17.11.2008
The international comparison ("key comparison") CCM.T-K1 which was started in 2005 for torques of 500 N·m and 1 kN·m was completed after the report had been agreed upon by the participants. Thereby, at best, the expanded (k = 2) measurement uncertainties of 2·10-5 stated by the partners could be confirmed, which also shows that the realization of static torques with smallest measurement uncertainties is mastered.
Participants in the international comparison CCM.T-K1, which was performed within the scope of the CIPM (Comité International des Poids et Mesures), were the national institutes of Brazil (INMETRO), Japan (NMIJ), Mexico (CENAM), Switzerland (METAS), Spain (CEM), South Korea (KRISS) and the United Kingdom (NPL) as well as - for Germany - the PTB. As pilot laboratory, PTB's Working Group "Torque Realization” was responsible for the organization and evaluation. Two torque transducers were sent to each participant for the performance of measurements. The transducers were measured once again at PTB after they had been returned. The report, which is now available and has been agreed upon by the participants, shows that at best, the indicated expanded (k = 2) measurement uncertainties of 2·10-5 could be evidenced. This is an excellent result insofar as the comparison was the first one worldwide for the quantity "torque", but also because of some other particularities.
Compared to force transfer standards, torque transducers are usually open systems which react more sensibly to environmental factors (such as, for example, the air humidity). For one of the two transducers used (TT1), this problem could be mastered only by sophisticated additional measurements and corrections. Fortunately, the second transducer (TB2) was hermetically encapsulated and, thus, clearly more stable (see No matching tab handler could be found for link handler key record:tt_news:1490.).
Another characteristic is the comparison of measuring devices with different spatial directions of the torque vector. Almost all participants have a device with horizontal measuring axis, as in this case the technical effort for a supported lever-mass system does not go too far. One participant (NPL), however, took on the challenge and realized a vertical measuring axis for which several air bearings had to be used. The result is quite satisfactory, as in some cases the deviations from the reference value are smaller than those of some devices with a horizontal measuring axis.
Unfortunately, one participant had problems with the measurement of anti-clockwise torques. For this reason, the respective values were regarded as outliers and were not included in the evaluation. For clockwise torque, the results were very good. As an example, Figure 1 shows one of eight diagrams which illustrate the deviations of the corrected results of the respective participants from the obtained reference value as well as the associated measurement uncertainties for the two transducers, the two torque steps and the senses of direction of the torque (clockwise or anti-clockwise torque). PTB is identified by an "H".
The comparison clearly showed that the realization of static torques in the mentioned range with smallest measurement uncertainties is mastered technically and can be regarded as a solid basis for the metrological back-up of torque measurements on an international scale. A publication of the results in a professional journal is in progress. Since this year, the measurements for the second comparison CCM.T-K2 for torques of 10 kN·m and 20 kN·m have been in progress, in which PTB's Working Group "Torque Realization" again acts as the pilot laboratory.
Figure 1: Result of the international comparison measurement for a torque of 500 N·m (Key Comparison CCM.T-K1). The figure shows the relative deviations of the corrected results (points) of the participants from the reference value (zero line) for the transducer TB2 and the torque 500 N·m (clockwise torque). The bars as well as the grey band show the associated expanded measurement uncertainties (k = 2), PTB = "H"
Contact person:
D. Röske, Department 1.2, WG 1.22, E-mail: dirk.roeske@ptb.de