Material requirements and common classifications for commonly used thermocouples in electric furnaces

Thermocouple is a widely used measuring instrument that has significant impact in multiple industries. The use of thermocouples in electric furnaces is also very important, playing a crucial role. The application of thermocouples in electric furnaces also has certain requirements for their data.

 

Information request for commonly used thermocouples in electric furnaces:

 

1. Low temperature resistance - The temperature measurement range of a thermocouple depends heavily on the low-temperature function of the thermoelectric electrode. In other words, the more stable the physical and chemical functions of the thermoelectric electrode in a low-temperature medium, the wider the temperature measurement range of the thermocouple composed of it.

 

2. More explicit - Use thermocouples with identical thermoelectric electrode data, requesting their electric heating functions to be stable and compatible, so that the thermocouples can be produced in batches and have good interchangeability;

 

3. High sensitivity and good linearity - the temperature difference and thermoelectric potential generated by the thermocouple should be sufficient and linearly correlated with temperature;

 

4. In addition to meeting the above requirements, we also hope that the resistance coefficient and resistance temperature coefficient of the thermoelectric material can be as small as possible, and that its price is cheap and the supply is sufficient.

 

Thermocouples used in electric furnaces should be selected based on the request when applied. How many types of thermocouples are commonly used now

 

[1) Platinum rhodium/platinum thermocouple - its graduation mark is S, the positive electrode is an alloy of 90% platinum and 10% rhodium, and the negative electrode is a pure platinum wire.

 

The advantage of this thermocouple is that it can easily prepare platinum rhodium alloy with a purity of *, making it easy to replicate, and has high temperature measurement accuracy. It can be used as a reference thermocouple in the temperature range of 630.74-1064.43 ℃. It has high physical and chemical stability and is suitable for application in oxidizing and neutral atmospheres; Its melting point is high, so the lower limit of temperature measurement is also high. In industrial measurement, it is occasionally used to measure temperatures above 1000 ℃, and can be temporarily and continuously applied below 1300 ℃. Short term temperature measurement can reach 1600 ℃.

 

The problem with platinum rhodium/platinum thermocouples is that they are expensive and have a low thermoelectric potential. When used in atmospheres such as restorative gases, metal vapors, metal oxides, silicon oxides, and sulfur oxides, they can quickly become contaminated and transform. Therefore, when used in these atmospheres, maintenance sleeves must be added. Additionally, the thermoelectric function of this thermocouple is highly nonlinear, and at low temperatures, its thermoelectric electrode will sublime, causing rhodium molecules to penetrate into the platinum electrode and contaminate it, resulting in unstable thermoelectric potential.

 

[2] Nickel chromium/nickel silicon thermocouple - with a division number of K, the positive electrode is 9-10% chromium and 0.4% silicon, the others are nickel, the negative electrode is 2.5-3% silicon,<0.6% chromium, and the others are nickel.

 

The advantages of this type of thermocouple are strong oxidation and corrosion resistance, good stability, large thermoelectric potential, good linear correlation between thermoelectric potential and temperature, and low cost of thermal electrode data. It can be temporarily and continuously used below 1000 ℃, and short-term temperature measurement can reach 1300 ℃.

 

The problem with nickel chromium/nickel silicon thermocouples is that they are easily corroded when exposed to restorative media at temperatures above 500 ℃ and in sulfur and chemical atmospheres. Therefore, maintenance sleeves must be added when working in these atmospheres, and their temperature measurement accuracy is also lower than that of platinum rhodium/platinum thermocouples.

 

[3] Nickel chromium/copper thermocouple - with a reading index of E, the positive nickel chromium identity is 9-10% chromium, 0.4% silicon, and the rest is nickel; The negative electrode contains 56% copper and 44% nickel.

 

The biggest advantage of nickel chromium/copper thermocouple is its high thermoelectric potential and low cost. The problem with this thermocouple is that it cannot be used to measure low temperatures. Its lower temperature limit is 800 ℃, and it is only suitable for temporary use below 600 ℃. Additionally, due to the susceptibility of copper alloys to oxidation and degradation, maintenance sleeves must be installed during application.

 

[4] Platinum rhodium 30/Platinum rhodium 6 thermocouple - referred to as dual platinum rhodium thermocouple, with division number B. The positive and negative electrodes of this thermocouple are both made of platinum rhodium alloy, with only a difference in alloy content ratio. The positive electrode contains 30% rhodium and the negative electrode contains 6% rhodium. The dual platinum rhodium thermocouple has strong anti fouling ability and still has good stability when measuring temperatures up to 1800 ℃. It has high temperature measurement accuracy and is practical for oxidizing and neutral media. It can temporarily and continuously measure low temperatures of 1400-1600 ℃, and short-term measurements can reach 1800 ℃.

 

The sensitivity of the dual platinum rhodium thermocouple is relatively low, and a performance instrument with high sensitivity should be equipped when applied. At room temperature, the effect of temperature on thermoelectric potential is minimal, so there is no need to stop temperature compensation in some applications.

 

[5] Copper/constantan thermocouple - with a division mark of T, the positive electrode is copper, and the negative electrode is an alloy of 60% copper/40% nickel.

 

Its advantages include high temperature sensitivity, easy replication of thermoelectric electrodes, low cost, good high-temperature performance, and the ability to measure temperatures up to -200 ℃. But its identity is that copper is prone to oxidation, so some individual temperature limits do not exceed 300 ℃.