MEMS flow sensors offer easier integratopm

The D6F-P 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.

Flow sensors are critical components in a variety of medical equipment.

They are used to monitor the output of gas delivery systems to ensure accurate gas flow rates as well as to monitor patients' breathing.

Devices requiring flow measurement include ventilators, anesthesia delivers, oxygen concentrators, spirometers, insufflators, sleep apnea diagnostic equipment, CPAP/BPAP sleep apnea treatment and pulmonary function test equipment.

Some of the flow sensing technology available includes differential pressure, positive displacement and turbine approaches.

Today's physicians are requiring electronic measurement to monitor gas flow, as this technology provides increased accuracy and reliability, as well as allowing for the capture of an accurate log of the progress of the treatment.

Many MEMS (micro electrical mechanical system) mass flow sensors offer easier integration and cost savings when compared to other flow measurement products that do not have built in signal amplification and temperature compensation.

Since these MEMS sensors are shipped individually calibrated at the factory, installation time is reduced because sorting is not required and final product calibration is greatly reduced compared to other flow sensor products.

By using MEMS Flow Sensors, highly accurate and stable mass flow measurements are delivered.

Although they offer many advantages over other technologies, MEMS mass flow sensors often had a higher cost because of high flow rate requirements.

A solution to reduce component costs and conserve space (and weight) is to use a low flow rate mass flow sensor in a bypass configuration to measure much higher flow rates.

A MEMS 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 mass flow sensors, however, deliver a higher level of resolution at very low flows when compared to differential pressure (DP) sensors.

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.

A basic bypass set-up is composed 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.

Items to be considered in designing the bypass set-up include 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 pressure drop created, but can also assist in straightening the flow or making it more laminar.

Turbulence in the gas flow can result in unstable readings.

The use of modeling with CFD (computational fluid dynamics) software can assist in this design process to optimise the design.

Some MEMS mass flow sensor manufacturers are able to provide this service for their customers.

MEMS mass flow sensors are in production and available now.

Omron offers two styles of 1 litre/min gas flow sensors; the D6F-P and D6F-01A1.

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.

This is an economical package with a guaranteed accuracy of +/-5% FS, 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.

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