Four laws of K-type thermocouple
Leave a message
1. Law of homogeneous conductors
K-type thermocouple wires are welded at both ends of the same homogeneous material (conductor or semiconductor) to form a closed loop. No matter what the conductor cross-section is and how the temperature is distributed, no contact potential will be generated, the temperature difference potential will cancel each other out, and the total potential in the loop is zero.
It can be seen that the K-type thermocouple must be composed of two different homogeneous conductors or semiconductors. If the thermocouple material is not uniform, additional thermoelectric potential will be generated due to the existence of a temperature gradient.
2. Law of intermediate conductors
When an intermediate conductor (third conductor) is connected to the K-type thermocouple loop, as long as the temperatures at both ends of the intermediate conductor are the same, the introduction of the intermediate conductor has no effect on the total potential of the thermocouple loop. This is the law of intermediate conductors.
Application: According to the law of intermediate conductors, in the actual temperature measurement application of thermocouples, the hot end is often welded and the cold end is open. The cold end is connected to the display instrument through the connecting wire to form a temperature measurement system.
Some people are worried that the contact potential generated at the connection between the wire and the thermocouple when the cold end of the thermocouple is connected to the instrument to read the mV value with a copper wire will cause additional errors in the measurement. According to this law, there is no such error!
3. Intermediate temperature law
The thermoelectric potential between the two junctions of the thermocouple circuit (temperature T, T0) is equal to the algebraic sum of the thermoelectric potential of the thermocouple at temperatures T, Tn and the thermoelectric potential at temperatures Tn, T0. Tn is called the intermediate temperature.
Application: Since the thermocouple E-T usually has a nonlinear relationship, when the cold end temperature is not 0 degrees Celsius, the actual thermoelectric potential E (t, t0) of the known circuit cannot be used to directly look up the table to obtain the hot end temperature value; nor can the temperature value obtained by directly looking up the table using the actual thermoelectric potential E (t, t0) of the known circuit, plus the cold end temperature to determine the measured temperature value of the hot end, which needs to be corrected according to the intermediate temperature law. Beginners often do not correct according to the intermediate temperature law!
4. Reference Electrode Law
This law is studied and paid attention to by professionals. People in general production and use links do not know much about it. A simple explanation is: use high-purity platinum wire as the standard electrode. Assuming that the positive and negative poles of the nickel-chromium-nickel-chromium thermocouple are paired with the standard electrode respectively, the sum of their values is equal to the value of this nickel-chromium-nickel-chromium.
