In May 2005, the first international key comparisons in the field of torque realization and torque measurement were initiated. Under the umbrella of the CIPM (Comité International des Poids et Mesures/International Committee for Weights and Measures), the best measuring facilities in the world are compared among one another to find out how well they agree and whether the measurement uncertainties specified for these facilities are plausible.

In view of the fact that since approximately the middle of the nineties torque has played a considerably more important role in metrology than the years before, and that in the last ten years a great number of new standard measuring devices with smallest relative measurement uncertainties of a few 10^-5 has been developed in the world, it has become necessary to metrologically ensure the properties of these devices also on the international level. The CIPM Key comparisons initiated in May 2005 serve this aim. In the first step, two comparisons have been agreed: CCM.T-K1 up to 1 kN·m and CCM.T-K2 up to 20 kN·m. In the first comparison, only facilities with lever-mass systems using air-cushioned levers or elastic fixed bearings (joints) are investigated. Such facilities can be found in America (INMETRO-Brazil, CENAM-Mexico), Asia (NMIJ-Japan, KRISS Korea) and Europe (PTB-Germany, METAS-Switzerland, CEM-Spain and NPL-Great Britain). As the number of measuring devices with lever-mass systems available in the world is clearly smaller, the second comparison also encompasses facilities working according to the reference principle. These are facilities in which a calibrated torque transducer is used as reference. Participants in this comparison are: SMERI (China), PTB (Germany) and NMIJ (Japan) with standard measuring devices with deadweight effect or MIKES-RAUTE (Finland), LNE (France) and CENAM (Mexico) with reference facilities.

Right from the start, PTB has played a pioneering role in the field of torque realization and measurement. It has investigated the most different influence quantities with correspondingly great research effort and collected extensive experience. This is why PTB Working Group 1.22 "Realization of Torque" was asked to assume the role of the pilot laboratory. The sequence of the measurements depends on the procedure which has already been successfully tested in force measurement. Maximally two torque steps are triggered (50% and/or 100% of the nominal torque); the separation between force in tension and force in compression (which here corresponds to clockwise torque and anti-clockwise torque) which is usual in force measurement is here not applicable, i.e. both directions are calibrated (for technical reasons, force and pressure ranges are separately compared in force measurement). For the measurements, high-precision torque transducers are used (e. g. of type TB2, see Figure 1) which are calibrated in the pilot laboratory, then at a participant's and subsequently again in the pilot laboratory. To minimize creeping influences of the transducer, a waiting time of six minutes is allowed for after the load has been applied. In addition, the mounting position of the transducer is turned twelve times through 60°, to be able to average geometrical deviations in the system out of the measurement results. For transport of the sensitive measuring instruments, a special temperature-stabilized box was developed and constructed which includes vacuum insulation panels with extremely good insulating properties as well as energy-storage plates which use the chemical conversion energy of the substance of content as energy buffer. In the case of a transport over fourteen days, this box can keep the temperature inside constant in the range from 19.5°C to 21.5°C.