Sodak:

If your water heater main flame comes on and goes off without outside electricity, it has a thermopile, not a thermocouple. It will keep working as long as the pilot flame is working. Most water heaters use thermocouples and need outside electrical power to open and close the main flame gas valve.

The following is from a web search of "THERMOPILES".

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A thermopile is serially-interconnected array of thermocouples, each of which consists of two dissimilar materials with a large thermoelectric power and opposite polarities. The thermocouples are placed across the hot and cold regions of a structure and the hot junctions are thermally isolated from the cold junctions. The cold junctions are typically placed on the silicon substrate to provide effective heat sink. In the hot regions, there is a black body for absorbing an infrared, which raises the temperature according to the intensity of the incident infrared. These thermopile employ two different thermoelectric materials which are placed on a thin diaphragm having a low thermal conductanceand capacitance. Since this creates a large temperature difference between the hot and cold regions, this structure enables improved detector performance. sp; The thermopile has some unique properties which cannot be duplicated by other detectors. It shows an inherently stable response to DC radiation and is not sensitive to ambient temperature variations.
It responds to a broad infrared spectrum, does not require a source of bias voltage or current.
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Bountyhunter