Have you ever noticed an unexpected reading from your thermocouple device? You then unplug your thermocouple and plug it back in and say hey, that’s better. What is happening here? What strange witchcraft is causing your thermocouple that was reading incorrectly, to indicate correctly suddenly?
Thermal Electromotive Force (Thermal EMF)
Thermal EMF’s are a common problem with low resistance and low voltage measurements. Issues with the reactions between dissimilar metals can cause changes in temperature to have unpredictable results when making these measurements. You’ve heard that dissimilar metals thing before, that’s the whole premise that a thermocouple works on. Two wires, different alloys, junction…Seebeck effect. Yep, it’s all coming back to me now. It turns out it’s a massive issue in making low voltage measurements with extreme precision. Typical lead-tin solder can introduce an error of 5µV/°C of temperature change. Here’s the real kicker in this research though. Connecting copper to copper-oxide can add an error greater than 1000µV/°C.
I Thought You Said this was About Thermocouples
I’m getting there. How would we end up with a connection between copper and copper-oxide? Corroded terminals. And if dirty connections can introduce an additional uncertainty of 1 mV, that is a huge deal when we are thinking about thermocouples. For a type R or S thermocouple that represents 5% of the entire millivolt range for that thermocouple, for just 1°C of temperature difference. Even for type K thermocouples that can introduce an error of 3°C per °C of temperature differential.
Have You Looked at Your Thermocouple Connectors Lately?
Whenever I look at the connectors with thermocouple calibrators or thermocouple thermometers that people hand me I’m often surprised they read anything close to what they should. Finding oxidation on connectors for types J, R, S, and T is ubiquitous. These connection types contain copper or iron and plugging and unplugging them creates surface scratches that make natural places for oxidation to form.
So Why Does Unplugging/Plugging Seem to Fix the Issue?
When we unplug and plug the thermocouple, it tends to scrape the oxidation off the connector. When I perform field calibrations with thermocouples, I typically carry a wire brush with me, and I look at my connections and give them a good scrape down if I see anything unpredictable. I’ve also been known to hit them with the wire wheel on the benchtop grinder when I’m in the shop from time to time.
The good thing is that given that removing the oxidation gets us to the correct reading, we can have some level of confidence that the oxidation was the cause of the error in the first place.