The selection of thermocouple should be based on comprehensive considerations such as the use temperature range, the required accuracy, the use atmosphere, the performance of the measurement object, the response time and the economic benefits.
First, the selection of measurement accuracy and temperature measurement range
When the operating temperature is 1300~1800℃ and the accuracy is relatively high, the B-type thermocouple is generally used, the accuracy is not high, and the atmosphere allows the use of tungsten-rhenium thermocouples, and the tungsten-rhenium thermocouple is generally used when the temperature is higher than 1800℃;
The operating temperature is 1000~1300℃, the accuracy is required and the high accuracy is available. S-type thermocouple and N-type thermocouple are available;
Generally use K-type thermocouple and N-type thermocouple below 1000℃;
Generally use E-type thermocouple below 400℃;
T-type thermocouples are generally used for measurement at 250°C and negative temperature. T-type thermocouples are stable and have high accuracy at low temperatures.
Second, the choice of atmosphere
S-type, B-type, and K-type thermocouples are suitable for use in strong oxidizing and weak reducing atmospheres;
J-type and T-type thermocouples are suitable for weak oxidizing and reducing atmospheres. If a protective tube with better air-tightness is used, the requirements for the atmosphere are not too strict.
Third, the choice of durability and thermal response
Thermocouples with large wire diameters have better durability, but have a slower response. For thermocouples with large heat capacity, the response is slow. When measuring temperatures with large gradients, the temperature control is poor in the case of temperature control. It requires a fast response time and a certain degree of durability, so it is more appropriate to choose an armored couple.
Fourth, the nature and state of the measurement object to choose the thermocouple
The temperature measurement of moving objects, vibrating objects, and high-pressure vessels requires high mechanical strength. Chemically polluted atmospheres require protective tubes. In the case of electrical interference, higher insulation is required.
Fifth, the selection process
Model—Graduation Number—Explosion-proof Level—Accuracy Level—Installation and Fixing Type—Protection Tube Material—Length or Insertion Depth
Product selection and ordering instructions:
1. Please specify when selecting and ordering
2. Product model
3. Indexing number
4. Protection tube material and diameter
5. Protection tube total length L and insertion depth
6. Fixing device type
7. The actual measurement range of the product
8. The type of threaded fixing device that is not marked when ordering is fixed external thread M27×2 (the rest of the threaded fixing type needs to be indicated)
9. Those who do not conform to the above-mentioned product models due to the special needs of users and need specially manufactured products. Please indicate your special technical requirements and contact the relevant responsible department of our company for special orders.
10. Selection skills of thermocouples at different temperatures
For the selection of thermocouples, firstly select the thermocouple and protective sleeve according to the upper limit of the measured temperature and the use environment, and secondly choose the specifications and dimensions of the thermocouple according to the structure and installation characteristics of the measured object.
Thermocouples can be divided into different types of structures: general industrial type, Kaizhuang type, high-performance entity type and special type thermocouple.
Sixth, there are the following types of common general industrial thermocouples:
(1) The nickel-chromium-constantan thermocouple is a low-cost metal thermocouple. Its positive electrode is nickel-chromium alloy (EP), and its negative electrode is copper-nickel alloy (EN). The long-term use temperature is 600 degrees, and the short-term use temperature is 800 degrees. The biggest feature is that the thermoelectromotive force rate is the largest, that is, the agility is the highest, but it is easily oxidized, and the index number is E.
(2) The copper-constantan thermocouple is a cheap metal thermocouple. Its positive electrode is pure copper (TP) and its negative electrode is constantan (copper-nickel) alloy (TN). A thermocouple commonly used in low temperature conditions. The temperature measurement range is -200~300℃. The thermoelectric wire has good averageness, high stability, good agility, low price, and the index number is T. Kaizhuang type thermocouple is to install the hot electrode wire in a metal sleeve with insulating material, and process it into a bendable solid combination, and then cut it according to the required length, and process its measuring end and reference end. Make the thermocouple of the Kay type. This kind of thermocouple can be made very thin and long, and can be bent according to the measurement needs in use. Its temperature measurement range is wide, the response speed is fast, the heat capacity at the hot junction is small, and the service life is long. At the same time, it has high mechanical strength, high pressure resistance, wide adaptability, great flexibility, convenient installation and use, and extremely wide application.
(3) Platinum-germanium 10—Platinum thermocouple is a precious metal thermocouple. The nominal composition of the positive electrode is platinum-germanium alloy (SP) containing 10% germanium, and the negative electrode is pure platinum (SN). The long-term use temperature is 1300℃, and the short-term use temperature is 1600℃. It has strong anti-oxidation function and should be used continuously in an oxidizing and inert environment. It is generally used for high temperature measurement with higher accuracy requirements, but it is expensive, poor mechanical strength, and low thermal electromotive force. It needs to be equipped with a display instrument with high agility, and the graduation number is S.
(4) The nickel-chromium-nickel-silicon thermocouple is a cheap metal thermocouple. Its positive electrode is nickel-chromium alloy (KP) containing 10%, and its negative electrode is nickel-silicon alloy (KN) containing 3% silicon. The negative electrode is magnetic. The long-term use temperature is 1000℃, and the short-term use temperature is 1200℃. The thermoelectric potential is 4~5 times larger than the S-type thermocouple. The linearity is excellent and the use temperature range is wide. It is the least chaotic one of the cheap metal thermocouples. It is suitable for continuous use in an oxidizing and inert environment. The price is cheap and the error is about 7°C. However, the thermoelectrode is not easy to be made too even and easy to oxidize, especially in the low temperature state, its stability is poor, and the index number is K.
