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If you have a water heater that has an electronic gas control, you have a thermopile. For water heaters with a standard gas control before 2010, you will have a standard thermocouple or a thermocouple with a thermal cut off.
A universal thermocouple generates about 75 millivolts to operate a gas control. A thermopile will generate about 750 millivolts to operate an electronic gas control.