I bought a 140 deg C Cantherm thermostat a while back. (F20A series) It didn't open up till ~170 C (much too hot for me.)
I bought a bunch and tested them. I did a rapid heat and then heat slowly near the set point temperature. Unit was on a piece of copper with a piece of foam over the top. Here's the set up with foam off to side.
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Without the foam the trip temperature was ~10-20 C higher than with the foam.Tolerance on the device is +/-5 C.
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Here's the data.
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Trip point is typically 10 C higher. I'll retest in vacuum sometime in the future. (For the 140C unit vacuum behavior was similar to foam covered on desk top.)
George H.