Differences among Four-wire, Three-wire and Two-wire Thermal Resistor

Thermal resistor is the most commonly used temperature detector in the medium and low temperature zone, covering two-wire, three-wire, and four-wire systems. Their difference is mainly reflected in the structure of sensor as follows:

1. Resistance compensation

Two-wire system: simple wiring without line resistance compensation, but with additional error arising from lead resistance.

Three-wire system: line resistance compensation can eliminate the influence of lead resistance, with higher measurement accuracy than the two-wire system.

Four-wire system: Two lead wires connected at both ends of the root of the thermal resistor provide a constant current I for the resistor, convert R into a voltage signal U, and then lead U to the PLC through the other two lead wires. This method can completely eliminate the influence of lead resistance.

2. Different principles

The two-wire system and three-wire system are measured by the bridge method, and finally the relationship between the temperature value and the analog output value is given.

Four-wire system not relates to electric bridge, performs transmission only by a constant current source, measured by a voltmeter, and finally the measured resistance value is given.

3. Different measurement accuracy

Four-wire system means that 4 wires are drawn from both ends of the thermal resistor. The circuit loop and voltage measurement loop are independently and separately wired. It has high measurement accuracy but requires more wires.

Three-wire system means three wires are provided and when Pt100B platinum resistance is connected, the reference terminal of current loop and the reference of voltage measurement loop are one wire (that is, the terminal I- and V- of testing equipment are short connected), providing slightly better accuracy.

The two-wire system means two wires are provided and when Pt100B platinum resistor is connected, the current loop and the voltage measurement loop are combined into one (i.e. the terminal I- and V-, and the terminal I+ and V+ of the testing equipment are short connected respectively) , providing poor measurement accuracy.