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Influence of humidity on precision torque transducer easily measurable

  • Metrology for Economy

Precision torque transducers based on strain gauges can, as far as they are not hermetically enclosed, be influenced by the humidity contained in the ambient air [1]. Measuring the humidity coefficient which describes this property has, so far, only been possible by means of very tedious procedures. Hence, accredited calibration laboratories were, for financial reasons, not able to measure this coefficient. Since, however, sometimes humidity-sensitive torque transducers are used as references in calibration laboratories, the contribution of humidity influences to the measurement uncertainty has to be determined. PTB has therefore tested simplified measurement methods for their suitability.

Previous measurements of the humidity coefficient required climate-controlled chambers which were adapted to the measuring facilities and which decoupled the tested transducer from the ambient air [2], [3]. Most calibration laboratories could – either from a financial or from a personnel viewpoint – not afford operating such chambers. For that reason, estimates are used to determine the influence of humidity on the measurement uncertainty (Fig. 1); these estimates are derived from exemplary investigations of PTB using climate-controlled chambers.

Figure 1: Temporal evolution of the sensitivity of a strain-gauge torque transducer that was exposed to stepwise humidity variations of the ambient air.

Highly simplified, continuous [4] measurement methods, where the transducers are exposed to certain material-dependent humidity fixed points in temporary enclosures without climate control, can considerably reduce the measurement complexity. PTB's Working Group "Realization of Torque" has now succeeded in demonstrating that such methods provide sufficiently precise statements for the purposes of calibration laboratories. This simplified method can, in future, be used by the laboratories concerned to measure humidity coefficients themselves. This will significantly improve the empirical verification of their measurement uncertainty budgets.

For the manufacturers of torque transducers, this opens up the opportunity to use the simplified procedures for investigations on how to improve the humidity tolerance of their products. In addition, also other measurands whose sensors can be influenced by humidity could benefit from these methods by adapting this principle to their conditions.

The results of the investigations were presented within the scope of a lecture held at the XXth IMEKO World Congress that took place in Busan, South Korea, from 9 to 14 September 2012. They also entered into a publication which is included in the proceedings of the meeting and was made available on IMEKO's website (www.imeko.org). Link


[1] Röske, D., Key comparisons in the field of torque measurement, 19th International Conference on Force, Mass and Torque (IMEKO TC3): "Theory and Application in Laboratories and Industries", 19-23 February 2005, Cairo, Egypt. Link

[2] Röske, D.; Mauersberger, D., On the stability of measuring devices for torque key comparisons, IMEKO XVIII World Congress and IV Brazilian Congress of Metrology, "Metrology for a Sustainable Development", 17-22 September 2006, Rio de Janeiro, Brazil. Link

[3] Röske, D., Final report on the torque key comparison CCM.T-K1: Measurand torque: 0 N · m, 500 N · m, 1000 N · m, Metrologia, 46 (2009), Tech. Suppl., 07006. Link

[4] Brüge, A., Fast Torque Calibrations Using Continuous Procedures, 18th Conference on Force, Mass and Torque, 24-26 September 2002, Celle, Germany.

Contact person:

Andreas Brüge, Dept. 1.2, WG 1.22, e-mail: andreas.bruege@ptb.de


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