Hydraulic thermostats use what is known as a "thermally expansible fluid." This is a highly reactive fluid that swells as the heat rises and shrinks as the heat falls. This fluid is contained within a capsule inside the thermostat, and a light metal rod is extended into the capsule. As the fluid rises, the rod is pushed up, which affects the thermostat dial.
This thermally expansive fluid needs to be very tightly contained--if any escapes, it severely compromises the readings of the thermostat. Very tight seals are placed to contain the fluid inside the capsule. The rod is fitted with a spring and lock system precisely made to react with the fluid, but natural fall back down into place as the fluid shrinks.
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