In article <4fb2op$> "IFOR dir" writes:

> Anyone heard of any OEM suppliers of a resistor heater sitting next to a thermistor
> (in, say, a small tube package) used to detect whether the pair were
> sitting in a gas or fluid?  I don't need to detect flow, just whether or
> not (say) air or water is present. 
> Any leads appreciated, Rob Peterson

Liquid detector:

      ----+------+------+      A Few Volts
          |      |      |
          |      R      |      Zener bias and base current resistor
          |      1      |
          |      |      |
          C      |      C
            B----+----B        Both NPN low power
          E      |      E      
          |      |      |
  (O1)----+      |      +----(02)
          |      +      |
          R      Z      R
          2      D      3
          |      |      |
      ----+------+------+      0V

Select ZD zener voltage and resistors R2 and R3 such that the transistors
are dissipating unequal amounts of heat, say a few milliwatts for one and
maybe a hundred or so for t'other.  Most of this heat will be generated
in the Collector areas and should be chosen to result in one transistor 
running fairly warm in air.  The Vbe drop varies from its nomunal 0.7V by 
about 2mV/K and thus there will be some small voltage measured between O1 
and O2.

If both transistors (or just the hot one if you prefer) are immersed in
some liquid, heat will be conducted away, reducing the junction 
temperature of (particularly) the hot transistor, and the Vbe difference
voltage will change accordingly.  The trick is essentially that the 
Collector regions do the heating while the Base-Emitter junction does
the temperature meaurement.  These regions are all, of course, in 
intimate thermal contact.

This works quite well.  In practice device variations mean that there 
will be some offset to be nulled out.  Ferranti ZTX300 type transistors
work well as the collector plate is pretty close to the outside world.

The Zener may be replaced by a couple of normal diodes in series 
connected the other way round giving about 1.4V drop.

This circuit detects just about any liquid with a time constant of about
a second upon immersion.  On drying out, the time constant depends 
greatly on the liquid used: a light organic solvent will dry out in a 
couple of seconds, while water is about the slowest to go having such a 
large specific heat capacity, and may take ten seconds or so.


Date: Sat, 10 Feb 96 14:53:44 GMT

Original Subject: Re: Air or fluid detector?

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