Physical Principles of Temperature Measurement

In many processes the recording of temperature is of great importance. For example in the melting process, in chemical reactions, in food processing, power generation or air conditioning. The requirements for temperature measurement and the technical design differ depending on the area of application and the individual measuring task.
Thermocouples and resistance thermometers are used on measuring objects where direct contact is possible to measure the temperature. They are often used for measurements in liquids, gases, melts or on solids.

The type of thermocouple or resistance thermometer used depends on the respective requirements for accuracy, response, temperature range and chemical properties of the measuring task.

One advantage of thermocouples over resistance thermometers is the higher upper temperature limit of up to several thousand degrees Celsius. In contrast, however, they have poorer long-term stability and lower measuring accuracy.

Temperature Measurement with Thermocouples

The measurement of temperature using thermocouples is based on the thermoelectric effect discovered by Seebeck in 1821. Here it is assumed that a voltage can be measured at the free ends of two wires made of different materials that are connected together, if the temperature at the junction of the wires is different from that at the free ends.

The temperature difference between the temperature at the measuring point and the temperature at the connections of the measuring instrument is always decisive for the measurement.

The temperature at the measuring point can be determined by the measured thermoelectric emf. More detailed information can be found in the standard DIN EN 60584-1 "Thermoelectric voltages and limiting deviations", which lists the basic value series of thermocouples.

Temperature Measurement with Resistance Thermometers

Temperature measurement with a resistance thermometer uses the linear relationship between the temperature and the electrical resistance of metal wires. Resistance materials are preferably pure metals, as they show a stronger change in resistance than alloys. The material most frequently used for this purpose is platinum.
The graph of DIN EN 60751 can be used to draw an exact conclusion about the ambient temperature from the measured electrical resistance value. You will find the characteristic curves of Pt100 and Pt1000 in the following table.
The designation of the resistance thermometers is derived from the electrical resistance at 0 °C present in each case. A resistance which has an electrical resistance of 100 Ohm at 0 °C is referred to as Pt100. Similarly, a resistor with the designation Pt500 has an electrical resistance of 500 Ohm at 0 °C, and a Pt1000 has an electrical resistance of 1000 Ohm
at 0 °C. The resistance of a Pt1000 at 0 °C is called Pt100.


Basic value series Resistance Thermometer

DIN EN 60751 defines the relationship between temperatures in °C and resistance in ohms for a platinum resistance thermometer with a resistance of 100 ohms or 1,000 ohms at 0 °C. The following tables show the basic value series and the characteristic curves of the resistance thermometers Pt100 and Pt 1000. In the following tables you will find these basic value series as well as the characteristic curves of the resistance thermometers Pt100 and Pt1000.

Temp. in [°C] Pt100 in [Ω] Pt 1000 in [Ω]
-200 18.5 185.2
-100 60.3 602.6
0 100.0 1000.0
100 138.5 1385.1
200 175.8 1758.6
300 212.1 2120.5
400 247.1 2470.9
500 280.9 2809.8
600 313.7 3137.1
700 345.3 3452.8