[phoenix-project] NTC sensor
- From: Georges Khaznadar <georges.khaznadar@xxxxxxx>
- To: phoenix-project@xxxxxxxxxxxxx
- Date: Sun, 14 Jun 2015 19:53:15 +0200
Hello Ozhan,
ozhan fenerci a écrit :
2-a) [...] I would like to write a GUI for that so students can use NTC100
thermistor before trying to use PT100. Has anybody written a GUI for NTC100?
Unless you buy expensive thermistors, you cannot rely on a precise
resistance value at a given temperature, without calibrating each
thermistor and keeping the calibration information attached to each of
them. The main reason is that thermistors are made with a
semi-conducting material surrounded by metallic plates, whose shape has
a direct effect on the resistance.
I can buy 5 thermistors for 2 € at my local store (which is
approximately US$ 0.35 per unit). However, the resistance is warranted
with a typical dispersion lower than 10%, between individual resistors.
The measured dispersion is generally much lower than 10%, probably
because a set of components bought together were manufactured in the
same run.
An easy way to do the calibration (in an approximative way), and keeping
it attached to the thermistor is to associate each thermistor with an
adjustable resitor, soldering them to make a voltage divider. Then you can
adjust all variable resistors to get a dividing ratio equal to 50%, near
the temperature of interest (let all thermistors be coupled to the same
temperature source, like melting ice, boiling water, or some eutectic
system at its melting point[1]). Then all the voltage dividers will give
very similar responses near this particular temperature.
I attach a short article about such temperature sensors. When
calibrated, their temperature behaviour can be related to fundamental
physics rather than to details of the thermistor's shape. Then you can
design a GUI with a single constant parameter to take in account the
temperature drift of the sensor's output.
Best regards, Georges.
P.S.: the short article was written wit TeXmacs, a wysiwyg editor able
to embed Yacas interactive sessions with well rendered formula, which
you paste further when necessary :)
Thanking you all for providing nice information how to make LM35 waterproof.
Making a thermistor waterproof is the same than for the LM35 (except the
connection : only two wires are required)
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