Why are thermoelectronic temperature measuring instruments used in a three-wire system?

The three-wire configuration for thermal resistors is intended to reduce the impact of lead resistance values and the fluctuations in these values due to changes in ambient temperature on measurement accuracy

Thermal resistors come in three-wire and four-wire versions, with the aim of eliminating the influence of wire resistance on temperature measurement.

A thermal resistor, right? The principle of temperature measurement using thermoresistors is based on the fact that resistance changes with temperature. Since there is a certain distance between the thermoresistor and the display instrument, wire resistance can affect the measurement; therefore, a three-wire system is used to eliminate the impact of wire resistance on the measurements.

There are two-wire, three-wire, and four-wire types. The two-wire lead method is simple, but it has lead resistance; this approach is only used in situations where lower measurement accuracy is sufficient. Three-wire system: This approach makes use of a bridge circuit, which helps to effectively eliminate the effects of lead resistance; it is the most commonly used system in industry. The four-wire system provides a constant current I to the thermistor through two wires at one end, converting R into a voltage U, which is then transmitted to the secondary instrument via the other two wires. This lead configuration enables the elimination of the impact of lead resistance, and is mainly used for high-precision detection.

How can this be explained? Most of them use a three-wire system

Three-wire system: Eliminates the resistance of the wires as well as the \"resistance changes\" caused by external temperatures. Four-wire system: It is a balanced bridge; as long as the 4 wires are of equal length, it is not affected by changes in resistance caused by temperature. @jiaguoyun

The three-wire system generally refers to platinum resistors, such as PT100 and PT101. By measuring the resistances between points A and B, as well as between A and C, it is possible to roughly calculate the temperature using mental arithmetic, as the resistance values are quite similar. The three-wire system is used to 1 eliminate the resistance of the wires and reduce the impact of such resistance, and 2 eliminate interference from external temperatures

I’d like to say a few words regarding Pt100 thermistors for general temperature measurement. 1. First of all, the temperature range must be below 500°C; this is a prerequisite. 2. For general temperature measurement where high precision is not required, the Pt100 resistance signal is fed directly into the DCS card. Eliminate intermediate steps. It is generally three-wire system. 3. When high precision is required, if thermal resistance signals are still fed into the DCS, the accuracy may not be guaranteed, as DCS temperature cards generally have only 16 bits of resolution, resulting in low precision; therefore this method is not suitable when high precision is needed. For this purpose, a temperature transmitter is needed; it can be integrated or separate. Personally, I believe that separate transmitters used on-site generally offer better quality and other advantages in terms of performance. However, for integrated temperature transmitters, it should be noted that the measured temperature should preferably not exceed 100 degrees, as the temperature experienced by the electronic components of the transmitter generally should not go above 65°C. This is why the operating temperature for most instruments is set at 65°C; with integrated temperature transmitters, the temperature of the medium is transmitted to the transmitter’s display through the measuring probe. I see that few people in the posts on this website have noticed this issue; this is also the real reason that limits the widespread use of integrated temperature transmitters. 4. As for whether to use a four-wire or three-wire system for thermoresistors, I think that if a four-wire system is necessary, it would be better to use a separate temperature transmitter on-site.

The three-wire system requires that the cross-sectional areas and lengths of the three wires used be identical. The circuit for measuring platinum resistors is typically an unbalanced bridge, with the platinum resistor serving as one of the resistance elements in the bridge. One wire is connected to the power supply of the bridge, while the other two wires are connected respectively to the bridge arm containing the platinum resistor and to the adjacent bridge arm. When the bridge is in balance, it can be calculated that Rt = R1R3/R2 + R1r/R2 – r. When R1 = R2, any change in the resistance of the wires has no effect on the measurement results, thereby eliminating the measurement errors caused by the resistance of the wires. However, it is necessary for the bridge to have equal-length arms; otherwise, it is not possible to completely eliminate the influence of wire resistance. It can be seen that using the three-wire system reduces the additional errors caused by wire resistance, and this is why the three-wire connection method is commonly used in industry.