Distinctions
Base Metal Thermocouples
Type J: Iron (+) vs Constantan (-), is the most commonly used calibration. It is suitable for use in a vacuum, inert, oxidizing with the iron leg protected or reducing atmosphere. If unprotected the iron wire may be attacked by ammonia, nitrogen and hydrogen atmospheres. In sub zero temperatures the iron wire may rust or become brittle. Type J should not be used in sulfurous atmospheres above 540°C.
Type T: Copper (+) vs Constantan (-), is commonly used for sub-zero to 700°F temperature. Preferred to Type J for sub-zero applications because of Copper’s higher moisture resistance, as compared to iron. If unprotected, it will still function in a vacuum, inert, oxidizing or reducing atmosphere.
Type K: Chromel (+) vs Alumel (-) is generally used to measure high temperature to 2300°F. It should not be used for accurate temperature measurements below 900°F or after prolonged exposure above 1400°F. If unprotected it can be used only in inert or oxidizing atmospheres. It has a short life in alternately oxidizing and reducing atmospheres and in reducing atmospheres, particularly in the 1500 to 1850°F range.
Type E: Chromel (+) vs Constantan (-) has the highest EMF output of any standardized metallic thermocouple. If used unprotected, Type E wires are NOT subject to corrosion at sub-zero temperatures. They can be used in inert, oxidizing or reducing atmospheres. Because they cover a wide range with a single calibration curve, Type E thermocouples are preferred for computer applications.
Type N: Nicrosil (+) vs Nisil(-), was developed for oxidation resistance and EMF stability superior to those of Type K thermocouples at elevated temperatures. These couples have been shown to have a longer life, than Type K thermocouples, in both laboratories and industrial applications.
19/20 Alloy: Nickel/Nickel Moly. 19/20 Alloy thermocouples provide a very stable reading in reducing atmospheres at elevated temperatures. Can be used in an oxidizing atmosphere with proper protection.
Noble Metal Thermocouples
Type S: Platinum-10% Rhodium (+) vs. Platinum (-). The Type S thermocouple is widely used in industrial laboratories as a standard for calibration of base metal thermocouples and other temperature sensing instruments.
Type R: Platinum-13% Rhodium (+) vs. Platinum (-). These thermoelements should always be protected from contamination by reduced oxides, metallic vapors or other oxides at high temperatures. Platinum protective sheaths are used at temperatures which preclude the use of base metal sheaths. Insulation should be silica free to prevent contamination. Type S is frequently used for calibration and checking. Type R has a slightly greater sensitivity and consequently is used more frequently in industrial applications.
Type B: Platinum-30% Rhodium (+) vs. Platinum-6% Rhodium (-). For use between 1000 and 3175°F. Intended to prevent the problems experienced with Types S and R such as: (1) weakening of the pure platinum leg due to excessive grain growth and (2) calibration shift due to the pure platinum wire picking up rhodium volatilized from the alloy wire at 1500°C. The flatness of the temperature-millivolt curve at normal reference junction ambient temperature permits the use of copper extension wire.
Refractory Thermocouples
These thermoelements possesses excellent stability at temperatures in the 3000°F to 4000°F range. For use at high temperatures a protective atmosphere must be provided such as hydrogen, inert gas or vacuum. They are extremely sensitive to mechanical damage and should be handled carefully to prevent breakage.
Type W: Tungsten (+) vs. Tungsten-26% Rhenium (-). Also identified as letter code type G. This was an early stage thermocouple capable of measuring high temperature with reasonable accuracy. However, one serious drawback was the positive leg became embrittled. Extension lead wire used is an alloy type 200/226.
Type W5: Tungsten-5% Rhenium (+) vs. Tungsten-26% Rhenium (-). Also identified as letter code type C. Adding 5% rhenium to the positive leg improved the ductility and produces a higher EMF output Extension lead wire used is an alloy type 405/426.
Type W3: Tungsten-3% Rhenium (+) vs. Tungsten-25% Rhenium (-). Also identified as letter code type D. The W3 provides the same ductility as the W5 with the highest EMF output of all three. Extension lead wire used is an alloy type 203/225.