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What is a thermocouple?
A thermocouple is a type of sensor that is used to measure or monitor temperature. Because of their relatively low cost and ability to measure wide ranges in temperature, using thermocouples in industrial control applications is common. Thermocouples are often used for measuring high temperatures where other types of sensors are unable to function.
How do Thermocouples work?
Thermocouples are manufactured from two electrical conductors using two dissimilar metal alloys joined together at one end that produce specific millivolt signal at a given temperature. This signal is then measured and interpreted by a thermocouple thermometer. The conductors are typically built into a cable with a heat-resistant sheath. At one end of the cable, the two conductors are joined together by welding. This end of the thermocouple (the hot junction) is thermally attached to the object to be measured. The other end (the cold, or reference junction) is connected to a temperature measurement system. The thermocouple gives you temperature readings at the hot junction point.
Are there different types of thermocouples?
Typically, Thermocouples are available in four different combinations of metals or calibrations: J, K, T and E. Each calibration has a different temperature range and environment. With each of the four combinations, the maximum temperature varies with the diameter of the wire used in the thermocouple.
What do I need to consider when selecting the thermocouple type?
Because thermocouples measure in wide temperature ranges they are widely used in many fields of industry. When selecting a thermocouple you must consider: temperature range; corrosive or chemical resistance of the sheath material; abrasion and vibration resistance and installation requirements (different equipment requires different compatibility considerations).
How do I know which junction type to choose?
Sheathed thermocouple probes are available with one of three junction types: grounded, ungrounded or exposed. At the tip of a grounded junction probe, the thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction. In an ungrounded probe, the thermocouple junction is detached from the probe wall. The thermocouple in the exposed junction style protrudes out of the tip of the sheath and is exposed to the surrounding environment. This type offers the best response time, but is limited in use to noncorrosive and moisture free applications. See the illustrations at the right for a full discussion of junction types.
What is response time?
A time constant has been defined as the time required by a sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Exposed junction thermocouples are the fastest responding. Also, the smaller the probe sheath diameter, the faster the response, but the maximum duration may be lower. Be aware, however, that sometimes the probe sheath cannot withstand the full temperature range of the thermocouple calibration.
What is a thermocouple, and what is the wire used for?
It is a sensor used for determining temperature. It consists of diverse metals joined at the ends in a single connecting point. This connecting point generates electric energy when the temperature of one end differs from the other. The wire is commonly used to link the thermocouple to control instrumentation. It may also be used to build the sensing point.
What is the difference between thermocouple grade wire and extension grade wire?
Thermocouple grade wire is wire that is used to make the sensing point of the instrument, where extension grade wire is only used to extend a thermocouple signal from a probe back to the instrument reading the signal.
Thermocouple Material Selection?
(Compacted MgO Thermocouples) All MgO insulated thermocouples are made using the highest purity MgO for temperatures up to 2300°F (1260°C). The thermo-elements are all ANSI special limits of error to give your measurements the best possible results. The various sheath materials are dependent upon the application. The following list will help you make the best selection:
304 SS
Maximum temperature of 1650°F (900°C) and is the most widely used low temperature sheath material. It offers good corrosion resistance but is subject to carbide precipitation in the 900°F to 1600°F (480 to 870°C) range.

310 SS
Maximum temperature of 2100°F (1150°C) and offers good mechanical and corrosion resistance similar to 304 SS. Very good heat resistance. Not as ductile as 304 SS.

316 S
Maximum temperature of 1650°F (900°C) and has the best corrosion resistance of the austenitic stainless steels. Subject to carbide precipitation in the 900°F to 1600°F (480 to 870°C)

Maximum temperature 2150°F (1175°C) and is the most widely used thermocouple sheath material. Good high temperature strength, corrosion resistance and is resistant to chloride-ion stress corrosion, cracking and oxidation. Do not use in sulfur bearing environments.

Hastelloy X
Maximum temperature 2200°F (1205°C) widely used in aerospace applications. Resistant to oxidizing, reducing and neutral atmospheric conditions. Excellent high temperature strength.
How are insulated wires identified?
The insulation is usually color coded for easy identification. There are some common guidelines for the colors. For example, the negative lead in insulated wire is red. The positive lead has the color of the thermocouple as well as the overall color of insulated extension grade wire. The outer jacket of thermocouple grade wire is typically brown. For high temperature wire, it is common to have a color coded tracer thread in the white material. For information on usable temperature ranges for the insulation see the "Wire Insulation Identification" table. Other tables at this link are available for information on the metals used in the wire, insulation color codes, and more.
How do I select thermocouple Sheath diameter?
Standard Sheath Diameters Suggested Upper Temperature Limits
.020” +.001 -.0005” 1290°F (700°C) 1290°F (700°C)
.032” +.001 -.0005” 1290°F (700°C) 1290°F (700°C)
.040” +.001 -.0005” 1290°F (700°C) 1290°F (700°C)
.063” ±.001” 1690°F (920°C) 1690°F (920°C)
.090” ±.001” 1830°F (1000°C) 1830°F (1000°C)
.125” +.002 -.001” 1960°F (1070°C) 1960°F (1070°C)
.188” +.002 -.001” 2100°F (1150°C) 2100°F (1150°C)
.250” +.003 -.001” 2100°F (1150°C) 2100°F (1150°C)