Resistance thermometers

What does a resistance thermometer do?

Resistance thermometers measure the temperature through the dependence of their electrical resistance on the temperature. Resistance thermometers are suitable for applications between -200 ... +600 °C (dependent on instrument model, sensor element, accuracy class etc.). Accuracy classes AA, A and B (per IEC 60751) apply to all resistance thermometers. A particularly suitable metal for precise temperature measurement is platinum (Pt). Platinum resistance thermometers are precise sensors with the greatest linearity, through which the best reproducibility is achieved during production.

Advantages of platinum:

• high chemical resistance
• Reproducibility
• Long-term stability
• simple processing

How does a Pt100 work?

The Pt100 sensor, like any resistance thermometer, uses the effect that metals change their resistance when the temperature changes. The measuring range of Pt100 sensors varies depending on the instrument, sensor element and accuracy class. At a temperature of 0 °C, a Pt100 measuring resistor has a nominal resistance of 100 Ω (ohm). As the temperature falls or rises, the electrical resistance also changes. This change occurs in line with a characteristic curve defined by international standards. Thus, the temperature can be determined very precisely on the basis of the measured resistance.

When is a Pt1000 needed?

The difference between the Pt100 and the Pt1000 is the resistance value of the sensor at 0 °C. The Pt1000 thus has an electrical resistance of 1,000 Ω (Ohm) at 0 °C. The characteristic curve of the Pt1000, defined by standards, is steeper than that of the Pt100, which means that the measured value has a higher resolution and enables more precise measurement results. With a Pt100, the temperature measurement is falsified by about 0.5 °C for each metre of connection lead. Since the base resistance of a Pt1000 is ten times that of a Pt100, the falsified value is also 10 times lower. That means that, when using a Pt1000, the temperature will only be falsified by about 0.05 °C for each metre of connection lead. For this reason, Pt1000 sensors are frequently used in a two-wire configuration.

Advantages and disadvantages of a Pt100/Pt1000 resistance thermometer

Since the Pt100 and Pt1000 sensors are manufactured using thin-film technology, the amount of platinum is reduced to a minimum, which also keeps the costs correspondingly low. One advantage over temperature measuring instruments, such as bimetal thermometers, is that the resistance thermometer can combine and evaluate the electrical signal with other data directly in measurement technology. WIKA resistance thermometers are also very robust and can be used in temperature ranges between -200 °C and +600 °C without losing any accuracy.

A disadvantage of resistance thermometers compared to thermocouples is the slower response behaviour, since measurements are taken over the entire volume of the measuring resistor.