The Operating Principle of Thermocouples

When two dissimilar metal wires are joined together at one end, a voltage is produced at the other end that is approximately proportional to temperature. That is to say, the junction of two different metals behaves like a temperature-sensitive battery. This form of electrical temperature sensor is called a thermocouple:

This phenomenon provides us with a simple way to electrically infer temperature: simply measure the voltage produced by the junction, and you can tell the temperature of that junction. And it would be that simple, if it were not for an unavoidable consequence of electric circuits: when we connect any kind of electrical instrument to the thermocouple wires, we inevitably produce another junction of dissimilar metals. The following schematic shows this fact, where the iron-copper junction J1 is necessarily complemented by a second iron-copper junction J2 of opposing polarity:

Junction J1 is a junction of iron and copper – two dissimilar metals – which will generate a voltage related to temperature. Note that junction J2, which is necessary for the simple fact that we must somehow connect our copper-wired voltmeter to the iron wire, is also a dissimilar-metal junction which will also generate a voltage related to temperature. Further note how the polarity of junction J2 stands opposed to the polarity of junction J1 (iron = positive ; copper = negative). A third junction (J3) also exists between wires, but it is of no consequence because it is a junction of two identical metals which does not generate a temperature-dependent voltage at all.

The presence of this second voltage-generating junction (J2) helps explain why the voltmeter registers 0 volts when the entire system is at room temperature: any voltage generated by the iron-copper junctions will be equal in magnitude and opposite in polarity, resulting in a net (series-total) voltage of zero. Only when the two junctions J1 and J2 are at different temperatures will the voltmeter register any voltage at all.

For more information about thermocouples (or any temperature sensor), contact BCE. They can be reached by calling 510-274-1990 or by visiting their web site at https://bcemfg.com.

Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

FAQ: Custom Heater Assemblies

Custom heater assembly that includes a sealed cartridge
heater, thermowell, and thermocouples (multiple).

What is a custom heater assembly?

In general terms, a custom heater assembly is a device that transforms electrical current to heat energy, is designed in a way that accommodates the needs of a unique heating requirement, and is used for the heating of gases, liquids, plastics, or metals.

What is a thermowell?

A thermowell is machined metallic tube used to house and protect sensors or heating elements. Thermowells not only protect heaters and sensors from erosive or corrosive media, they also allow for easy removal and replacement without exposing the process media.

What is a cartridge heater?

A cartridge heater is a cylindrically shaped electric heating element intended to be inserted in holes in metal platens or immersed in flowing media. The design incorporates a ceramic bobbin wound with nichrome resistance wire, carefully centered inside a metallic tube, and then backfilled with magnesium oxide (MgO), which provides electrical insulation. Cartridge heaters can be either swaged (compacted MgO) or un-swaged (loosely filled MgO). Swaging allows for maximum heat transfer to outside surfaces while keeping internal heater temperatures as low as possible, and preserving dielectric qualities.

What is a sealed cartridge heater?

A sealed cartridge heater incorporates all of the characteristics of a standard cartridge heater but is additionally sealed with BCE’s proprietary epoxy seal making it vacuum compatible. The seal additionally elongates heater life, especially in high humidity and moisture environments.

What electrical tests are performed on heaters?

Heaters undergo 3 main electrical tests: Resistance, MegOhm and Hipot.

A resistance test is performed using a fluke meter to ensure that the heater is manufactured within the correct tolerances of the electrical specifications. Heater leads are connected directly to the fluke meter leads to perform the test.

A MegOhm or Insulation Resistance test is performed using a megohmmeter. As its name implies it tests for any breakdown in a heater’s insulating material. The test is performed by supplying low to medium voltage to the insulating material for a small period of time.

A Hipot test is also used to assess the dielectric strength of the heater’s insulating material. In addition to performing a Dielectric Withstand Test like the Megohm, it can also perform a Dielectric Breakdown Test. This means that the heater insulating material is shocked at very high voltages until failure is achieved. This is more commonly performed on samples as it destroys the heater.

What is a thermocouple?

A thermocouple is a thermoelectric device that measures temperature. When the wire junction of its two dissimilar metals changes in temperature, a small voltage is created. The voltage is then used to calculate the temperature using the thermocouple’s reference table.

What is an RTD?

RTD stands for "resistance temperature detector". It is generally constructed of a fine and pure metallic wire wound around a ceramic or glass core. The relationship between wire resistance and temperature is used to sense the temperature of other devices.

Vacuum Thermowell and Heater Assembly Simultaneously Heats and Monitors Semiconductor Wafer's Temperature

BTU requirement and temperature control are critical in the fabrication of silicon wafers in the semiconductor industry. Seldom can a product provide both a thermally effective means to heat wafers and simultaneously monitor their respective temperature. BCE’s Vacuum Thermowell and Heater Assembly achieves both of these criteria. Its aluminum thermowell houses a cartridge heater capable of supplying uniform heat, which can be monitored through an embedded thermocouple.

Furthermore, a flange at the end of the thermowell allows for easy installation into any vacuum port and provides an effective seal via its dovetail O-ring groove. An aluminum construction further ensures that the entire assembly remains lightweight and inexpensive to machine, reducing overall product cost. Moreover, multiple RTDs are hermetically sealed into the thermowell to allow for precise temperature monitoring of wafers and other components while maintaining vacuum integrity. They can be positioned and designed for any application, as can the thermowell. In order to prevent any outgassing and minimize release of contaminants into the chamber, the RTDs are sealed using BCE’s proprietary epoxy meeting NASA’s low outgassing spec and are available with Kapton leads.

Check out the BCE Vacuum Thermowell & Heater Assembly web page for more detail.

Industrial Process Temperature Sensor Comparison

Array of thermoucouple assemblies
Courtesy Durex

It's always useful to have quick references for things. Durex, a globally recognized manufacturer of electric heating solutions, published the Temperature Sensor Element Selection Guide included below in a recent blog posting. It provides a consolidated comparison of the four primary temperature sensing devices used for industrial process control. Many will find it useful, so we share it with you here.

All the technical expertise needed for your process heating challenges is readily accessible at BCE. Share your heating challenges with experts, combining your process and application knowledge with their expertise to develop effective solutions.

Industrial Temperature Sensor Comparison and Selection Guide from Belilove Company-Engineers

Thermocouples, RTD's and Thermistors

This post explains the basic operation of the three most common temperature sensing elements - thermocouples, RTD's and thermistors.

A thermocouple is a temperature sensor that produces a micro-voltage from a phenomena called the Seebeck Effect. In simple terms, when the junction of two different (dissimilar) metals varies in temperature from a second junction (called the reference junction), a voltage is produced. When the reference junction temperature is known and maintained, the voltage produced by the sensing junction can be measured and directly applied to the change in the sensing junctions' temperature.