piclist 2000\06\03\150830a >
Thread: [PIC]: How to measure level of liquids and sense
picon face BY : Peter L. Peres email (remove spam text)

>Peter, can you more fully describe the makeup of this 10' length
>of sensor? I thought a thermistor was basically a resistive bead with
>copper wires attached. Are you referring here to some kind of distributed
>thermo-resistive element? Confusing.


normally thermistors are indeed bead-type using discoidal or cylindrical
active elements, but the original [tm] - ca. 1800's ? - way to do this, is
thermistor wire. Which comes on reels and is spec'd in ohms/meter. This is
different than resistor wire, as the latter has a near-zero temperature
coefficient, whereas the thermistor wire has a positive (and very linear vs.
other methods) coefficient. Tungsten wire comes to my mind as an inexpensive
<grin> candidate (note: tungsten wire: not tungsten steel. Chemical symbol
W, aka Wolfram in other parts of the world). This is what incandescent bulb
filaments are made of usually. In real life, complex and proprietary alloys
are used for the wire. The W wire is very fragile (it shatters like a
ferrite) so it has very limited uses. The tempco (for certain W alloys) is
about +0.03-0.05 ohms/K at around room temperature. You should really get
hold of a National Data Book that features the relevant chips, they go into
some depth with this (Automotive Section).

The wire is spanned in a frame (or a tube with openings), and immersed
vertically in the liquid. The liquid wets a part of the wire, and this part
will be cooled very efficiently. The part that sticks out heats up due to
the current passed through and this is what you measure. The wires used in
automotive apps. have a very high tempco (can't run them hot). The accuracy
depends directly on the temperature of the exposed wire and on the tempco of
the wire (both as high as possible to make it more accurate).



See also: www.piclist.com/techref/microchip/devices.htm?key=pic
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