[time-nuts] Temperature measurements
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
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
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
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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