Criteria for selecting the most suitable electrical temperature measuring point. — The temperature is one of the most important measured variables of a process medium and is predominantly monitored with electrical thermometers. But as simple as the task sounds, know-how is required when choosing the right sensor. We will give you some helpful tips.
The chemical and physical characteristics of a process medium almost always depend on the temperature. Electrical thermometers are the preferred method for monitoring this effectively. Owing to the complexity and individual nature of the processes, the necessary measuring instruments can only rarely be ordered from a catalogue.
When selecting instruments which are optimally tailored to the task at hand, particular consideration should be given to the following criteria:
The contact measurement method with resistance thermometers and thermocouples is first choice in industry and this article will therefore focus on these two device types. Resistance thermometers generally work with platinum sensors, for example Pt100 or Pt1000. The metal material is embedded within a ceramic body as a wire or applied to a ceramic substrate as a layer.
The temperature is measured by determining the electrical resistance of the platinum sensor. This changes as a function of the temperature according to characteristic curves defined, for instance, in IEC 60751 and corresponds to an absolute temperature value.
Temperature Measurement with Thermocouples
Thermocouples consist of two wires made of different metals and welded together. If a temperature difference occurs along these two phases, a thermoelectric voltage is produced between the two legs (Seebeck effect).
This can be measured at the ends of the metallic conductors and is in the region of a few µV per 1 °C. The voltage corresponds to a relative temperature value referred to the temperature at the cold junction.
Accuracy and Operating Temperature of Electrical Measurement
Resistance thermometers provide high accuracy and excellent long-term stability. The tolerance value for class AA sensors is 0.10 °C + 0.0017 | t |. However, the permissible measuring ranges for resistance sensors preclude use at higher temperatures. According to IEC 60751, classic ceramic sensors span a range from -200 °C to +600 °C.
Although the long-term stability of thermocouples is significantly lower, apart from a few exceptions they are capable of recording temperatures of up to 1700 °C. Some thermocouples — like tungsten-rhenium, gold-platinum or platinum-palladium — are indeed suitable for much higher values.
Since each measurement technology has its own benefits, the choice is inevitably a compromise between accuracy and long-term stability, influenced particularly by the required measuring range.
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