Thermal mass flow meters employ a simple, direct method of measuring flow without imposing a significant pressure loss. They work by sensing the temperature of air in a pipe and the amount of heat required to maintain a heated sensor at a fixed temperature difference above the air temperature. The faster the air flows, the more heat is required. The heat requirement depends on the mass velocity of the air, so the meter is measuring the mass of the air going by rather than its volume. As long as the compressed air has been filtered to remove lubricating oil and dried to remove water droplets, thermal mass flowmeters provide accurate flow data independent of temperature and with little dependence on pressure.