Do You Have a Thermocouple or an RTD? They are similar, but what are the exact differences between them? In this video, Thermal Corporation engineer, Kyle Otte, explains the differences between thermocouples and RTDs.
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
These tips will apply to both thermocouples and RTDs
#1 Avoid Tip Deformation
The actual sensing part of a temperature sensor is very close to the tip of the probe, so keeping the tip protected is extremely important. This is especially important when it comes to Resistance Temperature Detectors (RTDs). RTDs consist of a very small platinum resistor that is very fragile. Any hammering or mechanical force on the tip of an RTD can ruin the sensor. Thermocouple junctions can also be broken or become unreliable if they are deformed or disturbed. (Learn more on thermocouple junctions in our video on Youtube about the Different Types of Thermocouple Junctions.)
Make sure that the probe fits securely into the bore that it is meant to go in. Temperature sensors should not have to be forced into place. If the sensors are not easy to install there is most likely some contamination (plastic overrun) in the bore hole that needs to be removed.
Be careful with the temperature sensors when changing dies or servicing your machine. We see a lot of bent temperature sensors that no longer work correctly after being removed and reinstalled in a different die, because they were treated roughly during the transition.
#2 Avoid Contamination of Leads
While temperature sensors are not as prone to contamination as our other heating products, such as mica band heaters and cartridge heaters, it can still happen. Often what we see is plastic that has been melted and somehow made its way onto the temperature sensor leads. Depending on the type of lead wire insulation, and the type of plastic, the plastic can create a “short” between the leads. This is really a secondary junction. This causes problems in thermocouples and RTDs. It is very important to not allow the leads to be contaminated.
#3 Avoid Over Temperature
Published literature states that a J-type thermocouple is accurate up to 1382°F. However, keep in mind that this is the maximum temperature rating for the thermocouple junction. The insulation surrounding the thermocouple wire will not be rated to this high of a temperature. Often times what we see is that the fiberglass insulation around the thermocouple wire has got hot and burned through. This leads to a bare thermocouple wire that makes contact with the metal of a machine. This creates a secondary junction which then gives a bogus temperature reading and causes the machine to have problems.
This same phenomenon can happen when using an RTD. In that case, the resistance that the controller sees changes drastically, thus giving a false temperature reading.
If possible, run the thermocouple wire leads in open air where the temperature is much lower than the process temperature.
#4 Take Care with Lead Routing
A high percentage of the temperature sensors that we see fail have done so because the leads have been damaged in some way. Machines have sliding or moving parts, and can easily pinch or nick leads if care is not taken when routing the leads.
A stainless steel hose will help with deflecting mechanical force, but can still be crushed.
A stainless steel over braid can help with mechanical wear, or rubbing, but again it is best to avoid this scenario, if possible.
Teflon leads will be moisture resistant, but it is always better if temperature sensors can be kept dry. Thermocouple wire will rust if exposed to a high-moisture environment and, over time, will fail.
Written by Kyle Otte Edited by Shelby Reece Date Published: 02.20.2020 Last Updated: 02.20.2020
Learn ways to extend the life of your thermocouples and resistance temperature detectors (RTDs). Taking these tips, presented by Kyle Otte of the Thermal Corporation Engineering team, into consideration can lengthen the lifespan of a thermocouple and RTD.
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