In this video, one of our engineers, Kyle Otte, explains the different types of junctions available on thermocouples:
grounded
ungrounded
exposed
He also describes each of their uses and purposes for thermocouples. For more information on the different types of thermocouple junctions, view this blog post.
Last year, we visited a customer to make recommendations on testing thermocouples mounted on a probe that was inserted into a mold. They wanted to test the thermocouple by measuring the resistance of the probe, so the machine did not have to be disassembled to check the thermocouple.
In this particular example, heater wires are stainless steel overbraid. Thermocouple wires are inside the hose. Probes are wrapped in aluminum foil before being inserted into a mold that is the size of a small car. Sometimes, the thermocouple is damaged while being inserted into the mold. The customer’s thought was to measure the resistance of the thermocouple after insertion as an indication of whether the thermocouple was good.
There are several problems with this approach:
The first issue is that you are mostly just measuring the resistance of the wire. The thermocouple wire is metal, but it is not copper. So, the wire itself will have a relatively low ohms (maybe 5Ω to 25Ω or so, but not 1Ω to 2Ω like copper wire would be.)
The way an ohm meter measures a resistance is to place a voltage across the resistance and measure the current to calculate the resistance.
A typical circuit looks like this:
But, what we actually have in this example is this:
A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature. Thus, the circuit shown above actually has two voltage sources. This second voltage will cause the ohm meter current to change, and the perceived resistance changes. However, the resistance of the thermocouple junction did not actually change. But, because the thermocouple voltage changed, the voltage across the resistance changed, producing a change in the resistance reading. Thus, the resistance reading may increase 20% or so.
When using a handheld multimeter, the accuracy is specified something like 15% ± x ohms. For a low resistance reading, the error can be substantial. These meters are designed for troubleshooting, not measuring precise low resistance measurements.
What we recommend:
Rather than a single dual thermocouple, use two single underbolt thermocouples placed 180° from each other at the tip. One thermocouple would be the control. To check the thermocouples, connect the second thermocouple to a handheld thermocouple reader. Then, two thermocouples should read essentially the same. It would be unlikely that the hose would be damaged on each one creating identical secondary junctions.
Written by Jim Dixon and Shelby Reece Edited by Kyle Otte Date Published: 07.15.2019 Last Updated: 09.03.2019
There are three different types of junctions for thermocouples: exposed, grounded, and ungrounded. Each of these serves a different purpose and has different characteristics. Let’s look at each type of junction and where they should be utilized.
What Are Thermocouple Junctions and Why Are They Important?
Briefly, a thermocouple works utilizing the Seebeck Effect. The Seebeck Effect is the conversion of thermal energy into a voltage potential at the junction of different types of metals. If a continuous circuit is made using dissimilar metals joined at two junction points, and the two junction points are at different temperatures a small voltage will flow through the wire. By measuring this small voltage (usually tens of millionths of a volt), we can determine the temperature at the place where the two dissimilar metals are joined (the junction). The thermocouple junction is, therefore, extremely important because this is the point where we are actually measuring temperature.
Exposed Thermocouple Junctions
As depicted in the picture to the left, exposed junctions consist of two bare wires that are joined outside of a protective metal probe. Exposed junctions are used to measure the temperature of a gas. Because there is no protective cover, the junction is very fragile. No moisture, or liquid or solid contaminants should be present in the gas. Exposed junctions, however, do provide a faster response time because thermal energy does not have to travel through a metal sheath and/or compacted insulation to impart energy to the junction.
Grounded Thermocouple Junctions
Grounded junctions are the most common type of junction. In this application, the junction is connected directly to the probe tip usually via soldering or welding. This provides a relatively fast response time as heat only has to travel through a metal sheath or probe and weld, all of which have a high thermal conductivity. This also provides durable protection for the junction inside the probe tip.
Ungrounded Thermocouple Junctions
Ungrounded junctions are used to protect sensitive electronics from small feedback voltages. Ungrounded junctions are similar to grounded junctions in that they are inside of a protective probe. However, they are not welded or soldered to the probe tip; instead, they are surrounded by an insulator, usually magnesium oxide. Ungrounded junctions provide a slower response time because heat must travel through the probe tip and then through the magnesium oxide (which has a lower thermal conductivity than a metal) before reaching the junction.
So, Why Have an Ungrounded Junction-type Thermocouple?
One reason for an ungrounded junction-type thermocouple is if there is a small voltage difference between grounding points. For example, an electronically-sensitive control panel is grounded at 0V. This instrumentation is connected to a temperature sensor that is on a machine which has a small voltage leak through the ground circuit and thus is grounded at 1V. There exists the possibility that a small current could travel through the sensor back to the control panel and have an unwanted effect on the electronic equipment.
Main Points
Each junction is useful for different types of applications.
Exposed junctions have no protective cover and are very fragile.
Grounded junctions are the most common junction type and have a fast response time due to the highly thermal conductive material.
Ungrounded junctions are surrounded by an insulator and provide a slower response time, but are useful for protecting sensitive electronics from small feedback voltages.
Written by Kyle Otte Edited by Shelby Reece Date Published: 02.01.2019 Last Updated: 09.04.2019
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