The thermopile is used with 750 millivolt pilot systems. This system is generally found on gas fireplaces and gas logs that use wall switches or remote controls. 105 PCS household products: 100 x pressure diaphragm & 5 x thermopile Reliable sealing and anti-aging. This equipment is a kind of cobination type thermocouple, provides power for 250/750 millivolt self-generation control systems. Good helper for water heater, gas valve and other maintenance.