Just my cup of tea

Using two thermometers to estimate the temperature of my cup of tea. Which of them is right?

Using two thermometers to estimate the temperature of my cup of tea. Which of them is right?

I was in my office this weekend trying to finish off some work for the European Space Agency. So naturally I made a cup of tea to help me concentrate. But before I could drink it, I obviously needed to measure its temperature. So I reached for the two thermometers I happened to have lying around in the office.

One was a Fluke 62Max+, a thermometer that works my detecting the infrared light emitted by all objects. In the picture you can see two red dots on the surface of my tea. Most of the infrared radiation the device measures comes from a circle on which those dots would be on opposite sides.

The other was a thermocouple thermometer, a device which works by measuring the small voltage produced between the ends of two pieces of metal wire joined together in the middle. If the midpoint of the wire is heated – the junction – then a tiny voltage appears across the ends of the wire. It is only 40 microvolts for every degree temperature difference – or  40 millivolts for a 1000 °C temperature difference.

As you can see the infra red thermometer read 57.5 °C and the thermocouple thermometer read 59.8 °C, a difference of 2.3 °C. The question then arises of which thermometer I should believe?

Now I know you don’t care, and neither did I. But in a small way this experiment summarised exactly the kind of question I face every day. There is a temperature that someone wants to know, and uncertainty about the answer is bugging them, or costing them money. And a difference this large could easily be significant.

Answering such questions requires a fair amount of experience, an understanding of typical errors encountered with each type of thermometer, and an appreciation of the physical processes that can affect the answer. A neurotically-anxious personality coupled with some basic physical and mathematical training helps too.

In this case, there is the cooling of the liquid by the metal thermocouple probe, the infrared emission properties of hot water, and the temperature gradients within the water. And then there is the calibration of each device against national measurement standards.

And as I reflected on all the levels of complexity required to get the ‘right answer’, I had a moment of personal insight. For all my anxiety about my work, and for all my being behind on almost every project I am working on, I am well-suited to my work. In fact, it is just my cup of tea.

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