The use and maintenance of thermocouples?

1. Law of homogeneous conductor

A closed circuit is formed by welding two ends of the same homogeneous material (conductor or semiconductor), regardless of the cross-sectional area of the conductor and the distribution of temperature. There will be no contact potential, and the temperature difference potential will cancel out, resulting in a total potential of zero in the circuit.

It can be seen that thermocouples must be composed of two different homogeneous conductors or semiconductors. If the thermoelectric dipole material is uneven, an additional heating potential will be generated due to the existence of a temperature gradient.

2. Law of intermediate conductor

The intermediate conductor (third conductor) is connected to the thermocouple circuit. As long as the temperature at both ends of the intermediate conductor is the same, the introduction of the intermediate conductor has no effect on the total potential of the thermocouple circuit. This is the law of intermediate conductor.

Application: According to the law of intermediate conductor, in the actual temperature measurement application of thermocouples, the form of hot end welding and cold end open circuit is often used. The cold end is connected to the display instrument through a connecting wire to form a temperature measurement system.

Some people are concerned that using copper wires to connect the cold end of the thermocouple to the instrument to read the mV value may cause additional measurement errors due to the contact potential generated at the connection between the wire and the thermocouple. According to this law, there is no such error!

After reading this article, engineers have no worries about thermocouples

3. Law of Intermediate Temperature

The thermoelectric potential between the two contacts of a thermocouple circuit (at temperatures T and T0) is equal to the algebraic sum of the thermoelectric potential of the thermocouple at temperatures T and Tn and at temperatures Tn and T0. Tn refers to the intermediate temperature.

Application: Due to the non-linear relationship between thermocouples E-T, when the cold end temperature is not 0 degrees Celsius, the actual thermoelectric potential E (t, t0) of the known circuit cannot be directly used to lookup the table to obtain the temperature value of the hot end; It is also not possible to use the actual thermoelectric potential E (t, t0) of the known circuit to directly look up the temperature value from the table, and then add the cold end temperature to determine the measured temperature value of the hot end. It needs to be corrected according to the law of intermediate temperature. Beginners often do not correct according to the law of intermediate temperature!

4. Reference electrode law

This law is only studied and paid attention to by professionals, and is generally not well understood by those involved in production and use. Simply put, using high-purity platinum wire as the standard electrode, assuming that the positive and negative electrodes of a nickel chromium nickel silicon thermocouple are paired with the standard electrode, the sum of their values is equal to the value of this nickel chromium nickel silicon thermocouple

Thermocouple troubleshooting

After reading this article, engineers have no worries about thermocouples

1, After accurately wiring and powering on according to the instrument wiring diagram, the instrument first displays the thermocouple graduation number of the instrument, then displays the instrument range scale, and then the digital tube in the lower row of the instrument displays the set temperature, and the digital tube in the upper row of the instrument displays the measured temperature. If the digital display on the instrument panel shows a status such as "OVER", "0000", or "000" instead of the temperature of the heating element, it indicates a malfunction in the output part of the instrument. The following review should be conducted:

① Remove the thermocouple from the output end of the instrument thermocouple and short-circuit the output end of the instrument thermocouple with any wire. When powered on, if the value displayed on the digital display on the instrument panel is about room temperature, it indicates that the external connection of the thermocouple is open and should be replaced with a thermocouple of the same type. If the above state persists, explain that the output end of the instrument was damaged during transportation and needs to be replaced.

② Remove the thermocouple from the faulty instrument mentioned above and replace it with a thermocouple connected to the same type of scale instrument that is running normally next to it. After powering on, if the digital display on the upper row of the faulty instrument shows the temperature of the heating element, explain that the external connection of the thermocouple is open and replace it with a thermocouple of the same type. If the above state persists, explain that the output end of the instrument was damaged during transportation and needs to be replaced.

③ Remove the faulty thermocouple from the instrument and use a multimeter to measure ohms (R) * 1 (friends with conditions can use a signal generator or calibrator). Use two multimeter rods to measure the two ends of the thermocouple. If the resistance value displayed on the multimeter is high, it indicates that the external connection of the thermocouple is open, and replace it with a thermocouple of the same type. Otherwise, if there is a certain resistance value, it indicates that there is a problem with the output terminal of the instrument and the instrument should be replaced.

2, According to the instrument wiring diagram, the wiring should be accurate. If the digital display on the upper row of the instrument shows negative values or other phenomena after the instrument is powered on, explain that the thermocouple "+" and "-" connected to the instrument are connected incorrectly. Just replace it with a new one.

3, When the instrument is running with accurate wiring, the temperature displayed on the digital tube on the instrument panel differs from the actual measured temperature by 30oC~60oC. Even the difference is even greater, explaining that the scale of the instrument is incorrect compared to the scale of the thermocouple. According to the correspondence between the temperature (oC) and millivolt (MV) values of thermocouples with scale marks B, S, K, E, etc., under the same temperature (oC) state, the millivolt (MV) value that occurs is smaller for scale marks B, second for scale marks S, larger for scale marks K, and * for scale marks E. Follow this principle to judge.