Types of Temperature Sensors

In order to choose sensors correctly, it is necessary to understand for what measurements they are applied and how reliable is the result of measurement.

Mostly, the following four types of temperature sensors are used: thermistors, digital semiconductor sensors, thermocouples and resistance temperature detectors (RTDs). Let us dwell in more detail on their advantages and disadvantages.
 

Thermistors use the principle of conductor resistance change significantly with temperature. Thermistor sensors are made on the basis of various metal oxides.

The advantages of thermistors are a high sensitivity, a small size and a low price.

The main negatives are nonlinear characteristics, weak noise immunity, and a lack of interchangeability. The latter item needs more detail. Interchangeability is especially important in cases when the sensors are applied for measuring temperature of soil, compost or ready-made product where a low-skilled staff is used or the sensors are often pressured by loaders or broken by negligence. In fact, these sensors are turned into consumable items in real operating conditions. In these processes temperature thermistor sensors are not recommended to use. The point is that each manufacturer produces them with especially individual characteristics and in case of sensor’s failure you have to contact only the original manufacturer.  

Digital semiconductor temperature sensors have been actively promoted by the agricultural market not long ago. They come in the form of integrated circuits made by world leaders in the field of microelectronics.

The main advantages of these sensors are a convenient wiring diagram (which does not require an individual cabling directly to the sensor, sensors are connected to one common line), a digital signal allowing to avoid the use of different converters for the construction of measuring circuit, and a low price.

The disadvantages of these sensors are an extremely weak immunity (sensors can seriously lie, if a power line passes near or electric motors run) and a low accuracy (only 0.5 °C). The limited operating range of the sensor - up to 120 ° C – doesn’t allow to use them in a variety of heat transfer applications where the temperature can reach 130 -150 ° C.

Thermocouples are the most high temperature contact sensors. The principle of thermocouple operation is based on thermoelectric effect opened by the German physist Thomas Seebeck in 1821.  If you join two dissimilar conductors at one end and maintain the junction at different temperatures, the voltage is produced between the ends. The voltage value depends on the material of conductors and the temperature difference between the junctions. In a small temperature range voltage is proportional to the temperature difference.

The main thermocouple advantage is the wide range of operating temperature from -200°С to 2500°С, design simplicity and strength, as well as a low price.

The main limitation with thermocouples is accuracy (1°C)  tolerance, cold-junction compensation and nonlinear relationship between the measured temperature and the output voltage.  

The operating principle of Resistance Temperature Detectors (RTDs) is the same as for thermistors, - the change in resistance with temperature.

RTDs are remarkable for their own high accuracy (up to 0,1 ° C), stable indications, the proximity to the characteristics of the linear dependence and interchangeability.

RTDs are made from platinum by spraying a thin layer of platinum on a ceramic substrate. The technology allows the production of platinum sensors with extremely small dimensions and a minimum consumption of platinum (platinum content - 0,001 g). In this case, platinum fully retains its properties, such as resistance to high temperatures, thermal stability, and almost linear temperature dependence of the resistance.

Currently, RTD Pt100 and Pt1000 are the most common temperature sensors.

In Ukraine, PJSC TERA is one of the leading manufacturers of RTDs as well as of any other types of sensors.