"NY" wrote in message
o.uk...
"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.

Bit hard to do that with a bimetallic thing which is
entirely mechanical and which has say 8 set points tho.

And all done mechanically - on older cookers, anyway.

True.

Modern ones probably use temperature sensors and electronic logic

Yes, the manuals say you can set the temperature you want like with an
electric oven.

to replicate the mechanical "logic" that controlled the flow rate as the
temperature approached the one it was set at.

Wouldnt be hard to have it go flat out until it gets
to temperature and then back of the valve using a
stepper motor so it isnt on/off when at the set point,
set the valve to deliver a roughly constant temp that
still reacts when say the door is opened to check on
how whats being cooked is going by jabbing it with
a skewer etc