"Rod Speed" wrote in message
...
I've never come across a gas cooker with an oven which doesn't have a
thermostat and which relies on a constant flow of gas no matter whether
the oven is cold or up to temp. Even my mum's old 1962 cooker gave a big
flame when you lit it, which reduced to a smaller flame when the oven was
up to temperature.

Sure, but that wasnt done with a thermostat.

I'm not sure whether the temperature sensor reduced the gas flow to a
constant intermediate value when the oven was at temp,

Not in the thermostat sense, it basically reduces
the gas flow as the temperature increases.

I think we're in danger of splitting hairs here (as opposed to splitting
hares, as I once saw someone write, which is leporine butchery!). I tend to
think of a thermostat as something which keeps the temperature constant, by
means of some sort of feedback loop - whether that is achieved by gradually
reducing the heating effect until thermal equilibrium is reached, or whether
by cruder full-on/full-off control every so often.

Thats not possible with the traditional gas
oven which has no electronics involved at all.

You could design a gas-oven thermostat that had a bimetallic strip which
flipped between two states (as in a central heating thermostat) which turned
the gas full on or else to a minimum level that just kept the flame alight.
That is probably a classic thermostat that you are thinking of. But it so
happens that it is possible to control the flow of gas much more finely
without any energy loss by partially opening a valve, whereas doing the same
thing with electricity requires huge rheostats or big power transistors,
either way with a heat sink and significant energy loss when half-way on. So
electric cookers have a simple switch - and rely on plenty of thermal
inertia in the heating element and the air within the oven.

But either way it's temperature sensitive and reduces gas flow to achieve
constant temperature, rather than a simple partially-open valve which has no
thermal feedback to alter the gas flow once the temperature reaches the
desired level and to increase it temporarily if the oven door is opened and
the oven cools a bit more.

I'm not sure how rapid the cutoff is for a gas "regulo" but I think it's
fairly abrupt - full gas to get the oven up to temp as fast as possible,
then reduce to a fixed level appropriate to the temperature selected, with
minor adjustments in flow rate if the temperature happens to go a bit high
or low. And all done mechanically - on older cookers, anyway. Modern ones
probably use temperature sensors and electronic logic to replicate the
mechanical "logic" that controlled the flow rate as the temperature
approached the one it was set at.