Mostly, in the above mentioned range the following three types of sensors are used: thermistors, digital semiconductor sensors and RTDs. Let us dwell in more detail on their advantages and disadvantages.
Thermistors use the principle of conductor resistance change with temperature. Sensitive elements for thermistors are made on the basis of various metal oxides. The advantages of thermistors are a high sensitivity, a small size and low price. The main disadvantages are nonlinear characteristics, weak noise immunity, and a lack of interchangeability. Such characteristic, as 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 the 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 characteristic and in case of sensor’s failure one will have to apply to the original manufacturer.
Digital semiconductor temperature sensors have been actively promoted by the agricultural market not long ago. Sensitive elements for them are made by world leaders in the field of microelectronics. The sensitive element by itself is an integrated circuit containing a sensor and a converter to a digital signal. The main advantages of these sensors are a convenient wiring diagram (which does not require an individual cabling directly to the sensor, the sensors are connected to the 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 there is the line of force or running electric motors nearby) 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.
The operating principle of Resistance Temperature Detectors (RTD) 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), the stability of indications, the proximity to the characteristics of the linear dependence and interchangeability. RTDs are made 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, the TERA Company is one of the leading manufacturers of RTDs as well as any other types of sensors.