[time-nuts] Temperature measurements

Dr. David Kirkby drkirkby at medphys.ucl.ac.uk
Mon Jul 25 07:14:25 EDT 2005

Joseph Gray wrote:
> I was thinking about ordering one of these: 
> http://store.qkits.com/moreinfo.cfm/vk011 for measuring temperature 
> inside, outside, and wherever. I think tracking the temperature as I 
> take long term measurements would be a good idea.
> I was wondering which is best: measuring the room temperature that the 
> equipment is in, or measuring the temperature of the equipment itself. 
> If the equipment, which spot? Some parts are much hotter than others 
> (heatsinks for example). If room temperature, where in the room? The 
> corner with all the equipment is warmer.
> As for outside temperature, how should I house the sensor to protect it 
> from the elements, yet not interfere with its measuring capability? For 
> example, if I were to encase a sensor in epoxy, that would certainly 
> cause a delay between the actual temperature change and when it was 
> measured by the sensor.
> Any other thoughts on the topic also welcome.

As a sensor, the AD592AN, BN or CN are quite nice. They give a constant current, proportial to 
absolute temperaature (1 uA/K) it does not matter how long the leads are. But due to their packaging, 
the thermal time constant is not goingt be very short. (You can always average data later to get a 
slower response, so it is worth having a fast response and filtering later if needed).

Here's some data I collected with the TI counter working in its normal mode - not loopback. It looks OK.
That was done with a themmistor.

Here is some in loopback mode (look at the bottom two graphs).
There you can see a problem which seems to be correlated with temperature. Taking it out of its 
loopback mode seems to clear that.

I took all of that data with a thermistor at the input air vent of the TI counter. As doors are 
opened, so the temperature changes *rapidly*, which seems to cause problems with the TI counter in 
loopback mode.

If you want fast response, I would think a thermistor is probably the best bet, as they are very small 
and easy to use. A thermocouple would have a faster response, but they are not quite as easy to use.

I don't think covering the leads of a small thermistor in expoxy should not significantly change its 
time constant. You can get them hemetically sealed quite easily.
Dr. David Kirkby PhD CEng MIEE,
Senior Research Fellow,
Department of Medical Physics,
Mallet Place Engineering Building,
Gower St,
University College London,
London WC1E 6BT.

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