eli5 How do gas hobs know to stay on when on fire?
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What do you mean ‘know how to stay on’? You turn the knob, which starts the gas flow, and the gas will continue to flow until you turn the knob back to zero again. The heat from the burning gas will cause new gas flowing out to catch fire, until you shut it off and there’s no new gas to burn.
They have a little heat sensor that detects if the gas is burning. If it isn’t, it will turn the gas off. That’s why sometimes it doesn’t stay on when you first light it, because it takes a second or so to register the temperature.
There are two common techniques:
1. A sensor detects the heat from the fire and knows that it doesn’t need to keep trying to light it.
2. In a fire, the air conducts electricity a little better (ionization). Some higher end appliances have a sensor that can detect this difference in electrical conductivity and use that to decide when to create a spark or not. These have the added benefit of not needing to cool down if the fire goes out, so they can be used to reignite fire if it goes out as a safety mechanism so that the room doesn’t fill with gas.
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