What is a safety device?
An automatic shut-off device for a gas stove, and more particularly, a safety valve control device that can be retrofitted between the gas inlet pipe and the catch base of the stove. The device includes a coupling such that operation of the knob of the gas stove at the time operates the circuit of a gas safety valve control device. This operation causes the forward movement of a function shaft of the gas safety valve device and opens the gas intake valve to supply the gas to the stove burner. The function shaft is also subject to the control by an electromagnetic control rod to maintain the open state of the gas intake valve. In case the fire goes out accidently, the circuit device energizes an electromagnetic coil to attract upwardly an electromagnetic control rod, thereby disconnecting the function shaft, which is spring loaded, and which in turn operates the gas intake valve. This action thus disconnects the gas supply to the stove. Also, if the cooking time is too long, and the fire does not go out (e.g., one forgets to turn off the gas) or the gas at the stove burner can not be ignited within the given time, the device will also shut off automatically the gas intake valve.
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Many gas-fed heating appliances such as ovens and water heaters make use of a pilot flame to ignite the main gas burner when required. If it goes out gas may be released, which is a fire risk and a health hazard. To prevent this some appliances use a thermocouple in a fail-safe circuit to sense when the pilot light is burning. The tip of the thermocouple is placed in the pilot flame, generating a voltage which operates the supply valve which feeds gas to the pilot. So long as the pilot flame remains lit, the thermocouple remains hot, and the pilot gas valve is held open. If the pilot light goes out, the thermocouple temperature falls, causing the voltage across the thermocouple to drop and the valve to close.
Some systems, known as millivolt control systems, extend this concept to the main gas valve as well. Not only does the voltage created by the pilot thermocouple activate the pilot gas valve, it is also routed through a thermostat to power the main gas valve as well. Here, a larger voltage is needed than in a pilot flame safety system described above, and a thermopile is used rather than a single thermocouple. Such a system requires no external source of electricity for its operation and so can operate during a power failure, provided all the related system components allow for this. Note that this excludes common forced air furnaces because external power is required to operate the blower motor, but this feature is especially useful for un-powered convection heaters.
A similar gas shut-off safety mechanism using a thermocouple is sometimes employed to ensure that the main burner ignites within a certain time period, shutting off the main burner gas supply valve should that not happen.
Out of concern for energy wasted by the standing pilot, designers of many newer appliances have switched to an electronically controlled pilot-less ignition, also called intermittent ignition. With no standing pilot flame, there is no risk of gas buildup should the flame go out, so these appliances do not need thermocouple-based safety pilot safety switches. As these designs lose the benefit of operation without a continuous source of electricity, standing pilots are still used in some appliances. The exception is later model instantaneous water heaters that utilise the flow of water to generate the current required to ignite the gas burner, in conjunction with a thermocouple as a safety cut-off device in the event the gas fails to ignite, or the flame is extinguished.
Source: wikipedia
Also a thermoucouple is hard to fix and when you wantyou have to wait for the thermocouple to heat the metal before you can release your gas knobs, if you release it to early, the flames will just extinguish. Thermocouples are hard to fix once they are broken it is good to have one in the gas range but id rather have the manual type, because i can easily lit my burners without having to wait for the preheating time, yes its safe but it takes time.
