Thermocouple and thermocouple extension wires are flexible, insulated wires the first used to link a thermocouple to a temperature control device or make the sensing point (or probe) of the thermocouple while the second to extend a thermocouple signal from a probe back to the measuring instrument reading the signal over long distances.
The thermocouple wire determines the component's temperature and relays the information to the device. It works accurately over a wider temperature range than thermocouple extension wire. There are different types of thermocouples conctructed from 2 conductor solid core thermocouple grade specified gauge insulated wires with a positive and negative lead and an insulated outer jacket. Types N, K, E, J, T are base metal thermocouple wires mostly used while Types S, R and B are precious metal thermocouple wires. Common Thermocouple Wire Types are briefly described below.
K-Type thermocouple (nickel-chromium / nickel-silicon - nickel-aluminum)
Base metal thermocouple with strong oxidation resistance, which can measure the temperature of the medium from 0 to 1300 °C. It is suitable for continuous use in oxidizing and inert gases, with a short-term use temperature of 1200 °C and a long-term use temperature of 1000 °C. The relationship between its thermoelectric potential and temperature is approximately linear, and it is currently the most widely used thermocouple. However, it is not suitable for bare wire use in vacuum, sulfur-containing, carbon-containing atmospheres, and alternate redox atmospheres. When the oxygen partial pressure is low, the chromium in the nickel-chromium electrode will be preferentially oxidized, which will greatly change the thermoelectric potential, but the metal gas has little effect on it. Therefore, metal protection tubes are mostly used.
Disadvantages of type K thermocouples:
(1) The high-temperature stability of thermoelectric potential is worse than that of N-type thermocouples wire and precious metal thermocouples, and it is often damaged due to oxidation at higher temperatures (e.g. over 1000 °C).
(2) The short-term thermal cycle stability is low in the range of 250-500 °C, that is, at the same temperature point, in the process of heating and cooling, the thermoelectric potential is different, and the difference can reach 2-3 °C.
(3) The negative pole will undergo a magnetic transformation in the range of 150 to 200 °C causing the graduation value in the range of room temperature to 230 °C often to deviate from the graduation table. Especially when used in a magnetic field, there is often time-independent thermoelectric interference.
(4) Under the high-flux medium irradiation environment for a long time, due to the metamorphism of manganese (Mn), cobalt (Co), and other elements in the negative electrode, its stability is poor, resulting in a large change in thermoelectric potential.
T-Type thermocouple (copper-constantan)
In T-type thermocouple the positive electrode is of pure copper, and the negative electrode is of a copper-nickel alloy (also called constantan). T-type thermocouples exhibit the highest accuracy and good thermode uniformity among base metal thermocouples. Their operating temperature is between-200 and 350 ℃, as the copper hot electrode is easy to be oxidized. Therefore, when used in an oxidizing atmosphere, it should generally not exceed 300 °C. In the range of -200 to 300 °C, their sensitivity is relatively high. Copper-constantan thermocouples are low priced and commonly used.
J-Type thermocouple (iron-constantan)
The positive electrode of this duplex thermocouple is pure iron, and the negative electrode is constantan (copper-nickel alloy), characterized by its low price. It is suitable for reducing or inert atmosphere for vacuum oxidation, and the temperature range is from -200 to 800 °C. However, the commonly used temperature is only below 500 °C, because the oxidation rate of the iron hot electrode is accelerated after this temperature is exceeded. If the wire with a thick wire diameter is used, it can still be used at high temperatures and has a long life. The thermocouple is resistant to hydrogen (H2) and carbon monoxide (CO) gas corrosion, but cannot be used in an atmosphere containing sulfur (S) at high temperatures (eg 500°C).
N-Type thermocouple (NiCrSi-NiSi)
These thermocouples feature strong anti-oxidation ability in temperatures below 1300 °C, good long-term stability and short-term thermal cycle repeatability, good nuclear radiation resistance, and low-temperature resistance. In addition, in the range of 400 ~ 1300 ℃, the linearity of the thermoelectric characteristics of the N-type thermocouples is better than that of those of K-type. However, in the low-temperature range (-200 ~ 400 ℃), the nonlinear error is large, and at the same time, the material is hard and difficult to process.
E-Type thermocouple (nickel-chromium-copper-nickel (constantan)
Their positive electrode is of a nickel-chromium alloy, and the negative electrode of a copper-nickel alloy (constantan). Commonly used thermocouples, with highest thermoelectric potential and resulting sensitivity. Although their application range is not as wide as that of K-type , the E-Type thermocouples are often selected under conditions of high sensitivity, low thermal conductivity, and large resistance that can be tolerated. Restrictions in use are the same as type K but are less sensitive to corrosion in atmospheres containing higher humidity.
Precious metal thermocouple wires such as S-type or R-type are less comoonly used but briefly discussed below:
S-Type thermocouple (platinum rhodium 10-platinum)
The positive electrode is of a platinum-rhodium alloy containing 10% rhodium, and the negative electrode from pure platinum. Main features include:
- Stable thermoelectric performance, strong oxidation resistance, suitable for continuous use in an oxidizing atmosphere, long-term use temperature up to 1300 ℃, when it exceeds 1400 ℃, even in the air, the pure platinum wire will recrystallize, so that The grains are coarse and fractured.
- High precision, the highest level of accuracy among all thermocouples, is usually used as a standard or to measure higher temperatures.
- Wide range of use, good uniformity, and interchangeability.
Main disadvantages are that the differential thermoelectric potential is small, so the sensitivity is low. The price of these thermocouples is relatively high, the mechanical strength is low, and they are not suitable for use in a reducing atmosphere or under the conditions of metal vapor.
R-Type thermocouple (platinum rhodium 13-platinum)
The positive electrode of the thermocouple is of a platinum-rhodium alloy containing 13%, and the negative electrode is of pure platinum. They exhibit similar properties with S-type thermocouples. Mostly used as a high-temperature thermocouple.
