# Thermostat perceptions

[From Rick Marken (951130.2200 PST)]

Bill Leach (951130.23:15 U.S. Eastern Time Zone) --

Rick, it sounds to me as though maybe you do not know how a home
thermostat works (not really important to the current discussion).

The home thermostat is a binary device.

I don't think I know as well as you do; but I do know that the
current flow is either on or off; when you set the reference
you are actually setting a required coil size for the contacts
to hit and switch off (or on); is that right? Since the reference
and perception should be the same kind of variable, I suppose that
means that the thermostat perception that is actually controlled is
the size of the coil; is that right? It is compared to a reference
for desired coil size. So the binary electrical signal is really the
error signal; the current flows (driving the heater) when the coil is
too small and stops flowing (open circuit) when the coil is >= to the
reference size.

So the perceptual representation of heat in the thermostat is coil
size. Does that seem right?

Best

Rick

<[Bill Leach 951201.20:14 U.S. Eastern Time Zone]

[Rick Marken (951130.2200 PST)]

I don't think I know as well as you do; but I do know that the
current flow is either on or off; when you set the reference
you are actually setting a required coil size for the contacts
to hit and switch off (or on); is that right? Since the reference
and perception should be the same kind of variable, I suppose that
means that the thermostat perception that is actually controlled is
the size of the coil; is that right? It is compared to a reference
for desired coil size. So the binary electrical signal is really the
error signal; the current flows (driving the heater) when the coil is
too small and stops flowing (open circuit) when the coil is >= to the
reference size.

So the perceptual representation of heat in the thermostat is coil
size. Does that seem right?

Well most home thermostats (read cheap), use a flat coiled bi-metalic
sensor (which very much resembles the "balance wheel" or escapement wheel
spring in a mechanical watch.

The size of the coil really changes very little with temperature.
Typically the "inner" end of the coil is fixed in place and the "outer"
end (with an electrical contact attached) is free to "rotate" about an
axis perpendicular to the plane of the coil. So in essence, what is
controlled is the physical position of the "free" end of the coil.

Your analysis above applies directly to some industrial thermostats (many
of which indeed do use a sensing element that resembles a "coil spring").
Also, most home freezers and referigerators use a "bulb, fluid, and
bellows design) where what is controlled is the length of the bellows
(again in a fashion such as you described above).

-bill