Mass flowmeter

摘要

A mass flowmeter uses a transducer comprised of a small block of insulating material. The transducer has a bypass channel and a detector channel passing through it. The bypass channel is used to bypass most of the flow so that the flowmeter can handle a large flow range without overloading the detector. Depending on the flow range to be measured, the bypass channel may contain calibrated resistances to the flow to force appropriate flow amounts to the detector. The detector channel has a carefully controlled cross-section and contains three thermistor beads mounted along a line parallel to the flow direction. The center thermistor is heated by a constant current or constant temperature source. The remaining two thermistors are placed upstream and downstream of the heated thermistor bead at equal distances. The voltages developed across the two outside thermistors due to heat transfer caused by the flow are provided to a differential amplifier to generate a direct mass flow measurement. Linearization circuitry may also be provided to convert the raw differential amplifier output to a signal which varies linearly with the flow rate.