LMM-H03 is a thermodynamic sensing element for the bidirectional measurement of mass air flow in a well defined channel. It is dedicated for applications with high reliability requirements such as medical or industrial gas flow applications. Mounted directly within the flowing media the LMMH03 is sensitive against the heat transfer on a micrometer scale. This heat transfer is detected by a micro-machined silicon device which is mounted on ceramic substrate defining the geometry of the sensing element.
The functional principle of the LMM-H03 is a hot film anemometer, which is the thin film version of the hot wire anemometer. A thin film heating element (heater) is heated to a defined temperature difference with respect to the air. At zero flow there is heat dissipation due to the thermal conductivity of the air. With applied flow the heat dissipation increases steadily with the flow rate.
The sensing element can be operated at Constant Power and Constant Voltage mode. However, it is recommended to operate LMM-H03 in the described Constant Temperature Difference (CTD) mode. In the CTD mode the electronics detects any change of the thermal equilibrium very quickly and compensates it by adjusting the power settings through the heaters. This allows a very reliable mass flow measurement with a very short time constant which is in the order of milliseconds.
