Understanding the Importance of Thermocouples and How They Work

A thermocouple is a sensor used for measuring temperature. It comprises of two dissimilar metal wires, which are connected at one end and joined to a thermocouple-thermometer or another thermocouple-capable device at the other end. When thermocouples are properly configured, they can provide temperature measurements over a wide temperature range.

Why Use a Thermocouple instead of an RTD or Thermistor Sensor?

Thermocouples are temperature sensors that are known for their durability, versatility and cost- effectiveness. Therefore they are generally used in a wide variety of applications, ranging from heavy industrial and process industry usage to environmental and laboratory testing. Due to their wide range of permutations and technical specifications, it is necessary to understand their basic structure, the way they work and their limitations to determine the right kind of sensor for the desired application.

Choose the Right Thermocouple

Beaded Wire Thermocouple

The most basic type of thermocouple is the beaded wire thermocouple. It comprises of two thermocouple wires that are joined together with a welded bead. Since the bead of the thermocouple is exposed, there are several application limitations. The beaded wire thermometer should never be used with liquids that tend to oxidize or corrode the thermocouple alloy. Metal surfaces are also problematic. Usually, metal surfaces, particularly pipes are used to earth electrical systems. Thermocouple measurement can be affected by the indirect connection to an electrical system. Generally, beaded wire thermocouples are preferred for the measurement of gas temperatures. They also enable fast response times as they can be made extremely small.

Thermocouple Probe

A thermocouple probe comprises of a thermocouple wire integrated inside a ceramic or metal tube. The tube wall is called the sheath of the probe. Stainless steel and Inconel® are the common sheath materials. While Inconel supports higher temperature ranges than stainless steel, the latter is often preferred due to its wide chemical compatibility. Exotic metal or ceramic sheath materials are also available for extremely high temperatures. Customers can view OMEGA’s range of high temperature exotic thermocouple probes. The tip of the thermocouple probe comes in three different styles: insulated, grounded and exposed. With a grounded tip, the thermocouple comes in contact with the sheath wall. A grounded junction ensures a fast response time; however, it is highly susceptible to electrical earth loops. In insulated junctions, a layer of insulation separates the thermocouple from the sheath wall. The thermocouple’s tip protrudes beyond the sheath wall with an exposed junction. These exposed junction thermocouples are considered the fastest responding and the best suited for non-corrosive gas and dry air temperature measurements.

Surface Probe

For most types of temperature sensors it is very difficult to measure the temperature of a solid surface. In order to ensure accurate readings, the sensor’s entire measurement area has to be in good thermal contact with the surface. However, this is difficult to achieve when working with a rigid surface and a rigid sensor. To overcome this, the junction can be formed thin and flat to provide maximum contact with a rigid solid surface, as thermocouples are made out of pliable metals. Such thermocouples provide an excellent option for surface measurement. The thermocouples can also be built into a rotating mechanism, making it suited for measuring the temperature of a moving surface.

Wireless Thermocouples

Wi-Fi, Bluetooth and Zigbee wireless transmitters that connect to tablets, smartphones and laptops are used to log and monitor temperature measurements remotely.

How Does a Thermocouple Work?

When two wires consisting of dissimilar metals are connected at both ends and one of the two ends is heated, a continuous current flows in the thermoelectric circuit. When this circuit is broken at the center, the net open circuit voltage (the Seebeck voltage) becomes a function of the junction temperature and the composition of the two metals, i.e., when the junction of the two metals is cooled or heated, a voltage is generated that can be correlated back to the temperature.

Different Combinations of Metals

Thermocouples come in different combinations of metals, called calibrations or types. “Base Metal” thermocouples, known as Types K. J, E, N and T, are the most common types. In addition, there are high temperature calibrations, also known as Noble Metal Thermocouples –Types R, S, D, B and C.

Calibration	Tem Range	Std. Limits of Error	Spec. Limits of Error
	0 °C to 750 °C (32 °F to 13382 °F)	Greater of 2.2 °C or 0.75%	Greater of 1.1 °C or 0.4%
	-200 °C to 1250 °C (-328° F to 2282 °F)	Greater of 2.2 °C or 0.75%	Greater of 1.1 °C or 0.4%
	-200 °C to 900 °C (-328 ° F to 1652 °F)	Greater of 1.7 °C or 0.5%	Greater of 1.0 °C or 0.4%
	-250 °C to 350 °C (-328 °F to 662 °F)	Greater of 1.0 °C or 0.75%	Greater of 0.5 °C or 0.4%

Common Thermocouple Temperature ranges

Each type possesses a specific tolerance, temperature range and environmental considerations. While the thermocouple type decides the temperature range, the diameter of the thermocouple wire may also limit the maximum range, i.e. it may not be possible for a thin thermocouple to reach its full temperature range. K-type thermocouples are also called general purpose thermocouples because of their wide temperature measurement range and low costs.

How Do I Choose a Thermocouple?

1. Determine the application for which the thermocouple will be used.
2. Note the maximum and minimum temperatures the thermocouple will be exposed to.
3. Factor in any chemical resistance required for the thermocouple or sheath material.
4. Evaluate the need of abrasion and vibration resistance.
5. List any installation requirements.

How Do I Know Which Junction Type to Choose?

Sheathed thermocouple probes are available with one of three junction types: insulated, grounded or exposed. The thermocouple wires are physically connected to the inside of the probe wall, at the tip of the grounded junction probe. This leads to improved heat transfer to the thermocouple junction from the outside, through the probe wall. The thermocouple junction in an insulated probe is separated from the probe wall. Response time is slower than the grounded style, but electrical isolation is provided by the insulation.

In the exposed junction style, the thermocouple protrudes out of the sheath tip and is exposed to the surrounding environment. Although this type provides the fastest response time, it is limited in use to non-pressurized and non-corrosive applications. The above illustration can be seen for a complete discussion on junction types. The sheath insulation is sealed where the junction extends so as to prevent the penetration of gas or moisture, which could potentially lead to errors.

It is advised to use the grounded junction for the measurement of flowing or static corrosive liquid and gas temperatures, as well as for high pressure applications. The grounded junction is recommended. Moreover, the junction of a grounded thermocouple is joined to the protective sheath, enabling faster response times than that of the ungrounded junction type.

For measurements in corrosive environments, an insulated junction can be used. In such environments, it is preferred to have the thermocouple electrically isolated from and screened by the sheath. MgO powder physically insulates the welded wire thermocouple from the thermocouple sheath.

All OMEGA thermocouple wires, probes and connectors are available with either ANSI or IEC Color Codes. On the company’s website, model numbers reflect the IEC Color-Coded Product. Customers can contact sales or visit Omega’s website for instructions on how to order ANSI Color-Coded products.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.