Thermal mass flow meters measure the mass flow rate of fluids by measuring the heat conducted from a heated surface to the flowing fluid. Since a flowing fluid removes a known quantity of heat as it passes, the temperature difference between the heated sensor and the flow stream is directly proportional to the mass flow rate. Thermal mass flow meters are used almost entirely for gas flow applications. They feature no moving parts, have nearly unobstructed flow path, require no temperature or pressure corrections, and retain accuracy over a wide range of flow rates.