Freezer thermostat - how does sensor work?

My chest freezer just died. The thermostat dial seems to move a relay up/down and if I push the small stem that sticks out the end (Ranco type K50) the compressor cycles on. I guess I need a new thermostat. The dial won't move the level the extra 1-2 mm needed to activate the internal relay.
How does the sensing coil actually work? On air pressure? If I were to cut it open, I think it's hollow inside, similar to a furnace thermocouple. I presume the sensor somehow moves one end of the relay but I'd like to understand how the thing works since I'm curious.
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On Mon, 06 Aug 2007 13:58:50 -0400, Pat Coghlan wrote:

There is a gas/liquid inside the bulb at the end of the sensor tube that causes mechanical action on the other end. Don't know what the gas/liquid is but it may have properties similar to freon.
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In this case, there's no bulb, but it does appear to have a small cap on the end to seal it off.
Meat Plow wrote:

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On Tue, 07 Aug 2007 09:17:27 -0400, Pat Coghlan wrote:

Must be enough to not need the bulb which is reasonable considering the target temperature of your freezer being -10 to -20F.

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If it's a coil, such as in the older style (round) Honeywell thermostats... the coils is made of bi-metallic strip. One metal expands more, than the other, which causes bending. Most simple overtemp devices have some bi-metallic strip.

The bi-metal may be a conducting member itself, or push on something, or tilt a mercury switch (home thermostat)
RichK
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I referred to it as a coil, but only because when you buy one the (hollow?) tube is coiled up. Okay, rolled up.
You have to unroll it and insert it up a metal tube at the back of the freezer near the bottom.
It looks like this: http://morsun.ecvv.com/chinasuppliers/vp759857/THERMOSTAT.html
RichK wrote:

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