Electronic Design

Measuring Large Flows With Small Sensors Improves Accuracy

Flow sensors are critical components in a variety of medical applications, from monitoring the output of gas delivery systems to ensure accurate flow rates to monitoring a patient’s breathing. Ventilators, anesthesia delivery, oxygen concentrators, spirometers, insufflators, sleep apnea diagnostic and treatment equipment, pulmonary-function test equipment, and other critical devices all require flow measurement.

Some of the flow-sensing technology available includes differential pressure, positive displacement, and turbine approaches. Today, physicians are requiring electronic measurement to monitor gas flow, which provides greater accuracy and reliability. It also allows for the capture of an accurate log of treatment progress.

Compared to other flow-measurement components that don’t integrate signal amplification and temperature compensation, microelectrical-mechanical system (MEMS) mass flow sensors offer easier integration and cost savings. Individually precalibrated at the factory, these MEMS sensors reduce installation time by eliminating the sorting process (Fig. 1). Also, final product calibration often isn’t necessary.

Employing MEMS flow sensors, users can expect highly accurate and stable mass flow measurements. They offer many advantages over other technologies, though MEMS mass flow sensors often come with a higher price tag because of higher flow-rate requirements.

One solution for reducing cost, space requirements, and weight is to configure a low-flow-rate mass flow sensor in a bypass configuration to measure the higher flow rates. A MEMS flow-sensor bypass setup is similar to that of a differential pressure sensor, which is also an indirect method of measuring gas flow (Fig. 2).

The MEMS sensors also deliver a higher resolution at very low flow rates when compared to differential pressure (dP) sensors. Figure 3 illustrates the difference between typical mass flow sensors and 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 setup 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. As a result, the pressure drop over the sensor needs to be greater than or equal to that between the bypass ports (Fig. 4).

Items for consideration when designing a bypass setup include the flow rate, distance from the main flow path to the sensor, diameters of the main flow path and the bypass tube, and the amount of restriction and shape of the flow restrictor (Fig. 5). The design of the flow restrictor not only affects the pressure drop created, it also can assist in straightening the flow or making it more laminar.

Turbulence in the gas flow, another considration, can result in unstable readings. However, the use of modeling with computational fluid dynamics (CFD) software can assist in resolving this process to optimize the design. Some MEMS sensor manufacturers can provide this service.

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