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New temperature chamber for the testing of load cells up to 200 t

13.10.2005

A novel temperature chamber in combination with a deadweight force standard machine (FSM) allows the testing and certification of load cells with nominal loads up to 200 t by the Physikalisch-Technische Bundesanstalt (PTB). This facility is unique in Europe; for the first time, it offers manufacturers and customers the possibility of certifying load cells with very high nominal loads as used in the heavy industry for instance.

With an annual export of 500 million Euro (in 2004), the industry of measuring instruments represents one of the most important industrial sectors in Germany. Nowadays, the weighing technology reaches from "micro weighing instruments" with a maximal load of only 5 g and a digital resolution of 0.001 mg to huge crane weighers as used in the heavy industry with maximal loads of 500 t and typical resolutions of 200 kg. Numerous applications of weighing instruments, whether for business trade with goods or for the determination of mass for the billing of fees, are subject to verification in conformity with national or European legal prescriptions. The legally prescribed precondition for the national or European verification is the certification of weighing instruments for which the PTB department "Mass" is responsible in Germany. Since it is not always useful or feasible to test the whole weighing instrument, it is possible to test individual weighing modules.

The most important weighing module is the load cell as the core of modern electromechanical weighing instruments. In a load cell, the weight force applied usually causes an electrically evaluable resistance change in so-called strain gauges. Further measuring principles, such as the electromagnetic force compensation, are limited to applications in special-accuracy and high-accuracy weighing instruments. The metrological tests for the certification of load cells must take climatic influence factors into account and are documented in the internationally accepted OIML-Recommendation R60. The most important assessment criteria laid down in the OIML-Recommendation R60 are reproducibility, linearity and hysteresis as well as creeping and zero returning of load cells at temperatures of 20 °C, 40 °C and -10 °C.

The equipment available so far at PTB allows the testing of load cells with nominal loads of a few grams up to 100 t. But to meet the legal requirements also for the certification of load cells with very high nominal loads, it is necessary to extend the existing test range. By providing the 2-MN-FSM, which was put into operation in 2004, subsquently with an adapted temperature chamber, PTB keeps pace with ongoing developments in the field of weighing technology. In this way, the testing of load cells with nominal loads of up to 200 t, which had been required for a long time by the industry, becomes possible. Until now, such possibilities have not been available in Europe.

A photomontage and a schematic representation of the 2-MN-FSM and the fitted temperature chamber as well as the cooling unit installed on the crosshead are shown in figure 1.

Photomontage and schematic representation of the 2-MN-FSM and the fitted temperature chamber as well as the cooling unit installed on a crosshead

Fig. 1: Photomontage and schematic representation of the 2-MN-FSM and the fitted temperature chamber as well as the cooling unit installed on a crosshead

The maximum dimensions of the temperature chamber are determined in width by the two bars of the adjustable suspension gear of the FSM and in depth by a beam of the height-adjustable crosshead. The height of the chamber is limited by the maximum traverse path of the height-adjustable crosshead of the FSM. Due to the partitionable design of the chamber, easy mounting and demounting of the temperature chamber into and out of the FSM is ensured.

A very important criterion is the absolute protection of the FSM against fluid leakages. This is of vital importance since the deadweights of the FSM are made of steel. Corrosion of the deadweights due to leakages in the fluid circuit of the chamber can lead to weight force changes with disastrous consequences on the uncertainty of measurement. For this reason, the temperature adjustment of the chamber is carried out with air in connection with a direct vaporiser and an electrical heating system. The cooling medium of the vaporiser becomes immediately gaseous under ambient conditions and any contact with the deadweights is thus avoided. The cooling unit with fluid circuit is situated at a safe distance from the 2-MN-FSM on a crosshead. The connexion between the cooling unit and the vaporiser is realised with flexible tubes for cooling agent and special screw connexions so that a disconnection under pressure is possible and simultaneously a complementary protection against leakages of cooling agent is ensured.

An aggravating marginal condition for compliance with the metrological specifications of the temperature chamber are the parts of the FSM protruding into the inside of the chamber for the load transmission onto the load cell. These so-called loading pieces (see figure 2) cover the major part of the temperature chamber area. In order to reduce the heat conduction, they are not entirely made of steel but partly of special pressure-resistant plastics as thermal barriers.

temperature chamber with the thrust pieces partly made of special pressure-resistant plastics for the load transmission onto the load cells to be tested

Fig. 2: The inside of the temperature chamber with the thrust pieces partly made of special pressure-resistant plastics for the load transmission onto the load cells to be tested

Complementarily, the lower loading piece is provided with a lamella-like grill through which temperate air is introduced by means of air conduction shields. This allows not only a faster temperature adjustment but also the required spatial temperature gradients of < 0.2 K per 100 mm in the mounting area of the load cell. In order to ensure a temporal temperature constancy as high as possible, the cooling unit is operated with a constant power. The temperature is regulated by an electrical heating system which provides very short regulation times (of a few minutes) and maximum deviations from the nominal value of ±0.1 K in a temperature range from -20 °C to +55 °C. Thus, the temperature chamber meets all metrological requirements for the testing of load cells in conformity with the OIML -Recommendation R60.

Contact person:

Oliver Mack, FB 1.1, AG 1.12, E-Mail: Oliver.Mack@ptb.de