Sam" wrote:
had a question about how temperature is controlled in a gas oven.
till now i used to think that the dial(marked in degrees celcius)
regulates the amount of gas that can reach the oven. however i noticed
that along the insides are are two copper tubes which are closed at
one end. my guess is that they have something to do with temperature.
anyone know how this works?

Both copper tubes contain an medium (liquid/gas?) that expands as the
temperature goes up. The larger of the two copper tubes goes to the oven
compartment, and measures the temperature of the oven. When the oven reaches
the target temperature, it turns off the gas to the burner. When it drops
some amount below the target temperature, it turns the gas to the burner
back on. Normally you can hear the thermostat "click" as it turns the gas
off and on, and the burner flame can be heard going off and on after a short
delay.

The second and smaller of the two copper tubes is a safety device. The end
of this second one sits in the pilot light and turns the gas to the burner
off if there is no pilot flame.

There are variations on this basic scheme. I'm presuming on pilotless
(electronic) ignition ovens, this safety ensures that the main burner flame
starts, rather than making sure the pilot is lit. If the main burner flame
does not start, the gas to the burner would get cut off. I'd don't have a
pilotless oven, but my gas furnace with electronic ignition works this way.

Another variation is the use of an electric gas valve for the main burner.
Here instead of the gas to the burner being turned off and on directly by
the thermostat, it turns a switch off and on. The electric current is fed to
an electric gas valve which actually turns the gas on and off.

I have a 30 year old dual oven Magic Chef with pilot ignition for all
burners. The large bottom oven has the gas to the burner controlled directly
by the thermostat. The smaller top oven uses an electric gas valve. The top
oven will not operate during a power failure.

The same basic thermostat operation applies to electric ovens too. The
heating element is either on or off. The thermostat turns the electricity to
it on or off. In fact on electric surface elements, they are also mostly on
or off. The electricity to the element is cycled on and off for a variable
time period to produce variable heat output. More "on time" gives a higher
output. Cycling something on and off to vary the output is most obvious on a
microwave oven. There 60% power means the magnetron is on 60% of the time
and off 40% of the time... and you can hear this cycling happening loud and
clear.

I think gas surface burners are usually the only range heat sources that are
actually variable, where the amount of "input" (gas in this case) is
actually varied rather than cycled.