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.

Replaceable, Flanged, Multi-Thermocouple Vacuum Feedthroughs

(Click for larger view)

One of the biggest challenges facing the vacuum industry is to collect multiple readings from complex assemblies while being limited by the number of existing feedthrough ports in a vacuum chamber. With BCE’s Multi-TC, Flanged Feedthrough, companies no longer need to invest in adding more ports for a greater collection of temperatures from a vacuum environment as multiple thermocouples are embedded in just one feedthrough. The number, type and lengths of the thermocouples can be altered to match specific application requirements meeting each customers’ unique needs. As the thermocouples can be adapted to meet unique design constraints, so can this feedthrough’s flange. All sizes used are standard and are easily mountable with readily available hardware. No complicated assemblies are required for the most complicated of vacuum setups.

With most vacuum feedthroughs, troubleshooting can be difficult as well when multiple fixed elements are involved. If one element fails, generally the entire vacuum feedthrough is rendered useless. With BCE’s Multi-TC, Flanged Feedthrough, arduous troubleshooting procedures are a thing of the past. In fact, this feedthrough’s thermocouples are replaceable as they can be extracted from the compression fittings with great ease. This means that if thermocouples fail, they can be removed, tested and replaced within minutes. Thus, this no-hassle design allows for faster, more accurate and more cost-effective thermal data collection. Trust BCE’s Multi-TC, Flanged Feedthrough for your most complicated thermal detection needs.

Specifications

• Replaceable thermocouple(s) design 
• Vacuum Integrity: 10-8 atm.cc/sec 
• Multiple types and lengths available for thermocouple(s) 
• Multiple types and sizes available for flange(s) 
• Multiple types and sizes available for lead wires (KAPTON available for minimal contaminant release) 
• Compatible O-rings available for flange(s) 
• Can be adapted to virtually any temperature range

For more information, visit http://www.belilove.com/feedthrough.

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

Thermocouple Basics - Wire Type, Connectors, Construction

Industrial thermocouples, as the temperature sensor in a thermal system, are explained in the video below.

Seebeck Effect
(image courtesy of Wikipedia)

Thermocouple "Types", based upon standardized color designations is discussed, as well as thermocouple connectors, polarity and some aspects of construction (such as grounded vs. ungrounded vs. open tip).

Thermocouples are a fairly accurate, economic temperature sensor used in many industrial applications. They operate on the "Seebeck Effect" which is the phenomena of dissimilar metal conductors producing a measurable voltage difference between two substances.

Thermocouples are used widely in industrial thermal system design in industries such as power generation, primary metals, pulp and paper, petro-chemical, and OEM equipment.

For more information contact:
Belilove Company Engineers
21060 Corsair Blvd
Hayward, CA 94545
Phone: (510) 274-1990
Fax: (510) 274-1999
E-mail: sales@belilove.com

Bi-Metal Thermostats

Bi-metallic Thermostats

Bi-metallic (bi-met) temperature controls (thermostats) have been around for a very long time, but their simplicity, dependability, size and cost still make them a good choice for certain applications.

Also known as "thermoswitches", bi-metal thermostats come in  two primary styles - a "disk" type, which looks more like a button, and a "cartridge" style. Both operate on the same basic principle of differential expansion. Disk type devices are used in many household appliances, such as clothes dryers or coffee pots, as temperature control or as hi-limits. Cartridge style thermostats are used in more industrial applications and OEM equipment.

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.