DDM 900: Precision thermometer for laboratory and quality assurance - Dostmann electronic - #1

Dostmann electronic GmbH Waldenbergweg 3b 97877 Wertheim

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All temperature measurements by means of resistive sensors (RTD) consist of two steps which is due to its principle. In the first step the resistance value of the temperature sensitive sensor is taken. In the second step the measured temperature value must be calculated from the sensor's resistance value.

Apart from a sufficient accuracy a temperature measuring device should offer the interchangeability of sensors without calibrating the respective sensor with the measuring instrument. For this purpose it is necessary that the measuring of resistance occurs with a sufficiently small inaccuracy. The inaccuracy in the measuring of resistance is mainly determined by the reproducible resolution as well as by the linearity of the measurement and the accuracy of the reference element.

In the DDM 900 the high resolution and the excellent linearity are ensured by a analog/digital-converter which was developed especially for this instrument. The measuring of resistance is done with the help of switched direct currents. Thus the effects of thermoelectric voltage on the measured values are avoided without ending up with the problems concerning parasitic capacity and inductances which are typical of many AC-based devices. A hermetically sealed precision resistor integrated into the DDM 900 is used as a reference element. Doing so it is guaranteed that the measuring of resistance is ensured even without an external resistance-normal with an accuracy of up to ±0,0005 per cent.

The conversion of the measured resistance into a temperature value makes completely different demands on the measuring instrument than the measuring of resistance. Accuracy is of immediate importance for resistance measurements, whereas determining the appropriate temperature value often requires an input of sensor specific coefficients and thus has to be as user-friendly as possible. Additionally it has to be ensured that those sensor coefficients which have been set by the operator are not inadvertently deleted or changed.

In the DDM 900 the coefficients of standard sensors according to EN 60751 and ITS 90 are set by the manufacturer. In case other sensor coefficients have to be put in both data security and user-friendliness are most important. The key-controlled input of sensor specific coefficients (up to 12 coefficients for ITS 90-sensors) often proves to be quite laborious. Apart from that - in doing so - only a limited data security can be guaranteed. No matter whether the sensor coefficients or the value couples follow DIN or ITS 90, the resistance value for about 0° C usually must be put in, which is around 100 ohms for Pt-100 sensors. If, for example, it is only possible to put in this value with a resolution of 1.0 mohms, this leads to inaccuracies of up to 10 mK at a measured temperature of about 800° C. On top of that one has to add mistakes in digitization and linearity as well as mistakes of the reference resistor. For these reasons sensor specific coefficients for the DDM 900 are established with the help of easy-to-use software on the PC. Doing so automatically plausibility checks are carried out to avoid almost any mistakes. Afterwards the sensor data is transmitted from the PC into the DDM 900. As in any precison multichannel thermometer in the DDM 900 the sensor coefficients have to be assigned to the appropriate channels. In the DDM 900 this assignment is conveniently carried out with short user-defined descriptions of the sensors. The DDM 900 is able to store up to 21 (optionally up to 81) different sets of coeffients, a fact which has two substantial advantages: if multiplexers are used, the appropriate sensor coefficients can