Why you shouldn’t bother buying a thermometer

By Marcus Wilson 18/06/2012

Well, looks like winter has finally arrived here. There’s not much worse weather-wise than having a clear night with the cloud rolling in just as the sun rises. The clear night lets the temperature drop, as the ground radiates away more energy than it receives from the atmosphere and surrounding objects, and then the clouds stop the sun from warming the place up again.

Last weekend, we were getting odds and ends for the house, and I looked at thermometers for the baby’s room. (And, needless to add, still no baby.)  I was surprised to see that it is still legal to sell mercury thermometers here. They aren’t easy things to deal with at home  if you happen to break one. But, safety aside, I wouldn’t buy a mercury thermometer anyway, since they can be so unreliable. I illustrated this to Karen by pulling off five from their hooks, and looking at the range of temperatures they showed – the lowest was reading 16 degrees while the highest was recording 18.5 degrees C.  I suppose I could have got one that was reading in the middle.

I had a thermometer once that I calibrated by putting it into an ice-water mix, which should sit at 0 degrees if the water is reasonably pure. This particular thermometer read 3 degrees. With a calibration, at least you know where you are with it, but it still bugged me every time I read it.

Also, on a similar note, at one of our Osborne Physics and Engineering lectures a few years ago, a colleague of mine demonstrated this with a couple of mercury thermometers in a bucket of water. He asked the audience, a group of year 12 and year 13 school children, why they were reading different temperatures. There were some amazing answers put forward, involving some whacky thermodynamical thinking as to why one side of the bucket was strangely warmer than the other side. As far as I recall, no-one suggested that the thermometers could simply be lousy.

Which brings me to the point of all this. Next semester I teach my Experimental Physics paper, and one of the most useful things students will learn is never to blindly trust what a measurement device tells them.  Always ask, ‘how do I know it’s telling the truth?’




0 Responses to “Why you shouldn’t bother buying a thermometer”

  • We have one of those Liquid crystal strip thermometer for the babies room that will be terribly inaccurate, but has a picture of a cuddly bear or something on it.

    I don’t know if accuracy is very important for home use, relative temperatures much more important so you know the babies room is 5 degrees warmer than the lounge or outside for example (or this winter is colder than last…).

    Expect to get vetoed on your babies temperature accuracy measurements. Apparently a thermocouple attached to a multimeter is not a good way to check if she has a temperature.

  • Ah, the cuddly bear. That makes it perfectly fine, then.

    Yes, relative temperature is for the most part what you are interested in, and stuffing a thermometer in an ice-water mix to check its zero-point isn’t difficult, but if you are like me it is just PLAIN ANNOYING when you know your thermometer is not correct.