Mechanical structures of PRT sensors

PRT sensors are generally very delicate instruments and unfortunately, accuracy is almost without exception inversely proportional to mechanical robustness. To be an accurate thermometer, the platinum wire inside the element should be able to contract and expand as freely as possible as the temperature changes to avoid strain and deformation. The drawback is that this sort of sensor is very sensitive to mechanical shocks and vibration.

Standard Platinum Resistance Thermometer (SPRT)

Standard Platinum Resistance Thermometer (SPRT) sensors are highly accurate instruments for realizing the ITS-90 temperature scale between fixed points. They’re made from very pure (α = 3,926 x 10^-3 °C^-1) platinum and the wire support is designed to keep the wire as strain-free as possible. The “Guide to the Realization of the ITS-90” published by the Bureau International des Poids et Mesures (BIPM) defines the criteria that SPRT sensors must fulfill. Other sensors are not and should not be called SPRTs. There are glass, quartz, and metal-sheathed sensors for different applications. SPRTs are extremely sensitive to any kind of acceleration such as tiny shocks and vibrations, which limits their use to laboratories requiring the very highest accuracy measurements.

Partially supported PRTs

Partially supported PRTs are a compromise between thermometer performance and mechanical robustness.  The most accurate PRTs are often called Secondary Standard or Secondary Reference sensors. These sensors may adopt some structures from SPRTs and the wire grade may be the same or very close. Due to some wire support, they are less fragile than SPRTs and are even usable for field applications if handled with care, offering excellent stability and low hysteresis.

Industrial Platinum Resistance Thermometers, IPRTs

When the wire support is increased, the mechanical robustness increases, but so does the strain from drift and hysteresis issues. Fully supported Industrial Platinum Resistance Thermometers (IPRTs) have even more wire support and are mechanically very robust. The wire is encapsulated completely in ceramic or glass, making it highly resistant to vibration and mechanical shocks. The drawback is much poorer long-term stability and large hysteresis as the sensing platinum is bonded to the substrate, which has different thermal expansion characteristics.

Film PRTs

Film PRTs have evolved a lot in recent years and better ones are now available. They come in many forms for different applications. The platinum foil is sputtered onto the selected substrate; the resistance of the element is often laser-trimmed to the desired resistance value and eventually encapsulated for protection. Unlike wire elements, thin film elements make it easier to automate the manufacturing process, which makes them often cheaper than wire elements. The advantages and disadvantages are typically the same as with fully supported wire elements except that film elements often have a very low time constant, meaning that they react very fast to temperature changes.  As mentioned earlier, some manufacturers have developed techniques that better combine performance and robustness.

Other RTD sensors

Other platinum sensors

Although the Pt100 is the most common platinum RTD/PRT sensor, there are several others such as the Pt25, Pt50, Pt200, Pt500, and Pt1000. The main difference between these sensors is pretty easy to guess; it is the sensor's resistance at 0°C, which is mentioned in the name. For example, a Pt1000 sensor has a resistance of 1000 ohms at 0°C. The temperature coefficient is also important to know as it affects the resistance at other temperatures. If it is a Pt1000 (385), this means it has a temperature coefficient of 0.00385°C.

Other RTD sensors

Although platinum sensors are the most common, there are also RTD sensors made of other materials including nickel, nickel-iron, and copper. Common nickel sensors include the Ni100 and Ni120, an example of a nickel-iron sensor is the Ni-Fe 604-ohm and a common copper sensor is the Cu10. These materials each have their advantages in certain applications. Common disadvantages are rather narrow temperature ranges and susceptibility to corrosion compared to the noble metal platinum.

RTD sensors can also be made with other materials like gold, silver, tungsten, rhodium-iron or germanium. They excel in some applications but are very rare in normal industrial operations.

Since an RTD sensor’s resistance depends on temperature, we could also include all generic positive temperature coefficient (PTC) and negative temperature coefficient (NTC) sensors in this category. Examples of these are thermistors and semiconductors that are used for measuring temperature. NTC sensors are especially common to use for measuring temperature.