Dave M wrote:
Tony Hwang wrote:
Dave M wrote:
It's the weight of the mercury that moves the bimetal coil spring
such that it takes a higher temperature to bend the bimetal coil
spring so that the mercury flows back to the other end of the bulb.
When the mercury moves to one end of the bulb, it makes the spring
reposition itself so that it takes a much higher (or lower,
depending on heat or cool mode) for the spring to return back to its
original position. Here's a link to a web page that describes the
operation very well,
saving me a lot of typing.
http://inspectapedia.com/heat/Thermo...e_Response.php

Cheers,

Dave M

I think you are thinking backward. Spring controls the position of
bulb depending on temperature. Not the other way around.


Kirk Landaur wrote


Yep, I understand that the "Spring controls the position of bulb
depending on temperature". That was, I thought, my intention to
describe. When the spring bends far enough to tip the bulb, the
weight of the mercury in the bulb swings the bulb a bit farther ,
requiring the temperature to cause a greater swing in the other
direction to make it switch back. That's where the hysteresis comes
from. That was the question from the OP, which is what I was trying
to answer.
cheers,
Dave M

I forgot to add that, in more direct response to the OP's question, that the
hysteresis (temperature difference required to switch the HVAC unit from off
to on, and back to off) is created by the temperature characteristics of the
bimetal spring, the weight of the mercury ball (both unchangeable by the
user), and modified by the anticipator resistor.

More Cheers,
Dave M