MEMS improves flow measurement accuracy in medical devices

Flow sensors are key components in many medical devices, which are used to monitor gas output to ensure accurate flow. Currently available flow sensing technologies mainly include differential pressure sensing, positive displacement sensing, and impeller sensing. Microelectromechanical system (MEMS) large flow sensors are easier to integrate than those measurement components that do not include integrated signal amplification and temperature compensation circuits (Figure 1). Although MEMS large flow sensors have many advantages, they have always been expensive due to the large flow rates they measure.



One way to reduce cost, space, and weight is to set up a low-flow sensor in a bypass channel to measure the larger flow rate on the main channel. The bypass channel setting of a MEMS flow sensor is similar to that of a differential pressure sensor (indirectly measuring gas flow, see Figure 2). Compared with differential pressure sensors, MEMS sensors can still provide higher resolution at very low flow rates. Figure 3 describes the difference between ordinary large flow sensors and differential pressure sensors. When the flow rate is close to zero, the differential pressure curve becomes slow, making it difficult to determine whether the flow rate at that location is zero or negative.





Figure 4: The flow restriction on the main channel causes the liquid to flow to the bypass channel with the least resistance and pass through the MEMS flow sensor.

The simplest bypass channel setup consists of two ports inserted into the main channel with a restrictor between the ports. The restrictor directs the fluid to the channel of least resistance, into the shunt and through the flow sensor. Therefore, the pressure difference across the sensor will be greater than or equal to the pressure difference between the two shunt ports (Figure 4). When designing a shunt device, it is necessary to consider issues such as flow rate, distance from the main channel to the sensor, inner diameter of the main channel and bypass channel, and the restrictive flow and shape of the restrictor (Figure 5). The restrictor not only creates a pressure difference in the channel, but also creates a laminar flow in the channel.