The majority of thermal flow meters are used for gas flow applications; although there are several designs offered for liquid measurement applications. Thermal mass flow meters place a heat source into the flow stream and measure the heat dissipation using one or more temperature sensors.
Thermal flow meters fall into three main families. All of them measure fluid mass flow rate by measuring the heat loss from a heated surface to the flowing fluid. In the case of the thermal dispersion, or immersible, type of flow meter, the heat is transferred to the boundary layer of the fluid flowing over the heated surface. In the case of the capillary-tube type, the heat is transferred to the fluid flowing through a small heated capillary tube that is parallel to the main fluid flow path. Many mass flow controllers (MFC) which combine a mass flow meter, electronics and a valve are based on the capillary design. A third type contains a heated and an unheated RTD attached to the outside of the tube. The temperature differential between the two RTDs provides the primary flow signal; at high flow rates the differential is lower as flow removes more heat, and at low flow rates the differential is higher, as flow removes less heat.

While all thermal flow meters use heat to make their flow measurements, there are two different electronic schemes for measuring the amount of heat dissipated. One method is called the constant temperature differential. Thermal flow meters using this method have two temperature sensors — a heated sensor and another sensor that measures the temperature of the gas. Mass flow rate is computed based on the amount of electrical power required to maintain a constant difference in temperature between the two temperature sensors. A second, and less popular method, is called a constant current method. Thermal mass flow meters using this method also have a heated sensor and another one that senses the temperature of the flow stream. The power to the heated sensor is kept constant. Mass flow is measured as a function of the difference between the temperature of the heated sensor and the temperature of the flow stream. Both methods are based on the principle that higher velocity flow rates result in a greater cooling effect. Both measure mass flow based on the measured effects of cooling in the flow stream.

The thermal mass flow meter operates independent of density, pressure, and viscosity. Accuracy of thermal mass flow meters is typically 1 to 2% of reading. Thermal mass flow meters benefit from having no moving parts, and a nearly unobstructed flow path.
Sensors for gas measurement applications are available for a couple of hundred dollars, up to one thousand dollars. Sensors for liquid measurement applications usually cost a couple thousand dollars.

-Presented by Max Machinery, Inc. the Positive Displacement Meter Experts