Why does my digital thermometer take so long to reach a reading?

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Nufcdixie asked the Naked Scientists:
Why a digital thermometer on a watch takes 20-30 mins to reach a true reading.

Why is it not instantaneous like IRed ones.

What do you think?
« Last Edit: 28/02/2011 18:30:04 by _system »


Offline Soul Surfer

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A normal digital thermometer just like a traditional mercury thermometer depends on reaching a thermal equilibrium with its surroundings before it gives an accurate reading.  this can take quite a long time particularly if like in a watch it is buried inside a container and isolated from its environment by air.  An infra red radiation thermometer measures temperature by the actual radiation from the hot body being measured so there is no need for anything to reach thermal equilibrium and the response time is just the length of time it takes to make the radiation measurement.
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Offline syhprum

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Can you tell me what temperature the watch mounted thermometer is measuring, is it your body temperature ?.
Have you compared the case temperature of the watch directly with an IR thermometer ? 20 Min's is an awful long delay and a bad design fault if the watch is meant to be a useful thermometer or maybe it is just a cheap device with a temperature measuring facility added as a gimmick (apologies if you paid a lot for it !)