Temperature measurement with thermocouples
Thermocouple measurement is particularly suitable for high and rapidly changing temperatures, such as in a furnace profile. Temperature measurement with thermocouples uses a thermoelectric effect that occurs when two metallic conductors of different materials are connected. With thermocouples, however, a number of factors must be taken into account in order to rule out measurement errors.
During thermocouple measurement, a voltage is generated at the junction of the two metallic conductors (measuring point), which is measured. However, this electrical voltage is relatively low at a few 10 µV per 1 °C temperature difference. Depending on the material and temperature range, around ten different types of thermocouple are used, which are identified by a capital letter. The most commonly used thermocouple alloy is nickel-chromium / nickel (type K) for a temperature range of -270 °C to 1,300 °C.
Typical areas of application for thermocouple measurement
- High-temperature measurements, e.g. during heat treatment in steel and aluminum processing.
- For temperature distribution in flames for monitoring firing systems or in industrial furnace construction.
- In cooling systems or for temperature monitoring in warehouses.
- In the areas of experimental, test and test bench automation as well as temperature measurements in the laboratory.
- Also in mechanical and plant engineering, waste recycling, energy generation, the packaging industry, injection molding tools, transformer construction, etc.
What is important when measuring thermocouples
Measuring accuracy of thermocouples: Thermocouples are inexpensive and very robust. The junction of the two metallic conductors is very small and can be used very flexibly. Due to the system, however, the measuring accuracy is lower than with Pt100 measurement, for example. Due to the low electrical voltages in the microvolt range, the metallic conductors should be shielded in order to avoid errors caused by interference signals.
Measuring point and reference point: The thermocouple measures the temperature difference between the measuring point (the junction of the two metallic conductors) and a transfer point (reference junction). It is therefore important to reliably determine the temperature at the transfer point, e.g. at the connection terminals of the measuring device. To do this, the measuring device should be characterized by the lowest possible self-heating and measure the temperature at each terminal strip independently of each other.
Signal conditioning and calibration: So that the thermocouples can be connected directly to the analog input of the measuring device, the device should already have a converter for signal conditioning. As the characteristic curve of the thermocouple changes over the course of its service life due to external influences and ageing, the temperature sensor should be calibrated at regular intervals.
Measuring device and software support for thermocouple measurement
The Loggito Logger - is a very compact measuring device with four or eight universal analog inputs, making it ideal for cost-effective thermocouple measurement. Thanks to low-power technology, the power consumption of the Loggito logger is low and self-heating is very low. The setup and configuration is menu-guided and is already prepared for all thermocouple types. The different colored connections of the thermocouples are visually displayed in the configurator and the connection to the correct terminals is indicated.
The ProfiSignal software facilitates the analysis and visualization of thermocouple measurements with statistical functions for displaying and analysing the temperature curve (such as minimum value, maximum value, average value, increase). Both offline and online values can be displayed. Thanks to very effective measurement data management, the user can also scroll and zoom through historical data without delay. A recorder function also allows individual test measurements to be recorded and saved using a recording button.
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