Principle and selection of thermocouple compensation wire

1. Why use thermocouples to compensate for wires?

Since thermocouples can achieve temperature measurement, why use compensating wires

Let's take a look at the practical application of measurement.

Generally speaking, for the convenience of monitoring and operating measuring instruments, the temperature of the measuring instrument is almost constant, which is very convenient

Place the workstation at a certain distance from the actual temperature measurement point. At this distance, the situation is generally complex

For example, through power sources, there may be electromagnetic interference, which can frequently be subjected to external forces. The use of thermocouple wires can lead to measurement errors, easy breakage during use, and frequent replacement issues. In addition, the material of thermocouple wires is generally expensive, and the cost is also very high for long-distance use. These are all reasons for using compensating wires.

In summary, using compensating wires has the following advantages

1) Improve the physical and mechanical properties of the temperature measurement circuit of thermocouples. Using multi-core wires or small-diameter compensating wires can enhance the flexibility of the circuit, facilitate wiring, and also adjust the circuit resistance or block interference

2) Reduce the cost of measurement lines. When the distance between thermocouples and measuring devices is long, using compensating wires can save a lot of thermocouple materials, especially when using precious metal thermocouples.

2. Classification of thermocouple compensation wires

Thermocouple compensation codes can be divided into two categories:

1) Extended compensation wire

Extended compensation lead, also known as extended lead, has the same nominal chemical composition and thermoelectric potential as the matched thermocouple in its alloy wire, with the letter“ X” Attach after the thermocouple calibration code, for example,“ KX” Indicating the use of extended compensating wires for K-type thermocouples.

2) Compensation type compensating wire

Compensation type compensation lead, also known as compensation type lead, has a nominal chemical composition of its alloy wire that is different from that of the matched thermocouple, but its thermoelectric potential value is the same as the nominal thermoelectric potential value of the matched thermocouple when it is between 0-100 ℃ or 0-200 ℃, with the letter“ C” Attach after the thermocouple calibration code, for example,“ KC” The hyperbolic cosine value. Different alloy wires are suitable for thermocouples with the same index number and can be distinguished by letters.

3. The correct method for selecting thermocouple compensation wires

The division code of the compensating wire must be consistent with the thermocouple used. For example, K-type thermocouples must use K-type thermocouple compensation wires.

When selecting based on the temperature of the usage environment, the working temperature range of the compensation wire is relatively low, generally divided into two types: 0-100 ℃ and 0-200 ℃. The price of 0-100 ℃ is lower and can be selected according to actual needs.

When there are many sources of interference on site, shielded compensation wires with strong anti-interference properties can be selected.

When there are multiple temperature measurement points at the same time, cables with multiple sets of compensation wires can be selected.

4 . Precautions for using compensating wires

1) The selection of compensation wires must be based on the type of thermocouple used and the correct choice made during use. For example, k type even or k type even compensation wires should be selected.

2) The two contacts of the contact connection and thermocouple terminal should be as close as possible to ensure that the temperature of the two contacts is consistent. The connection part with the instrument terminal should be kept at the same temperature as much as possible to protect the areas with fans on the instrument panel and the contact parts from direct contact with the fan.

3) Due to the low signal of the thermocouple with a length of microvolts, when used for a long distance, signal attenuation and strong electric field interference coupling in the environment can cause distortion of the thermocouple signal, incorrect temperature measurement and control, and in severe cases, temperature fluctuations. In general, it is better to use thermocouples to control the wire length within 15 meters, but if it exceeds 15 meters, it is recommended to use a temperature transmitter to send signals. The temperature transmitter converts the potential value corresponding to the temperature into direct current and transmits it, thus having strong noise resistance.

4) The wiring of compensation wires must be kept away from power lines and noise sources. In unavoidable places, try to use a crossover approach and avoid parallelism.

5) Shielded compensation wires can be used to improve the anti-interference performance of thermocouple connection wires. When there are many sources of noise on site, the effect is better. However, the shielding layer must be tightly grounded. The shielding layer not only fails to provide shielding, but also enhances noise