Mass Flow Sensors Fit Respiratory & Medical Applications
Thu, 06/21/2007 - 7:58am
With the continued expansion of the respiratory industry, MEMs flow sensors will play an important role.By Steve Massie, Omron Electronic Components, LLC
The world market for respiratory products is exploding. Applications include devices such as pulmonary function testing equipment, spirometers, sleep apnea diagnostic
As OEMs respond to these market requirements, engineers are creating a demand for components to accurately measure gas flow rates in this equipment. The requirements include smaller, lighter, more accurate flow sensing devices. Some manufacturers have responded to these challenges through the utilization of MEMS technology (Micro-Electro Mechanical System). This strategy has enabled the development of some of the smallest, most precise mass-flow elements.
OEMs are demanding that sensor packages be able to measure levels as low as several ml/minute, to beyond 200 liters/minute. With cost considerations always present, these flow sensors need to also be developed in consideration of
The concept of the MEMS flow sensor design is to measure the change of temperature across two thermal sensors (see diagram 2). These measurements are then calculated to a flow rate. The flow rate correlates to a specific output voltage, from 1 to 5 VDC (see chart 1). The MEMS sensor design allows the device to measure true mass-flow with an output stated in standard liters per minute.
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Diagram 3. Crepe structure etching allows the sensing area to be enlarged while maintaining the same cubic volume of the silicon substrate thus enabling improved performance while minimizing size.
To assure the flow rate is measured properly, a variety of sensor bodies with different cross-sectional areas are available. Additionally, larger flow rates can be measured utilizing a relatively small flow sensor by incorporating a bypass channel design.
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Chart 1. The MEMS flow sensor element measures the change of temperature across two thermal sensors. The temperature difference is then correlated to a specific flow rate with an output voltage, from 1 to 5 VDC.
While anesthesia gases are typically a mono-directional flow, respirators, CPAP, and ventilators require a bi-directional sensor to detect inhalation and exhalation. The MEMS mass-flow sensors can be produced to perform either function by changing the bias in the internal electronics package. These flow sensors can be calibrated for most of the gases used in medical applications, such as O2, air, N20, NO, He, HeO2, and CO2 by adjusting for the density (thermal absorption) of the gas.
With expectations of the continued expansion of the respiratory industry, component suppliers are working closely with these OEMs to assure their mass-flow detection device requirements are met. MEMS flow sensors are one such example of a product developed to meet today’s ever-changing needs and those of the future.
About the Author
Steve Massie is product manager, Omron Electronic Components, LLC, New Technologies; (847) 882-2288; firstname.lastname@example.org.