Measure Large Flows with Small Sensors

Here is a typical MEMS flow sensor bypass set-up.

Many Micro Electrical Mechanical System (MEMS) mass flow sensors are easier to integrate and cost less than other flow measurement products that do not have built in signal amplification and temperature compensation.  These MEMS sensors are shipped individually calibrated at the factory, which eliminates sorting and often reduces or eliminates final product calibration.

MEMS Flow Sensors deliver accurate and stable mass flow measurements.  Early versions were
expensive because of high flow rate requirements.  However, a low flow rate mass flow sensor in a
bypass configuration can measure much higher flow rates; yet reduce cost and weight.  Such a flow sensor bypass set-up is similar to that of a differential pressure sensor, which is also an indirect method of measuring gas flow.  The MEMS sensors, however, deliver a high resolution at very low flows compared to differential pressure (dP) sensors.

A basic bypass set-up consists of two ports inserted into the main flow path with an orifice or some other type of flow restrictor between them.  The restriction in the main flow path causes the flow to follow the path of least resistance into the bypass channel and through the flow sensor.  The pressure drop over the sensor needs to be greater than or equal to that between the bypass ports.

This chart illustrates the signal resolution of MEMS mass flow sensors over differential pressure sensors, even at very low flow rates. At flow rates close to zero the dP curve flattens out making it difficult to distinguish low flow readings from no flow or negative flow.

In the design of a bypass set-up, you need to consider the flow rate, the distance from the main flow path to the flow sensor, the diameters of the main flow path and the bypass tube, and the amount of restriction and shape of the flow restrictor.  The design of the flow restrictor not only affects the created pressure drop, it can also assist in straightening the flow or make it more laminar.  Turbulence in the gas flow can result in unstable readings.  You can use computational fluid dynamics (CFD) software to correct for turbulence.  Some sensor manufacturers are able to provide this service for their customers. 

The restriction in the main flow path causes the flow to follow the path of least resistance into the bypass channel and through the flow sensor. 

Omron offers two styles of 1 LPM gas flow sensors, the D6F-P and D6F-01A1 series.  The D6F-P series is available with both bidirectional and unidirectional flow calibrations, with a negative flow indication on the unidirectional version.  This series has an integrated dust segregation system that uses symmetrical centrifugal flow paths to keep particulate matter away from the flow sensor element regardless of flow direction.  It is available with either PCB terminals or a plug type connector.  Accuracy is guaranteed to be ‚±5% F.S., with typical results within ‚±2% F.S.  The D6F-01A1 series has a higher guaranteed accuracy of ‚±3%.  The straight through flow path incorporates a series of screens to evenly distribute the flow and an internal orifice that is often helpful in eliminating pump pulsing.  Both sensors have excellent low flow resolution.