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So this is where I write a long rant about thermal dynamics, and
somebody comes along to tell me how completely wrong I am...
My mum's old oven was always too hot. It would burn anything you tried
to cook to a cinder. My mum remains convinced this is because the
thermostat was faulty. This is nonesense because... IT DOESN'T HAVE A
THERMOSTAT.
The temparature control on the front just adjusts the size of the flame
(by adjusting how fast the gas is let in). There's no temparature sensor
beyond the one used to check that the gas has ignited.
I've told her this repeatedly. Her responce has always been "oh yeah? So
how come the oven doesn't just constantly heat up until it melts?"
The answer, of course, is: BECAUSE IT'S NOT A PERPETUAL MOTION MACHINE!
This afternoon, I sat down and worked out the whole thing. See, you
light a fire in the back of the oven, and it injects heat at a constant
rate. An oven is well insulated - that's why even with a relatively tiny
flame, it can reach a few hundred degrees Centigrade inside. But any hot
object inevitably looses heat.
As the oven gets hotter, it looses heat faster. Eventually, the rate at
which heat is lost equals the rate at which the flame is adding heat. At
this point, the oven now holds a constant temparature. No thermostat
required.
Adjusting the size of the flame changes the rate of heat injection, and
hence moves the steady-state temparature.
It's the same physics that causes an object to reach terminal velocity
in freefall, and why your car doesn't have an infinite top speed.
"Oh yeah? So how come the central heating system has a thermostat then?"
Well, our oven adjusts temparature by changing the size of the flame.
The boiler for the heating does NOT adjust the size of the flame - only
the duty cycle. Switching a flame on and off requires a thermostat.
More importantly, the inside of an oven is a few hundred degrees hotter
than the outside. If the outside temparature moves 10 degrees, it's not
going to change the steady-state temparature inside by much. On the
other hand, the temparature outside your house is normally much closer
to the temparature inside. So variations in outside temparature will
have a bigger effect.
On top of that, it doesn't really matter if your oven is 10 degrees
colder than you asked for, but it makes a pretty important difference if
your house is. So an oven needs less exact control than a house, and a
house has more significant variations to take into account.
I notice that electric cookers also use a thermostat. Again, this
appears to be because they modulate duty cycle rather than heat
injection rate, for whatever reason. (I presume it's just awkward to
smoothly adjust the level of a multi-kW electric pathway?)
Small question: The rate at which an object looses heat. Clearly this is
proportional to how much hotter it is than its surroundings. But is this
relationship linear?
From what I can remember, the temparature of an object approaches that
of its surroundings according to an exponential curve ("cooling curve").
The derrivative of an exponential is also an exponential, so that would
indicate that as an object gets hotter, it looses exponentially more
heat. Right?
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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On Sun, 01 Jun 2008 19:14:43 +0100, Orchid XP v8 <voi### [at] dev null>
wrote:
>So this is where I write a long rant about thermal dynamics, and
>somebody comes along to tell me how completely wrong I am...
I remember thinking the same as you when I lived with my parents
(quite a while ago) and I actually had a look inside. What I saw was
when you put the oven on the flame was quite large, between one and
two inches, after the oven had heated up. The size of the flame had
shrunk to being about a quarter of an inch. You probably cannot see an
open flame in modern cookers, health and safety etc.
Oh and if you Google "gas oven thermostats" you might be surprised as
you can find how to fix faulty ones ;)
--
Regards
Stephen
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> My mum's old oven was always too hot. It would burn anything you tried to
> cook to a cinder. My mum remains convinced this is because the thermostat
> was faulty. This is nonesense because... IT DOESN'T HAVE A THERMOSTAT.
Ermm, actually it does have a thermostat. Simply tested by wiggling the
knob while the oven is cold and observing that the flame size doesn't
change.
If it didn't have a thermostat then to set the temperature at eg 200 degrees
(I don't know what gas mark that is) the flame size would be so small it
would take 10 hours to get to anywhere near 200 degrees.
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scott wrote:
> Ermm, actually it does have a thermostat. Simply tested by wiggling the
> knob while the oven is cold and observing that the flame size doesn't
> change.
Interesting. On my oven, it does.
[There is some kind of sensor that detects whether the gas has ignited.
When you turn the oven on, it lets in a tiny amount of gas and ignites
it. Then, after a few seconds, the flame suddenly grows to full size.
Thereafter it remains at that size until you turn the oven off.]
> If it didn't have a thermostat then to set the temperature at eg 200
> degrees (I don't know what gas mark that is) the flame size would be so
> small it would take 10 hours to get to anywhere near 200 degrees.
