piclist 2000\06\02\061225a >
Thread: [PIC]: How to measure level of liquids and sense
www.piclist.com/techref/microchip/devices.htm?key=pic
picon face BY : Peter L. Peres email (remove spam text)



> Not that I've ever tried this, but I figure the temperature variation
> would distribute itself along the tube in some manner [to be measured
> and calibrated empirically], and the math would be some weighted
> averaging routine. Just a thought. The sorta thing you try once, and
> immediately get a strong hunch whether it will ever work or not.

The way to do this is with 2 thermistors buried in a full cylinder such that
their sizes are negligible wrt to the cylinder diameter and whose
(cylinder's) heat conductivity approximates that of the liquid. The relation
that governs the output is so hard to match to reality that a lookup table
must be used. The output depends on the heat conductivity of the liquid and
the thermistors must be run very hot to get decent results. At the same time
the liquid must not boil or convect on the cylinder. If the liquid moves
(pump, drain, convection) without level change then the readout changes (the
conductivity changes).

However, there is a similar method, that is used, and works well. It is the
one with the thermistor wire that is partially immersed. This is what
measures the fuel level in your car's tank probably. The sensors are
available in lengths of up to 10 ft (can you say marine diesel tank ?).
Check out a National catalog for chips that interface to these, although you
can roll your own. Note that most sensors are 'bare' wires but there are
such sensors with teflon or ceramic clad wire which can go into evil liquids.
These are expensive and fragile. The fun part is, that this method is
imprevious to sloshing as it integrates the 'waves' away ;-)

bye,

       Peter

<20000601202556.A4C242A388@plp.plp.home.org>

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