What is a Thermocouple ?

A thermocouple is a thermoelectric device used to make accurate measurements of temperatures, especially at high temperatures. It consists of a circuit having two wires of different metals or metal alloys welded together. A temperature gradient across the junction of the wires gives rise to an electric potential by the Seebeck effect. This potential varies with the strength of the temperature gradient and can be measured in milivolts. Thermocouples can also be used to generate small amounts of electricity for powering other micro devices.

Thermocouple junction types

Basically, they are 3 junction types, grounded, ungrounded and exposed. For grounded thermocouples it has a metal sheath and the thermocouple wires are welded to the casing. Usually, it is done with a tig welding machine, but it can be done by soldering the wires. However, it may cause contamination to the thermocouple wires.

Usually, grounded thermocouples are not filled with MgO powder unless it is intended for higher temperature applications. Alternatively, to mineral insulated thermocouples, the wires can be insulated with ceramic insulators for high temperature applications.

Why grounded thermocouples? Grounded thermocouples temperature sensors are widely used, because it offer faster respond time, more accurate reading at short distant. It is a preferred junction type for high temperature applications. Precaution for ground loop at long distances and at low temperature usage.

Why ungrounded Thermocouples? Ungrounded thermocouples are used primarily for isolating the control system from the sensor and to prevent ground loop. It is more inaccurate and slow respond time.

Exposed Junction, offer the fastest respond time. It is not intended for pressurized or corrosive environments.




Thermocouple Sensitivity

Type K --- Chromel ( Nickel-Chromium Alloy ) / Alumel ( Nickel-Aluminium Alloy )
This is the most commomly used "general purpose" thermocouples. They are available in the ~ -200 °C to +1200 °C range. The characteristic of the thermocouple undergoes a step change when a magnetic material reaches its Curie point. This occurs for this thermocouple at ~ 352°C. Sensitivity is ~ 41 µV/°C.

Type J --- Iron / Constantan

Limited range (~ -40 to +750 °C) makes type J less popular than type K. The main application is with old equipment that cannot accept modern thermocouples. J types cannot be used above 760 °C as an abrupt magnetic transformation causes permanent decalibration. The magnetic properties also prevent use in some applications. Type J's have a sensitivity of ~ 53µV/°C.

Type T --- Copper / Constantan
Good for temperature in the ~ -200 to 350 °C range. Non-magnetic, type T thermocouples are a popular choice for strong magnetic fields and subzero environments. Type T thermocouples have a sensitivity of ~ 43 µV/°C.

Type E --- Chromel / Constantan ( Copper-Nickel Alloy )
Type E has a high output ~ 68µV/°C which makes it well suited to cryogenic use. It is also non-magnetic.

Type N --- Nicrosil (Nickel-Chromium - Silicon Alloy) / Nisil (Nicke l- Silicon Alloy)
High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B, R, S) types. They can withstand temperatures above ~1200 °C. Sensitivity ~ 38 µV/°C at 900°C.

Thermocouple types R, S and B are all noble metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity ~ 10 µV/°C they are usually used for high temperature measurement (>300 °C).

Type B --- Platinum 30% Rhodium / Platinum 6% Rhodium
Good for high temperature measurements up to ~ 1800 °C. Type B thermocouples (due to the shape of their temperature to voltage curve) give the same output at 0 °C and 42 °C. This makes them useless below 50°C.

Type C --- Tungsten 5% Rhenium / Tungsten 26% Rhenium
Good for temperature ~ 35 to 4200°F. This thermocouple is well suited for vacuum furnaces at extremely high temperatures and must never be used in the presence of oxygen at temperatures above 500°F.

Type R --- Platinum 13% Rhodium / Platinum
Good for high temperature up to ~1600 °C. Low sensitivity ~10 µV/°C and high cost makes them unsuitable for general purpose use.

Type S --- Platinum 10% Rhodium / Platinum
Good for high temperature measurements up to 1600 °C. Low sensitivity ~10 µV/°C and high cost makes them unsuitable for general purpose use. Due to its high stability, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

Type M --- Nickel Alloy 19 / Nickel-Molybdenum Alloy 20
This type is used in the vacuum furnaces as well for the same reasons as with type C above. Upper temperature is limited to 2490°F.

E1129/E1129M-98 Standard Specification for Thermocouple Connectors
E 1159-98 Standard Specification for Thermocouple Materials, Platinum-Rhodium Alloys and Platinum
E 1350-97(2001) Standard Test Methods for Testing Sheathed Thermocouples Prior to, During and After Installation
E 1652-00 Standard Specification for Magnesium Oxide and Aluminum Oxide Powder and Crushable Insulators Used in the Manufacture of Metal-Sheathed Platinum Resistance Thermometers, Base Metal Thermocouples, and Noble Metal Thermocouples
E 1684-00 Standard Specification for Miniature Thermocouple Connectors
E 1751-00 Standard Guide for Temperature Electromotive Force (emf) Tables for Non-Letter Designated Thermocouple Combinations
E 2181/E 2181M-01 Standard Specification for Compacted Mineral-Insulated, Metal-Sheathed, Noble Metal Thermocouples and Thermocouple Cable.

Thermocouple Cable
E 601-81(1997)..Test Method for Comparing EMF Stability of Single-Element Base-Metal Thermocouples Materials in Air
E 608/E 608M-00. Standard Specification for Mineral-Insulated, Metal-Sheathed Base-Metal Thermocouples
E 696-00 Standard Specification for Tungsten-Rhenium Alloy Thermocouple Wire
E 710-86(1997) Standard Test Method for Comparing EMF Stabilities of Base-Metal Thermo elements in Air Using Dual, Simultaneous, Thermal-EMF Indicators
E 780-92(1998) Standard Test Method for Measuring the Insulation Resistance of Sheathed Thermocouple Material at Room Temperature
E 839-96 Standard Test Method for Sheathed Thermocouples and Sheathed Thermocouple Material
E 988-96(2002) Standard Temperature-Electromotive Force (EMF) Tables for Tungsten-Rheniu

ASTM Standards Related to Thermocouples
E 207-00...Method of Thermal EMF Test of Single Thermo element Materials by Comparison with a Secondary Standard of Similar EMF-Temperature Properties
E 220-02 Standard Test Method for Calibration of Thermocouples By Comparison Techniques
E 230-98e1..Temperature Electromotive Force (EMF) Tables for Standardized Thermocouples
E 235-88(1996)e1..Specification for Thermocouples, Sheathed, Type K, for Nuclear or Other High-Reliability

E 452-02..Test Method for Calibration of Refractory Metal Thermocouples Using a Radiation Thermometer
E 574-00..Specification for Duplex, Base-Metal Thermocouple Wire with Glass Fiber or Silica Fiber Insulation
E 585/E 585M-01a ..Standard Specification for Compacted Mineral-Insulated, Metal-Sheathed, Base Metal


Thermocouple junction


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