[time-nuts] Temperature measurements

David Forbes dforbes at dakotacom.net
Sun Jul 24 23:40:44 EDT 2005

At 8:46 PM -0600 7/24/05, 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.


I had the joy of measuring the temperature of a big spectrometer 
system over the last 6 months to learn about its instabilities. I 
used self-adhesive chip RTD sensors from Minco feeding a National 
Instruments FieldPoint box. My experiences led me to the following:

1. The temperature of the equipment tends to be similar to its 
performance instabilities, but the time lag of the electronics is 
widely dependent on the airflow and mounting of the circuitry. I saw 
time lags of anywhere from 5 seconds to an hour in different parts of 
my spectrometer.

2. You may learn lots of interesting things about your air 
conditioning system. In my case, I learned that the building chiller 
system runs 24 hours a day, but the office heater coils were shut off 
from 11PM to 6AM (presumably to save power). This provided some nice 
temperature step functions to evaluate the time lags of the various 
electronics boxes.

3. The mounting of sensors with regard to time lag is only important 
if the circuitry is forced-air cooled. The time lag of the stuff in 
the box will be longer than that of the sensor if there's no air 
passing through. Room air is best measured at the air inlet to one of 
your electronics boxes.

4. To achieve decent stability of some moderately power-hungry 
electronics on the order of a minute, I had to modify the forced-air 
cooling to isolate room air from the air inside the box. This is 
perhaps not interesting to you, but it's very useful to know if you 
have circuit boards that dissipate a fair amount of heat yet need to 
be temperature-stabilized.

5. Outside air is best measured with a sensor glued to a small sheet 
of aluminum that's shielded from the sun yet has ready access to 
airflow. The size of the sheet determines the time lag of the sensor. 
Alternately, a small sensor with a ~1 second time lag may be hung in 
the air by its leads. Last year I built a gadget to measure 
atmospheric turbulence for telescope seeing, using micro 
thermocouples suspended in air. It had a couple hundred Hertz 
frequency response. Two of the sensors were destroyed in a rainstorm, 
at $40 each for repair.


--David Forbes, Tucson, AZ

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