Thermocouple are widely used type of temperature sensor and can be used as a mean to convert thermal energy into electrical energy. A thermopile is a group of thermocouples connected in series. Thermocouples measure the temperature difference between two points and not absolute temperature. Hydrometer is an instrument used for determining the specific gravity of liquids. A barometer is an instrument used to measure atmospheric pressure. While a pyrometer is a non-contact temperature measuring device, generally the term is applied to instruments measuring temperature above 600∘C.