On 2018-08-08 14:48:54
Standard for conductor used in thermal resistance
Two wire system: two wire system refers to the way of connecting a wire at both ends of the thermal resistance to lead out the resistance signal: this method of thermal resistance leads is very easy, but there must be a lead resistance R in the connecting wire. The size of R is related to the material and length of the wire, so this way of lead is only applicable to the place with low measurement accuracy.
Four wire system: the method of connecting two wires at both ends of the root of the thermal resistance is called four wire system. Two wires provide constant current I for the thermal resistance, convert r into voltage signal U, and then lead u to the secondary instrument through the other two wires. It can be seen that this kind of lead way can eliminate the influence of the thermal resistance of the lead, and is important for high-precision temperature detection.
Because the output of thermocouple is thermoelectric potential, if the lead of another material is connected, it will produce another potential, which will affect the measurement accuracy. Therefore, it is necessary to connect compensation wire, extend the cold end to the instrument, so that the influence of room temperature on Zui is small. In fact, the output of thermal resistance is resistance value, and the influence of conductor material is conductor resistance and its change with temperature. The solution is to adopt three wire system or four wire system, commonly used three wire system. In essence, the conductor with the same material section and length and thermal resistance value are differential processed to eliminate the influence of conductor. Therefore, it basically has nothing to do with the conductor material, but requires that the conductor material section and length are the same.
The compensation wire used is the same as the thermocouple compensation wire! That is to say, GB / t4989-94 compensation wire for thermocouple, the signal wire of thermal resistance does not seem to have corresponding standard. For the signal transmission line of thermal resistance, as long as the material section and length of the wire are the same, the PT100 thermal resistance is used in our factory. Signal lead is a common, shielded copper core KVV control cable.