Russell McMahon <apptech <at> paradise.net.nz> writes:
> Convection ...
> Aaagh aaagh aagh

More to the point: adding something with leads to something with leads means
adding a radiator. To actually measure the temperature of a point without
cooling or heating it one sets up a gradient meter. That's like a Wheatstone
bridge for heat. It works like so:

     insulation
       ======
  H/C--T1--T2--Target
       ======
     insulation

H/C is a heater and/or cooler, T1 and T2 are thermometers (or thermistors) with
an insulating sleeve. the '-' is a thermal link (copper rod or direct connection,
i.e. thermal grease and pressure). How it works:

T1 and T2 are calibrated and both outputs are applied to an error amplifier. 
This drives the H/C (for heating only a resistor), until T1 and T2 have the same
readout. Then the heat flow in the assembly is nearly zero and T1 and T2's 
outputs indicate the temperature of the target with most errors deleted. To 
avoid oscillation of the error amplifier some thinking is required (hint: the
assembly works as an integrator and there is 'phase shift' between the two
differential inputs).

Peter P.


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