Thermocouple Manufacturers

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About Thermocouples

A thermocouple is a type of temperature measuring device. It is made of two dissimilar metals that are joined at junction points. A potential difference is formed at the contact junction and is calibrated to accurately measure temperature. They are used to measure temperature ranges from -270°C to more than 2000°C. They give accuracy up to 1°C.

Thermocouples provide a very economical means of measuring temperature. They do not need any power to operate. Thermocouples can function independent of batteries or stand by devices. They can continuously relay information about the temperature without needing much of maintenance.

Thermocouple Data Loggers are small hand held devices. They record low-level millivolt thermocouple signals, process them, take the reference of the built in cold junction temperature sensor and give accurate readouts. The data can be later transferred to other data storage devices. Multiple thermocouple probes can be connected to the data loggers.

How Thermocouples Work

Thermocouples work on the principle of Seebeck effect. Two electrical conductor wires are joined together that show a voltage difference depending on the ambient temperature. One of the junction points in a thermocouple forms the hot junction, while the other is a cold junction, which acts as a reference point.

When the junction for measurement is exposed to heat, a voltage is produced. This voltage is measured against a standard reference. Thus the exact temperature at the junction can be computed. The standard reference can be either a calibration source of known temperature or it can be a chart. The chart denotes a voltage (mV) against a particular temperature rating (degrees centigrade). Devices like voltmeter, analog to digital converted or other devices can be used to give a representation of the voltage into temperature.

Types of Thermocouples

Thermocouples are classified as per the metals used, the type of junction, covering sheath used and the temperature to be measured. The wire diameter also plays an important role in the range of temperature measurement. Wires with larger diameter can be used for higher temperatures. ASTM Standard E230 gives Standards for different thermocouple types.

Thermocouples are color coded for easy identification in colors like red, blue, purple and so on.

Types of Thermocouples are:

• Beaded Wire Thermocouple: It is made of two pieces of thermocouple wires. A welded bead joins these pieces. They are used for gas temperature measurement.
• Thermocouple Probe (Tube and Wire type): This is made of thermocouple wire placed in a metal tube called the sheath probe. Sheath is made of metals like Inconel and stainless steel, fiberglass and mineral insulators. Tube and Wire type are also called as soft wire and bayonet thermocouples. They are available in different styles like PP Style Tube and Wire, BT Style Beaded Base Metal, ST Style Brass Shim, PL Style Fixed Bayonet, WT Style Flexible Wire, NT Style Nozzle, MB Style Melt Bolt and so on. Melting point of the sheath also varies from 1405°C for steel to 1260°C for Hastelloy.
  • Metal Probes: Inconel is used for higher temperature ranges while steel is used for its corrosion resistance. Other material like Hastealloy is also used.
  • Mineral Coated: Mineral coated thermocouple is made of a sheath of fiberglass or Pyrex. They are used for continuous operations. They are available in different styles like with protective cap.

Different types of probe tips for thermocouples are also available:

• Grounded: In this type, thermocouple is in physical contact with the wall of the sheath. It has a higher response time but errors can occur due to electrical ground loops.
• Ungrounded: In this type, a layer of insulation is paced between the thermocouple and the sheath.
• Exposed: The thermocouple has an exposed junction and this extends beyond the Sheath. This type is suited for air temperature measurement.
• Surface Probe: The have a flat and thin junction and are used for surface temperature measurement.

Thermocouple Metal Wires

Thermocouple wires are made of a number of different metal alloys. They are given specifications for different metal pairs and these are specified using letters B, E, J, K, T, R, S. Each type can be used for a specific temperature range and in different media.

• Type B: Made of 70% Platinum/ 30% Rhodium and 94% Platinum/ 6% Rhodium, it can be used in Inert and Oxidizing media at a temperature of -50 to 1750°C. This is also called a Noble metal thermocouple.
• Type E: This type is made of a combination of Chromel (Ni-Cr) and Constantan (Cu-Ni) alloys. They have a temperature range of -270 to 1000°C and can be used in inert and oxidizing media. This type gives the largest output in millivolt per degree of temperature change.
• Type J: It is made of Iron & Constantan (Fe & Cu-Ni) alloys. They can be used in different types of media like inert, oxidizing, reducing and vacuum. The temperature range for this type is -270 to 1200°C. The presence of iron can cause rusting and oxidation problems.
• Type K: Made of Chromel (Ni-Cr) and Alumel (Ni-Al) alloys, they are suitable for inert and oxidizing media in the temperature range of -270 to 1350°C.
• Type N: Made of Nicrosil (Ni-Cr-Si) and Nisil (Ni-Si) alloys, it is becoming more popular and replacing type K thermocouples. Since they have a low sensitivity, they are generally only used for high temperatures greater than 300°C. It is suitable up to a temperature of 1200°C in both inert and oxidizing media.
• Type R: Made of Platinum and 87% Platinum/13% Rhodium alloys. It can be used in inert and oxidizing media at a temperature range of -50 to 1750°C. This is also called a Noble metal thermocouple.
• Type S: Made of Platinum and 90% Platinum/10% Rhodium alloy, it is used in a temperature range of -50 to 1750°C in both inert and oxidizing media.
• Type T: Made from alloys of Copper & Constantan (Cu- Ni), it has a temperature range of -270 to 400°C. It is suitable for use in inert, oxidizing, reducing and vacuum media.

Other types are RS/ SX, N, CX K and N, etc. Thermocouples are also classified informally as: Base metal type (Type E, J, K, N and T), rare metal and precious metal (Type B, S, and R), rare earth type made of refractory materials and the exotic class used for calibration and in experimental appliances (Type W).

Using Thermocouples

The following points should be well remembered when using thermocouples or when handling thermocouple applications:

• Thermocouples are electrical conductors and a high voltage is preset in them. They should be handled with the attention given to handling live electrical wires.
• With prolonged exposure to heat and temperature, they may give errors in reading the temperature. In critical operations, Thermocouples must be calibrated regularly.
• Errors can happen when the thermocouple junction leads that connect to voltmeter make two extra thermoelectric junctions that subtract voltage from the signal.
• While selecting a thermocouple, linearity of the voltage drop should be a function of temperature and the thermocouple should have a large voltage to temperature ratio.
• Thermocouple wires should not be twisted to form a junction as they give inaccuracies but they should be welded or soldered using a good flux.
• Wire Sizes: Diameter of Thermocouple wires is specified in Gage Number or AWG and SWG sizes. Gage numbers vary from 0 (8.25 millimeters) to 45 (0.046 millimeters). Lower the Gage Number, larger is the diameter and higher the temperature range of the thermocouple. However, smaller diameter wires have a faster response time.

Applications of Thermocouples

Thermocouples are widely used in a number of industries and applications:

• Heat treatment plants, cryogenic plants, kilns, steel and metal melting industries are the primary consumers of thermocouples.
• Thermocouples are used in the dairy industry to heat milk to a sufficient temperature.
• They are used in food preparation and the bakery industry to heat ingredients to the correct temperature.
• They are also used in automobiles, aircrafts, space vehicles and marine and submarine vessels.
• Thermocouples are also used in air conditioners and environment control devices to correctly measure ambient temperatures.
• They are also used in gas and vapor manufacturing industries to monitor gases like Hydrogen, Nitrogen, Methane and others.
• Other areas are Plastic Injection Molding Machines, Plating Baths, Industrial Processing equipment, Medical and Packaging Equipment, Liquid Temperature Measurement, DEW point temperature and humidity of gases.

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