Principle of Thermistor Temperature Measurement?

The principle of temperature measurement: Thermistors (such as Pt100) are temperature sensors that convert temperature into resistance based on the principle that their resistance value changes with temperature. A temperature transmitter obtains the resistance value (voltage/current) by applying a known excitation current to a thermal resistor to measure its voltage at both ends, and then converts the resistance value into a temperature value to achieve temperature measurement. There are three wiring methods between thermistor and temperature transmitter: two-wire system, three wire system, and four wire system. The two-line system is shown in Figure 1. The transmitter applies excitation current I to the thermistor through wires L1 and L2, and measures potential V1, V2... Temperature measurement principle. Thermistors (such as Pt100) are temperature sensors that convert temperature into resistance based on the principle that their resistance value changes with temperature. A temperature transmitter obtains the resistance value (voltage/current) by applying a known excitation current to a thermal resistor to measure its voltage at both ends, and then converts the resistance value into a temperature value to achieve temperature measurement. There are three wiring methods between thermistor and temperature transmitter: two-wire system, three wire system, and four wire system.

Because A/D converters cannot directly measure resistance, and Ohmic routing tells us R=U ÷ I, we let a constant current flow through the resistor and indirectly measure resistance by measuring the voltage drop of the current across the resistor.

Due to the principle of thermistor temperature measurement, there is a one-to-one correspondence between temperature and resistance value. When displaying temperature, it is necessary to connect the thermistor and the display instrument with wires. The resistance value of the connecting wires may cause additional system errors in the measurement system, and the longer and thinner the wires, the greater the error. The two-wire connection method cannot overcome this error, so it is necessary to set the line resistance in the software to eliminate this error through software compensation. However, this set value is a constant, and in reality, the line resistance is not fixed and unchanged with factors such as line aging and environmental temperature changes. Therefore, software compensation methods still have errors. The three wire connection method can reduce or eliminate this systematic error in real time with changes in line resistance, but this is also based on the premise that the line resistance of the two lines at both ends of the thermistor is equal. Of course, under the condition of the same material and equal environmental temperature, the line resistance of the two lines at both ends of the thermistor is basically equal. Therefore, in order to further improve accuracy, completely eliminate wire materials, environmental temperature, etc. The influence of factors on measurement accuracy and error, We adopt a four wire measurement method, as shown in the following figure. It is recommended to use the four wire measurement method as much as possible in situations where high-precision measurement is required. The principles of first, second, and third wire measurement

Under the two-wire system: operational amplifier input differential voltage: Therefore, after the software measures U ÷ I, it needs to subtract the pre-set constant (line resistance 2RL) to obtain the resistance value Rt of the thermal resistor.

Under the three wire measurement method: Since the two constant current sources have equal currents and are known as I, according to the superposition theorem, the operational amplifier inputs a differential voltage: [2) ()] ((2) LtLLLLLUIRRRIIRRIIIRRU=´ ´+´ - ´ ´=+´=∈+When using a three wire measurement method, due to the use of dual constant current sources, in order to eliminate the error caused by line resistance, it is required that the currents of the two constant current sources must be equal, and the line resistance at both ends of the thermistor must be equal. In fact, due to the fact that the currents of these two constant current sources and the line resistance at both ends of the thermistor are often not equal, there is a small amount of error, so the line resistance cannot completely cancel out (eliminate), but it can already meet the requirements within a certain accuracy range