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nuckolls.bob(at)aeroelect
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 Posted: Tue Feb 16, 2010 1:06 pm Post subject: Are home grown temperature probes feasible? |
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At 11:17 AM 2/16/2010, you wrote:
Dont waste your time dinking with zeners!
Nice for an experiment to demonstrate principles but insane for
production use. Especially in your case where you want several, all
needing to be matched!
As I suggested for OAT, use a temp chip like for example:
http://www.analog.com/en/temperature-sensing-and-thermal-management/analog-temperature-sensors/tmp36/products/product.html
It's $0.45 in quantity! why would you even think about zeners. Jim's
spent too much time in the classroom teaching EE techs. Feasible yes,
best choice? not a chance.
===================================
Good lick! You beat me to it Ira.
There is indeed a whole class of out-of-the-box, calibrated,
easy to use temperature sensors. They tend to behave like a
zener with a calibrated temperature coefficient . . . but are
indeed rather complex integrated circuits. One such device
suited to the task is:
Part Listing:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-7324-5-ND
Datasheet:
http://www.st.com/stonline/books/pdf/docs/2158.pdf
I've used the big brothers to this device in dozens of applications
over the years. Here's one recipe for success in wiring, packaging
and mounting this and similar devices:
http://aeroelectric.com/Pictures/Instruments/Temperature/LM135_Temp_Sensor/
I've mostly used the super-spec devices like the LM135 . . . but
for our purposes the lower cost, less accurate devices will
suffice.
The rudimentary wiring diagram illustrates the manner in which
we used these devices to feed directly into a data acquisition
system . . .
http://aeroelectric.com/Pictures/Data_Acquisition/Weeder_Module_DAS_2.jpg
monitored by an ordinary lap-top to record a multitude of
phenomenon including temperatures.
These sensors have been potted into heat sinks, 8AWG terminals
for surface mounting, drilled into the heads of bolts, glued
to surfaces (E-6000 or J-B Weld works good), etc. Many applications
simply hung the device in the open air for an ambient measurement.
If you want a direct reading, degrees C or even degrees F display,
there are low cost, LCD displays (the last picture) with a 200mV
full scale basic sensitivity. With the addition of a few 1% resistors,
perhaps a couple of pots, and a precision voltage reference, you
can re-scale and offset the reading to display degrees C directly.
But by simply scaling the device for 20 volts full scale (100:1
divider) the device will read out in degrees K. A K-degree is
the same size as a C-degree, 273K is freezing. The simpler display
isn't too hard to learn to use. Room temp is 300K, boiling is 373K.
This approach is good for about +/- 2 degrees C accuracy without
going to more sophisticated components, wiring and bench calibration.
The DAS system illustrated was supported by a PC based utility that
permitted scaling and offset adjustment to get about 0.5 degree C
accuracy.
Bob . . .
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rampil
Joined: 04 May 2007
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