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From: Spehro Pefhany
Subject: Re: Water sensor
References: <firstname.lastname@example.org> <email@example.com>
X-Newsreader: Forte Agent 1.92/32.572
Date: Tue, 14 Jan 2003 16:39:20 GMT
NNTP-Posting-Date: Tue, 14 Jan 2003 11:39:20 EST
On Tue, 14 Jan 2003 16:15:06 GMT, the renowned Marc_F_Hult
>On Tue, 14 Jan 2003 09:20:57 +0000 (GMT), Tony Williams
> wrote in message
>>In article ,
>> Bruce Murphy wrote:
>>> Tony Williams writes:
>>> > When in water the thermistor will struggle to get
>>> > above 100C as it tries to evaporate the water off
>>> > (540 cals/gm?). At that point its resistance will
>>> > be stuck down at the 100C value.
>>> While this is a cute idea, since the sensors have to be small, this
>>> strikes me as replacing one hazard with another.
>> It's a cheap (and cruder) version of what is
>> used for oil-level sensing in gearboxes, etc.
>> In that app the thermistor resistance is measured
>> at low power, then it is given a high power for
>> a fixed time, and the resistance measured again.
>> (T1 - T2) determines whether the thermistor is
>> immersed in oil or in air. Widely used in the
>> automotive and avionics industries... there are
>> even ic's to do the job.
>You also need to measure temperature because thermistors are 'notorious' for
>changing value with temperature ;-) One way is to use two thermistors, one of
>which is always submerged (or not) and in same temperature/heat conductance
I think you misread it. Tony's method works because it measures the
difference in thermal resistance to the ambient, which is mol
independent of temperature. The nonlinearity of the thermistor R(T)
might add some fun to the equation though, both in the measurement and
in getting the "high power" to the thermistor. This would be a very
good test of thermal fatigue in the thermistor assembly and I'd worry
about the R(t) as well.
>In 1985 or so I presented a paper on a thermistor-based method to assess oil,
>water and air saturation in porous media such as sand based on both static and
>heat pulse measurements. The latter provides a waveform which contains more
>information for interpretation than two static measurements.
Which is what Tony's method does, using the sensor as a heater.
Probably an offshoot of his ground-breaking work on the Williams
>Do you know of a replacement for the now-discontinued LM1820 Fluid Detector IC?
I did one in a microcontroller, but never got around to marketing it
generally. It was for detecting fluid in equipment cabinets.
Hydrogeologist, eh? Interesting work.
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