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:
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.
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 1000 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.
Since the Pt100 and Pt1000 sensors are manufactured with thin-film technology, the platinum content 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.
A strap-on thermometer, also known as a strap-on temperature sensor, is a non-invasive measuring method for the surface temperature. The strap-on temperature sensor is used for measuring temperatures on pipe surfaces in the measuring range -50 and +200 °C (-58 and +392 °F). The advantage compared to a threaded thermometer is that the strap-on temperature sensor has no direct contact with the medium and, due to this indirect temperature measurement, any influence on the medium can be excluded. Another advantage is that aggressive media have no effect on the service life of the thermometer. In our blog post “Strap-on thermometer or threaded thermometer? Temperature measurement on pipes", you'll find further differentiating factors for temperature measurement with strap-on temperature sensors.
With the pipe-mounted probe, unlike with the threaded thermometer, the pipe does not need to be opened. The flat shape of the temperature probe's measuring point enables it to be used, and easily mounted, in confined spaces. The strap-on temperature sensors can be mounted without tools using quick-mounting brackets, which are available for pipe diameters between 12 and 42 millimetres. In the case of resistance thermometers, the Pt100 strap-on temperature sensors and Pt1000 strap-on probes can thus be easily attached to the surface of the pipe.
The WIKA model TF44 strap-on temperature sensor can easily be fixed to the pipe with a quick-mounting bracket. The WIKA strap-on temperature sensor consists of a measuring element within an aluminium sleeve. In usual strap-on thermometers, the contact surface of the sensor housing is rounded and usually adapted to the pipeline diameter, which often requires different instrument designs. The WIKA model TF44 strap-on temperature sensor, on the other hand, has an edge length of only six millimetres and is therefore suitable for all nominal widths. The aluminium case of the WIKA model TF44 strap-on temperature sensor has excellent thermal conductivity. The probe is connected to the evaluation electronics via a connection line made of PVC or silicone.