Selection and installation of industrial thermocouple thermometers

In industrial applications, the selection of thermocouples is first based on the upper limit of the measured temperature, the structure and installation characteristics of the measured object that require the correct selection of the thermocouple's thermoelectric electrode and protective sleeve, and the specifications and dimensions of the thermocouple.

Thermocouples can be divided into ordinary industrial type, aluminum bracket type, special type, etc. in terms of structure.

The general industrial thermocouple is as follows:

1 . Platinum rhodium 10 platinum thermocouple: a precious metal thermocouple with platinum rhodium alloy as the positive electrode and platinum as the negative electrode. It has a short-term working temperature of 1600 ℃ and a long-term working temperature of 1300 ℃. It has good physical and chemical stability and is generally used for high-precision high-temperature measurements. But the material is high, the thermoelectric potential is small, and the division code is s.

2. Nickel chromium alloy thermocouple: not the most stable type of precious metal in terms of performance, widely used, extremely nickel chromium alloy. The short-term working temperature is 1200 ℃, and the long-term working temperature is 900 ℃. The thermoelectric potential of this thermocouple is 4 to 5 times greater than the previous thermocouple, and it has good linearity with an error of generally (6-8) ℃. However, thermoelectric electrodes are difficult to produce uniformly, prone to oxidation, and have poor stability. The division code is k.

3 . Nickel chromium copper thermocouple: The positive weight is nickel chromium alloy, with a short-term working temperature of 800 ℃ and a long-term working temperature of 60 ℃. Thermocouples with the highest thermoelectric potential have high measurement accuracy, but are prone to oxidation. The division code is e.

4 . Copper Kang Copper Thermocouple: This is a commonly used thermocouple for low temperature measurement, with a temperature range of (-200-+200) ℃, good stability, high low temperature sensitivity, and low price. The division code is t.

Aluminum thermocouples are processed by combining hot electrodes, insulating materials, and metal sleeves. They have the characteristics of being slender, bendable according to measurement needs during use, low heat capacity, small heat capacity of hot contacts, long lifespan, and strong adaptability. They are widely used.

Thermocouples should be placed as close as possible to the temperature control point to be measured. In order to prevent heat transfer along the thermocouple or to prevent the protective tube from affecting the measured temperature, the thermocouple must be immersed in the measuring fluid to a depth of at least 10 times the diameter. When measuring solid temperature, thermocouples must be in close contact or in close contact with the material. To minimize the thermal conduction error, please reduce the temperature gradient near the junction.

When measuring the temperature of gas inside a pipeline with a thermocouple, if the temperature of the pipeline wall is significantly high or low, the thermocouple will emit or absorb heat, causing a significant change in the measured temperature. In this case, a so-called shielded thermocouple that uses radiation shielding to bring the temperature close to the gas temperature can be used.

When selecting temperature measurement points, they must be representative. For example, when measuring the fluid temperature in piping, the measuring end of the thermocouple is located at the location where the flow velocity is highest in the piping. Generally speaking, the end of the thermocouple protective cover should cross the centerline of the flow rate.

In practical use, special attention should be paid to using compensation codes. Usually, the compensation wire connected between the instrument and the terminal box is the same or close to the thermocouple used for thermoelectric characteristics, so connecting to the thermocouple will not generate a large additional thermoelectric potential and will not affect the total thermoelectric potential of the thermocouple circuit. If ordinary wires are used instead of compensating wires, there will be no compensation effect and the accuracy of temperature measurement will be reduced. Therefore, when installing measurement wiring in units of use, do not reverse polarity when connecting compensation wires to thermocouples. On the contrary, the temperature measurement error increases.

In actual measurement, if the measured value deviates from the actual value, the installation position of the thermocouple will become inappropriate. In addition, the wires of the thermocouple may be oxidized or gravel may appear at the welding point of the thermocouple measurement end.