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Cooking Equipment (rec.food.equipment) Discussion of food-related equipment. Includes items used in food preparation and storage, including major and minor appliances, gadgets and utensils, infrastructure, and food- and recipe-related software. |
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temp control in a gas oven
hi folks,
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? thanks, Sam |
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temp control in a gas oven
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. |
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temp control in a gas oven
wff_ng_2 wrote: > > 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. Electronic ignition systems usually use the electronic ignition to start a pilot first.. The controls must sense that this pilot is started before the main gas valve is opened. This is done so if for some reason their is an ignition failure (low gas pressure, ignition system failure, whatever) that large amounts of gas are not released while waiting a resonable time (usually 30 seconds) for igntion to occur and it being sensed. Once the immediate heating needs are met both the main and the pilot are shut off until there is again a call for heat. This is the advantage of the system. no standing pilot to waste gas when it is not needed. |
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