1. Two wire system
The lead form of a wire connected to both ends of the thermal resistance temperature sensing element is two-wire system. This lead way will bring in additional error of lead resistance, and the lead should not be too long in use. It is generally applicable to the occasions with low precision requirements.
2. Three wire system
One end of the thermal resistance temperature sensing element is connected with two leads, and the other end is connected with a lead. This lead is called three wire system.
It can eliminate the influence of the resistance of the inner lead, and the measurement accuracy is higher than that of the two-wire system. The circuit for measuring three wire platinum resistance is generally unbalanced
The platinum resistance is used as a bridge arm resistance of the bridge. When the bridge circuit is balanced, Rt=(R1R3/R2)+(R1r/R2) - r When R1=R2, the change of wire resistance has no effect on the measurement results, thus eliminating the measurement error caused by wire line resistance. The premise is that it must be a full arm bridge, otherwise it is impossible to completely eliminate the impact on the wire resistance. According to the analysis, the adoption of the three-tier system will greatly reduce
For the additional error caused by small wire resistance, the three wire wiring mode is generally adopted in industry.
3. Four wire system
Two leads are connected at both ends of the thermal resistance temperature sensing element, and the four wire system shall be adopted for high-precision measurement. When the measured resistance value is very small, the resistance of the test line may introduce obvious errors. For four wire measurement, two attached test lines are used to provide constant current, and the other two test lines measure the voltage drop of the unknown resistance. When the input impedance of the voltmeter is high enough, the current almost does not flow through the voltmeter, so that the voltage drop on the unknown resistance can be accurately measured, and the resistance value can be calculated.