That's an interesting theory.
OOC, any idea how much energy it takes to warm up 0.25 m^2 of air from
methane...)
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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On Mon, 02 Jun 2008 09:00:15 +0100, Invisible <voi### [at] devnull> wrote:
>
>Interesting. On my oven, it does.
>
>[There is some kind of sensor that detects whether the gas has ignited.
>When you turn the oven on, it lets in a tiny amount of gas and ignites
>it. Then, after a few seconds, the flame suddenly grows to full size.
>Thereafter it remains at that size until you turn the oven off.]
That is why the oven gets too hot. The thermostat is not working
properly.
You have found the fault, now all you have to do is fix it.
Well done.
--
Regards
Stephen
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> Interesting. On my oven, it does.
It's broken then. In a cold oven, any temperature setting significantly
above room temperature should cause the maximum size flame to be used. Once
the oven gets to that setting the thermostat should then regulate the gas
flow to keep that temperature.
> [There is some kind of sensor that detects whether the gas has ignited.
> When you turn the oven on, it lets in a tiny amount of gas and ignites it.
> Then, after a few seconds, the flame suddenly grows to full size.
> Thereafter it remains at that size until you turn the oven off.]
That is a different system, that is there to stop the gas coming out full
whack until the oven knows that it's actually ignited. Otherwise if there
is a fault (and the gas is not ignited) then you might end up filling your
house with gas, which is not such a good idea.
> OOC, any idea how much energy it takes to warm up 0.25 m^2 of air from
The specific heat capacity of air is roughly 1 Joule per gram per degree
Kelvin. So I make that about 45 kJ.
You probably want to heat up some of the metal inside the oven too...
> (And how much energy is released when you burn some methane...)
Googling "energy density methane" seems to give 55 MJ / kg.
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>> Interesting. On my oven, it does.
>
> It's broken then. In a cold oven, any temperature setting significantly
> above room temperature should cause the maximum size flame to be used.
> Once the oven gets to that setting the thermostat should then regulate
> the gas flow to keep that temperature.
Well, we've since thrown the old oven away. But the brand new one we've
got does exactly the same thing. And that has a mechanical thermometer
inside which shows me it's working correctly.
>> [There is some kind of sensor that detects whether the gas has
>> ignited. When you turn the oven on, it lets in a tiny amount of gas
>> and ignites it. Then, after a few seconds, the flame suddenly grows to
>> full size. Thereafter it remains at that size until you turn the oven
>> off.]
>
> That is a different system, that is there to stop the gas coming out
> full whack until the oven knows that it's actually ignited. Otherwise
> if there is a fault (and the gas is not ignited) then you might end up
> filling your house with gas, which is not such a good idea.
Uh, yah. I figured. ;-)
>> OOC, any idea how much energy it takes to warm up 0.25 m^2 of air from
>
> The specific heat capacity of air is roughly 1 Joule per gram per degree
> Kelvin. So I make that about 45 kJ.
Right. I had no idea what the technical term for that is.
Now I simply *must* know if the specific heat capacity of oil is
significantly different from water... (Every time I make cheese
toasties, the cheese inside is hotter than lava! They should fill
storage heaters with that stuff or something...)
> You probably want to heat up some of the metal inside the oven too...
Oh, for sure. And I assume that there's some kind of air pathway to let
more oxygen into the oven too - and hence a similar pathway that lets
the hot air escape. I just wanted to get some idea "how big" the number is.
>> (And how much energy is released when you burn some methane...)
>
> Googling "energy density methane" seems to give 55 MJ / kg.
On the other hand, what the HELL volume of methane gas weighs 1 kg? ;-)
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Stephen wrote:
> You have found the fault, now all you have to do is fix it.
>
> Well done.
Nah, we threw that oven away. ;-)
[Ever tried throwing an oven? It's ****ing heavy! And loud...]
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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On Mon, 02 Jun 2008 11:39:11 +0100, Invisible <voi### [at] devnull> wrote:
>Stephen wrote:
>
>> You have found the fault, now all you have to do is fix it.
>>
>> Well done.
>
>Nah, we threw that oven away. ;-)
LOL
>[Ever tried throwing an oven? It's ****ing heavy! And loud...]
No I just move house :-)
--
Regards
Stephen
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> Now I simply *must* know if the specific heat capacity of oil is
> significantly different from water...
I think it's actually lower than water.
> (Every time I make cheese toasties, the cheese inside is hotter than lava!
> They should fill storage heaters with that stuff or something...)
They use it for storage heaters and cooling electrical stuff because it
doesn't boil at 100 degrees and is an insulator.
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