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Thermocouples/RTD


Types Of Thermocouples
Certain combinations of alloys have become popular as industry standards. Selection of the combination is driven by cost, availability, convenience, melting point, chemical properties, stability, and output. Different types are best suited for different applications. They are usually selected on the basis of the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the chemical inertness of the thermocouple material, and whether it is magnetic or not. Standard thermocouple types are listed below with the positive electrode (assuming ) first, followed by the negative electrode.

K
Type K (chromel {90% nickel and 10% chromium}-alumel {95% nickel, 2% manganese, 2% aluminium and 1% silicon}) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C (chromel positive relative to alumel when the junction temperature is higher than the reference temperature).[8] It is inexpensive, and a wide variety of probes are available in its ?200 °C to +1350 °C / -330 °F to +2460 °F range. Type K was specified at a time when metallurgy was less advanced than it is today, and consequently characteristics may vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a deviation in output when the material reaches its Curie point; this occurs for type K thermocouples at around 350 °C . Wire color standard is yellow (+) and red (-).

E
Type E (chromel-constantan)[5] has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic. Wide range is ?50 to 740 °C and Narrow range is ?110 to 140 °C. Wire color standard is purple (+) and red (-).

J
Type J (iron-constantan) has a more restricted range than type K (?40 to +750 °C), but higher sensitivity of about 50 µV/°C.[2] The Curie point of the iron (770 °C)[9] causes a smooth change in the characteristic, which determines the upper temperature limit. Wire color standard is white (+) and red (-).

N
Type N (Nicrosil-Nisil) (nickel-chromium-silicon/nickel-silicon) thermocouples are suitable for use between ?270 °C and 1300 °C owing to its stability and oxidation resistance. Sensitivity is about 39 µV/°C at 900 °C, slightly lower compared to type K.
Platinum types B, R, and S
Types B, R, and S thermocouples use platinum or a platinum-rhodium alloy for each conductor. These are among the most stable thermocouples, but have lower sensitivity than other types, approximately 10 µV/°C. Type B, R, and S thermocouples are usually used only for high temperature measurements due to their high cost and low sensitivity.

B
Type B thermocouples use a platinum-rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800 °C. Type B thermocouples produce the same output at 0 °C and 42 °C, limiting their use below about 50 °C.

R
Type R thermocouples use a platinum-rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600 °C.

S
Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

T
Type T (copper - constantan) thermocouples are suited for measurements in the ?200 to 350 °C range. Often used as a differential measurement since only copper wire touches the probes. Since both conductors are non-magnetic, there is no Curie point and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about 43 µV/°C.

C
Type C (tungsten 5% rhenium - tungsten 26% rhenium) thermocouples are suited for measurements in the 0 °C to 2320 °C range. This thermocouple is well-suited for vacuum furnaces at extremely high temperatures. It must never be used in the presence of oxygen at temperatures above 260 °C.

M
Type M thermocouples use a nickel alloy for each wire. The positive wire (20 Alloy) contains 18% molybdenum while the negative wire (19 Alloy) contains 0.8% cobalt. These thermocouples are used in vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400 °C. It is less commonly used than other types.
Chromel-Gold/Iron
In chromel-gold/iron thermocouples, the positive wire is chromel and the negative wire is gold with a small fraction (0.03-0.15 atom percent) of iron. It can be used for cryogenic applications (1.2-300 K and even up to 600 K). Both the sensitivity and the temperature range depend on the iron concentration. The sensitivity is typically around 15 µV/K at low temperatures, and the lowest usable temperature varies between 1.2 and 4.2 K.
Thermocouple Comparison
The table below describes properties of several different thermocouple types. Within the tolerance columns, T represents the temperature of the hot junction, in degrees Celsius. For example, a thermocouple with a tolerance of ±0.0025×T would have a tolerance of ±2.5 °C at 1000 °C.
Type Temperature range °C (continuous) Temperature range °C (short term) Tolerance class one (°C) Tolerance class two (°C)
K 0 to +1100 -180 to +1300 ±1.5 between -40 °C and 375 °C
±0.004×T between 375 °C and 1000 °C
±2.5 between -40 °C and 333 °C
±0.0075×T between 333 °C and 1200 °C
J 0 to +750 -180 to +800 ±1.5 between -40 °C and 375 °C
±0.004×T between 375 °C and 750 °C
±2.5 between -40 °C and 333 °C
±0.0075×T between 333 °C and 750 °C
N 0 to +1100 -270 to +1300 ±1.5 between -40 °C and 375 °C
±0.004×T between 375 °C and 1000 °C
±2.5 between -40 °C and 333 °C
±0.0075×T between 333 °C and 1200 °C
R 0 to +1600 -50 to +1700 ±1.0 between 0 °C and 1100 °C
±[1 + 0.003×(T - 1100)] between 1100 °C and 1600 °C
±1.5 between 0 °C and 600 °C
±0.0025×T between 600 °C and 1600 °C
S 0 to +1600 -50 to +1750 ±1.0 between 0 °C and 1100 °C
±[1 + 0.003×(T - 1100)] between 1100 °C and 1600 °C
±1.5 between 0 °C and 600 °C
±0.0025×T between 600 °C and 1600 °C
B +200 to +1700 0 to +1820 Not Available ±0.0025×T between 600 °C and 1700 °C
T -185 to +300 -250 to +400 ±0.5 between -40 °C and 125 °C
±0.004×T between 125 °C and 350 °C
±1.0 between -40 °C and 133 °C
±0.0075×T between 133 °C and 350 °C
E 0 to +800 -40 to +900 ±1.5 between -40 °C and 375 °C
±0.004×T between 375 °C and 800 °C
±2.5 between -40 °C and 333 °C
±0.0075×T between 333 °C and 900 °C
Chromel/AuFe -272 to +300 n/a Reproducibility 0.2% of the voltage; each sensor needs individual calibration.