joemac: you may be right in that the resistance of a TEC doesn't vary, but what I was trying to say, is that it's irrelevant: we can still measure the power output of a resistor/heater, by multiplying the amps and volts going to it. Simple.
Yes, the resistance of a heater/resistor changes, as you apply more power to it. Other than than, maybe some vibrations might change it, during the test, but that would still show up in the voltage and amp measurements.
As for the TEC, how would you know, for sure, that you have the same exact clamping pressure, from one day to the next? Are you going to "torque" the coldplate on it, every single time? What kind of errors do you think that would give you? How could you measure that error?
Also, you still don't know how much heat the pelt is outputting through its hot side. Although it's proportional to the power that you give it, it's not "directly" proportional. So how would you measure the power?
A tec is great if you're trying to get to a specific temperature, but that's not the case here: we want to get to a specific power output.
I like your diagram. You'd have to extend the insulation, to make sure that the cold air doesn't find it's way back up. If you use metallic screws, some of the output is going to end up being dissipated on the cold side. So you'd still be back to measuring it.
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