Dissimilar Metal Junctions (How Thermocouples Work)

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.

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.

Hotfoil-EHS Heat Treating Power Consoles

Precise control over your pre-weld and post-weld heat treatment parameters are critical. Accurate temperature control, specific soak times, uniformity, and controlled heat up and cool down times are required to ensure strong welds. Hotfoil-EHS power consoles are designed to provide the best control, easiest user interface, and longest lasting operation, even in the toughest environments. Using only the highest quality components, Hotfoil-EHS power consoles are field-tested and application proven. Control systems can be specified with or without recorders or ramping controllers, and are standardly available in 6, 9, 12, 18, and 24 zone configurations.


The ICE Advanced Heat Treatment Control System

The ICE IS System, is developed for precise, reliable and efficient heat treatment control. It consists of IS controllers and ISPort software. With ISPort software you can define process parameters as temperatures, rates, tolerances etc; operate and control one or many processes from one or several controllers; edit PID values; fill in needed information, ex. customer info, work info etc.; print all work documents and heat treatment certificates. For more information contact Hotfoil-EHS,


Common Temperature Sensors Used in Industry

Temperature SensorsTHERMOCOUPLE
Due to their simplicity, reliability, and relatively low cost, thermocouples are widely used. They are self-powered, eliminating the need for a separate power supply to the sensor. Thermocouples are fairly durable when they are appropriately chosen for a given application. Thermocouples also can be used in high-temperature applications.

Thermocouple Advantages:
  • Self-powered
  • Simple
  • Rugged
  • Inexpensive
  • Many applications
  • Wide temperature range
  • Fast response
Thermocouple Disadvantages:
  • Nonlinear output signal
  • Low voltage
  • Reference required
  • Accuracy is function of two separate measurements
  • Least sensitive
  • Sensor cannot be recalibrated
  • Least stable
Resistance temperature detectors are attractive alternatives to thermocouples when high accuracy, stability, and linearity (i.e., how closely the calibration curve resembles a straight line) of output are desired. The superior linearity of relative resistance response to temperature allows simpler signal processing devices to be used with RTD’s than with thermocouples. Resistance Temperature Detector’s can withstand temperatures up to approximately 800 C (~1500 F).

RTD Advantages:
  • More stable at moderate temperatures
  • High levels of accuracy
  • Relatively linear output signal
RTD Disadvantages:
  • Expensive
  • Self-heating
  • Lower temperature range
Thermistors work similarly to RTD’s in that they are a resistance measuring device, but instead of using pure metal, thermistors use a very inexpensive polymer or ceramic material as the element.

Thermistor Advantages:
  • High output
  • Fast
  • Two-wire ohms measurement
Thermistor Disadvantages:
  • Nonlinear
  • Limited temperature range
  • Fragile
  • Current source required
  • Self-heating

Industrial Heat Treatment Furnaces, Big and Small

Here at Hotfoil-EHS, we are capable of building furnaces of many sizes, big and small. From small, low-throughput furnaces, to much larger high yield furnaces, to rail-driven furnaces designed to move back and forth over the materials being heated, Hotfoil-EHS Design Engineers and Fabrication Shop has done it all. Gas or electric, Hotfoil-EHS has the experience to build a custom furnace to your exact specification.  If you can think it, we can build it!


Indirect Resistance Heating

weld preheat heater
Indirect resistance heating example:
Weld preheat ceramic mat heater.
With indirect resistance heating, a heating element transfers heat to the material by radiation, convection, or conduction. The element is made of a high- resistance material such as graphite, silicon carbide, or nickel chrome. Heating is usually done in a furnace, with a lining and interior that varies depending on the target material. Typical furnace linings are ceramic, brick, and fiber batting, while furnace interiors can be air, inert gas, or a vacuum.

Indirect resistance heating can also be done with an encased heater, in which the resistive element is encased in an insulator. Called metal sheath heaters this type of heater can be placed directly in liquid to be heated or close to a solid that requires heating. Numerous other types of resistance heating equipment are used throughout industry, including strip heaters, cartridge heaters, and tubular heaters.

Clamp-on pre-weld electric heater
Indirect resistance heating example: 
Clamp-on pre-weld electric heater.
Resistance heaters that rely on convection as the primary heat transfer method are primarily used for temperatures below 1,250 ̊F. Those that employ radiation are used for higher temperatures, sometimes in vacuum furnaces.

Indirect resistance furnaces are made in a variety of materials and configurations. Some are small enough to fit on a counter top, and others are as large as a freight car. This method of heating can be used in a wide range of applications. Resistance heating applications are precisely controlled, easily automated, and have low maintenance. Because resistance heating is used for so many different types of applications, there are a wide variety of fuel-based process heating systems, as well as steam-based systems, that perform the same operations. In many cases, resistance heating is chosen because of its simplicity and efficiency.

Electric hopper heater
Indirect resistance heating example: 
Electric hopper heaters.
Indirect resistance heaters are used for a variety of applications, including heating water, sintering ceramics, heat pressing fabrics, brazing and preheating metal for forging, stress relieving, and sintering. This method is also used to heat liquids, including water, paraffin, acids, and caustic solutions. Applications in the food industry are also common, including keeping oils, fats, and other food products at the proper temperature. Heating is
typically done with immersion heaters, circulation heaters, or band heaters. In the glassmaking industry, indirect resistance provides a means of temperature control. Many hybrid applications also exist, including “boosting” in fuel-fired furnaces to increase production capacity.

Resistance heating applications are precisely controlled, easily automated, and have low maintenance. Because resistance heating is used for so many different types of applications, there are a wide variety of fuel-based process heating systems, as well as steam-based systems, that perform the same operations. In many cases, resistance heating is chosen because of its simplicity and efficiency.

Contact Hotfoil-EHS for any industrial resistance heating project. With decades of application experience, Hotfoil-EHS engineers can help you design a system tailored to your exact needs.