Oven with temperature readout

[Sam]

Not when you do it with a compressor with the fridge instead of an electrical heater with the oven.

[The Other Mike]

A similar temperature probe thing was fitted on a Belling (made by Brother) microwave in the early 80's Connection was by something very closely resembling a 1/4" mono audio jack and socket that penetrated the cavity wall. The turntable having the capability to be switched off. The oven (touch wood) is still going strong at maybe 35 years old and in fairly regular use in the workshop.

They really built them like brick shithouses in those days.

[Dave Plowman (News)]

Not quite sure why. At basics, both are simply switched off when they reach the required temperature, and on again when needed to maintain that.

[whisky-dave]

Well most temp sensors use metal in their construction. Which is why you couldn't just put a LCD display in a microwave oven. I doubt you could just place a thermomerter in their either un less you has a suitable one, a meriry one might explode as would a standard alcohol fil led one.

In such an oven you get really rapidily changing temperatures too, and it s o much depends on what you are heating. I checked the o/p of my old oven ye ars ago. What yuo do is heat about 200ml of water for say a minute or two t hen measure the temerature rise and apply the specific heat capacity of wat er to work out teh power of the oven. You can't just stick a standard themo meter in the oven.

[whisky-dave]

Not bad but it's more than I paid for my microwave oven, I'd expect to see them in poundshops like an other thermometer.

I wonder where else you can stick it and get a valid reading ?

[Sam]

You get much finer control with the electrical heater.

[Dave Plowman (News)]

That is simply down to the thermostat design - hysteresis, etc.

[Jimmy Wilkinson Knife]

Or the compressor taking a while to get going? If I turn on a resistive element, I get heat immediately. If I turn on a compressor, a few minutes pass before cooling takes place, and another few minutes for it to stop cooling when I turn it off. Even if a fridge stat had a hysteresis of 0.00001C and the air inside the fridge was blown around by a fan, so perfectly even, it would still continue to cool after the compressor was turned off due to the thermal capacity of the coolant in the internal pipes.

[Dave Plowman (News)]

Thanks for confirming you don't know what hysteresis is.

[Jimmy Wilkinson Knife]

I know exactly what it is, but the sentence above suggests we're talking about THERMOSTAT hysteresis.

[Dave Plowman (News)]

If you're using a sensor with electronics to set the temperature as many do rather than a mechanical thermostat these day, the hysteresis is what you decide it to be.

[Jimmy Wilkinson Knife]

The user doesn't always get access to that setting. I do on a thermostat I assembled myself in kit form. But one on the front of an oven, I'd not expect the manufacturer of the cooker to be confusing the user with settings like hysteresis. They'll have determined what's best for the working of the oven and the life of the heating element.

[Dave Plowman (News)]

Just as well with the likes of you.

[Jimmy Wilkinson Knife]

Clearly the manufacturer would limit it to a sensible range, so the user couldn't set "200C +/- 1 billion".

[Jimmy Wilkinson Knife]

Actually, shouldn't as fine a hysteresis as possible be used? Take it to the extreme and say the stat could measure 0.001C. Why not do that? What you'd end up with is the equivalent of a dimmer switch on the heating element, and once it reached temperature it would be on say a 30% duty cycle. I'm assuming an electronic stat here and not a bimetallic strip (do they still use those?) as you'd wear it out damn quick. Not that a bimetallic strip could have an adjustable hysteresis. Of course if you're driving a compressor in a fridge, you want it to stop and start less often.

[Sam]

Down to how the heat is added or removed, actually.

[Sam]

Not with something like a fridge or freezer where its better to have the compressor start and stop less frequently.

Take it to the extreme and say the stat could measure 0.001C. Why not do that? What you'd end up with is the equivalent of a dimmer switch on the heating element, and once it reached temperature it would be on say a 30% duty cycle. I'm assuming an electronic stat here and not a bimetallic strip (do they still use those?) as you'd wear it out damn quick. Not that a bimetallic strip could have an adjustable hysteresis. Of course if you're driving a compressor in a fridge, you want it to stop and start less often.

[Dave Plowman (News)]

I have a fridge here. Badged Hoover. Must be over 30 years old. So doubt it is a problem in practice.

Nor is there any need to control temperature to within an inch of its life.

[Jimmy Wilkinson Knife]

A ridge takes a while to drop a C or three. A cooker can heat 30C in a very short time. So it needs to be turned on and off more quickly than a compressor.

[Steve Walker]

My marine fish-tank used to have the heaters driven by a pulse-width modulated supply - total overkill for simple heating of a large thermal mass by pretty low powered heaters, but I had a spare industrial PID controller and it gave me an integrated temperature display for free.

SteveW