A MEMS flow sensor in a bypass set-up is similar to that of a differential pressure sensor, which is an indirect method of measuring gas flow. (Model D6F-P shown)

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 delivery, oxygen concentrators, spirometers, insufflators, sleep apnea diagnostic equipment, CPAP/BPAP sleep apnea treatment, and pulmonary function test equipment.

Some of the flow sensing technologies available include: differential pressure, positive displacement, and turbine approaches. Today physicians are requiring electronic measurements to monitor gas flow, which provides increased accuracy and reliability as well as allowing for the capture of data for an accurate log of the treatment progress.

Measure Large Flows with Small Sensors

By utilizing one of Omron’s low flow rate MEMS (Micro Electro Mechanical System) mass flow sensors, such as the D6F-P or D6F-01, in a bypass configuration to measure much higher flow rates, a reduction in component costs and space savings would be realized. A MEMS flow sensor bypass set-up, as shown in Figure 1, is similar to that of a differential pressure sensor, which is an indirect method of measuring gas flow.

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.

The MEMS Mass Flow Sensors, however, provide a higher level of resolution at very low flows when compared to differential pressure (dP) sensors. Figure 2 illustrates the difference between typical flow and dP sensors. Notice that 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. As shown in Figure 3, 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 pressure drop over the sensor needs to be greater than or equal to that between the bypass ports.

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. Figure 4 shows some typical flow restrictor shapes.

Omron is ready to assist you with your design challenges. Through the utilization of computer modeling with CFD (computational fluid dynamics) software Omron engineers can analyze your system and provide a starting point for your orifice design. Testing of the actual physical system is recommended for final optimization.  

Omron’s MEMS 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 and final product calibration time is greatly reduced and often eliminated compared to other flow sensor products.

MEMS Mass Flow Sensors are in production and available now. 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.

A look at some typical flow restrictor shapes.

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 (see video clip below). It is available with either PCB terminals or a plug type connector. This is an economical package with a guaranteed accuracy of ±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. 

For additional information please contact Omron Electronic Components at 847-882-2288, by emailing components@omron.com, or by visiting www.components.omron.com.