Temperature sensors are divided into three categories: thermocouples, thermal resistance and thermistors according to the characteristics of temperature sensing elements.
Let's look at the picture first, the following three pictures are the usual appearance of thermocouples, RTDs and thermistors.
Thermocouple. Thermocouples are the most commonly used in industry and are based on the principle of soldering two different conductors or any end of a semiconductor together to form a thermocouple. In other words, two different conductive materials hand in hand become thermocouples.
Just like the spouse, because his physical principle is to use the thermoelectric potential to detect temperature, hence the name thermocouple. The red and blue lines in the diagram below are two different materials.
The conductor or semiconductor that makes up the thermocouple is called a thermoelectrode, and the one end that is soldered together is inserted into the temperature measurement site and becomes the working end, and the other end is called the cold end and serves as the reference end.
If the temperature at both ends is different, this temperature difference causes a thermoelectric potential to be generated at the other two ends of the conductor or semiconductor, which can be converted to the corresponding temperature using voltage sampling.
There are two ways to measure thermocouples:
Set the reference point to 0 (cold junction compensation) and read the temperature directly. The air temperature of the reference contact is measured (reference contact compensation) and the temperature difference t is taken into account. 
Where is the temperature sensing part of the thermocouple?
As shown in the diagram above, the thermocouple part inside the liquid does not produce thermal EMF, which is only present in the part with a temperature gradient.
Since the thermocouple material is generally the best of the best, it is generally not designed between the temperature measurement point and the instrument, but the compensation lead is used to extend to the instrument side, so as to save the thermocouple material and reduce the cost.
The thermoelectric potential of the thermocouple increases with the increase of temperature, and the magnitude of the thermoelectric potential is related to the material of the thermocouple and the temperature value of both ends of the thermocouple, but not the length and diameter of the thermoelectrode.
The temperature measurement principle of RTD is based on the fact that the resistance value of a metal conductor increases with temperature. Most of the thermal resistance is made of pure metal materials, such as platinum, nickel, copper, etc., and the selected metal materials must have a certain special effect that the resistance value changes with the change of temperature, that is, the temperature coefficient of the resistance must be large enough to make the resistance change with the temperature more easy to measure.
Resistance leads are generally divided into two-wire, three-wire, and four-wire systems, and the reason for this is that more accurate resistance values need to be measured. The schematic diagram of the connection measurement of the four-wire system is as follows:
In the diagram, L4 and L3 are connected to the high-impedance input to sense the voltage, and V1 is input to a constant-current drive RTD.
Thermistors include a positive temperature coefficient (PTC) and a negative temperature coefficient (NTC), of which temperature sensing is mostly negative, that is, the resistance decreases with increasing temperature.
NTC thermistors are made of metal oxides such as manganese, cobalt, nickel and copper as the main materials, and are manufactured by ceramic process, and these metal oxide materials have semiconductor properties, because they are completely similar to semiconductor materials such as germanium and silicon in the way of conductivity.
When the temperature is low, the number of carriers (electrons and holes) of the oxide material is small, and its resistance value is high, and as the temperature increases, the number of carriers increases, and the resistance value decreases. The main electrical parameters are as follows:
The zero power resistance value is r25
Refers to the DC resistance value measured at the specified temperature (25°C), and the change in resistance value due to self-heating is negligible to the total measurement error.
b constant. It is calculated from two resistance values at a specific ambient temperature and characterizes how quickly the resistance changes with temperature, i.e., the higher the b value, the faster the resistance decreases with temperature, and vice versa.
Dissipation factor δ
Refers to the amount of power required to increase its temperature by 1°C by heating itself.
Thermal time constant
The thermal time constant, measured in seconds, refers to the temperature difference between the initial temperature t0 and the final temperature t1 of the thermistor element at zero load when the ambient temperature changes dramatically2 of the time it takes for the temperature to change.
NTC's Application Circuit: