in all points by the overwhelming majority.
A particularity of the comparison consisted in the fact that the laboratories had pursued different paths of tracing back their measuring equipment: on the one hand, traceability to the base quantities "mass", "length" and "time" and, on the other hand, link-up with PTB's standard facility (by means of calibration of the own facility or by calibration of a reference transducer). Moreover, various measurement principles – such as direct deadweight effect at the constant lever arm, pendulum mass principle, sliding poise device with constant mass and variable lever arm as well as horizontal and vertical reference installations – were compared with each other. This – and also the result of the comparison – were reported at an international conference (22nd IMEKO TC3 Conference [3]) at the beginning of 2014.
Another outstanding result could be derived from the intermediate measurements which had been carried out at PTB during the round robin period – which, after all, had taken a total of 16 months : the stability of the transfer standards used was fairly good and almost completely lay within the measurement uncertainty of the used standard facility of 0.002% (see Figure 1).
Figure 1: Stability of the TT1 sensor over a period of 16 months, dashed line: mean value · (1 ± 0.002%).
The report [4] which has now been presented by the DKD technical committee documents that the realization and the dissemination work excellently in this field and thus contribute to the metrological confirmation of the accreditations granted.
References:
[1] DIN 51309:2005-12 (E) - Calibration of static torque measuring devices, issue 2005-12
[2] DKD: http://www.ptb.de/cms/dienstleistungen/dkd.html
[3] D. Röske, A German Torque Comparison From 20 N·m To 200 N·m And Its Results, Proc. of the 22nd IMEKO TC3 Conference, 3 to 5 February, 2014, Cape Town, South Africa
http://www.imeko.org/publications/tc3-2014/IMEKO-TC3-2014-015.pdf
[4] DKD-V 10-1: Bericht über den DKD-Ringvergleich nach DIN 51309:2005-12: statische Drehmomente von 20 N·m bis 200 N·m (in German)
Contact person:
Dirk Röske, Dept. 1.2, WG 1.22, e-mail: dirk.roeske@ptb.de