What are the advantages and limitations of an RTD sensor?

How do RTD elements work?

When a voltage is applied to a metal, at the ends electrons will start moving to the positive pole.
This movement of electrons is interfered with by imperfections in the metals crystal structure which is non temperature-dependent. This phenomenon creates a constant resistance. While the temperature rises, the vibrations of the metal atoms on their roster increases. This restricts the movement of the conducting electrons. The vibrations increase linearly with temperature which results in the increase of resistance. The increase in resistance is directly proportional to the temperature. This effect is referred to as the positive temperature coefficient.

The ideal temperature measurement would have a high temperature coefficient which would result in de biggest change in resistance with temperature. Also the ideal temperature measurement would not change in characteristic properties over a long period of time. The ideal measurement can also not be influenced by chemical effects.

What are the advantages of an RTD probe?

The relationship between resistance and temperature is relatively linear, resulting in Pt100 sensors to be extremely accurate for a certain set temperature. Due to its linearity, Pt sensors are very stable. Furthermore, RTD sensors have high repeatability.

Usually, Pt sensors are preferred over thermocouples when the high precision measurement is required in a temperature range up to 600°C.

What are the limitations of an RTD probe?

In general, it is impossible to conclude whether RTDs or thermocouples are better. Instead, it is more useful to compare the performance of RTDs and thermocouples by addressing other factors, such as measuring range, accuracy, costs, and response time.

1. Temperature range
   RTDs are a good fit for temperatures ranging from -200°C to +600°C. However, outside this temperature range, RTDs are not the best fit.
2. Response time
   On average, RTDs have longer response time compared to thermocouples.
3. Costs
   On average, thermocouples tend to be less expensive than RTDs. However, when RTDs are used in MI cables, it demands more specialism to correctly weld the RTD element to the bare conductors. Depending on the perspective you are looking for, an RTD is more expensive.
4. Size of element
   Thermocouples can be made with diameters of 0.25 mm. Although thin film RTD elements are very thin, it is still challenging to fit them in small diameter MI cables.

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