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Improvements in Pressure Sensors Help Medical Equipment Design

Fri, 03/11/2011 - 11:17am
David Ezekiel

Goals of new medical equipment design include small size, low cost, low power consumption, and a quiet user experience. Evolving technology embedded in pressure sensors and signal conditioning ICs allows for design improvements and the inclusion of new features in medical equipment. This article highlights specific examples in the design of breathing equipment and oxygen concentrators.  

Pressure sensor qualities affect all types of medical equipment, including blood pressure measurement devices, hospital beds, infusion pumps, dialysis machines, deep vein thrombosis equipment, and more. Recent improvements to these components enhance the flexibility medical device designers have when developing new products.

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Fully compensated, signal-conditioned sensors with either an analog or digital output

Pressure Sensors for Breathing Applications
The most obvious feature for pressure sensors in breathing applications, such as ventilators, respirators, and sleep apnea equipment, is the accurate and sensitive measurement of breathing rate. Detection of low pressures for these applications has traditionally been expensive, with difficulty in finding a cost-effective sensor sensitive enough for low breath detection.

Two new design improvements in pressure sensing have alleviated design concerns for low breath detection sensing. First, sensing technologies with sophisticated signal conditioning or MEMs-based thermo-anemometry allow for the measurement of extremely low pressures under one inch of water and come with bi-directional measurement capabilities. Previously, these sensors were cost-prohibitive or did not exist. These pressure sensors are excellent for applications demanding the lowest sensitivity. Next, the availability of low cost signal conditioning chips allow designers of breath detection equipment to use a less expensive pressure sensor, such as one measuring four inches of water, and amplify the output for one to two inches of water measurement range.

Low power pressure sensors also provide advantages for the medical equipment designer. Combined with the lower sensor costs and improved sensitivity previously described, new variations of breath detection equipment reaching new markets and applications are created. Breath detection equipment is more cost-effective and more portable, allowing for use by more patients.

Oxygen Concentrators and Conservers
Oxygen concentrators and conservers are being designed for more portability and for use at the patient’s bedside. Size, low power, and reduced sound are important design elements for the medical designer.

 

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Pressure sensor with MEMS-based thermo-aneomometry easily measure under 1 in. of water

How have trends in pressure sensor technology helped with these more frequent design requests? Overall sensor packaging has become smaller. Sensor die size is smaller due to improvements in manufacturing technologies, and integrated signal processing and calibration translate into few overall components within the sensor itself. Smaller sensors with many packaging options, such as surface mount, low profile, and with or without ports, eliminates the need for tubing and allows for the use of manifolds. Fewer components within the oxygen concentrator are needed to integrate the pressure sensor, causing less mechanical stress and better long-term reliability. Ease of field service is also improved if care is taken by the designer ahead of time to allow for easy access to critical components.

Another trend in sensing technology is the availability of more flexible sensor signal conditioning chips. The designer can now take a standard pressure sensor and, with the use of an SSC, customize a very narrow pressure measurement range as well as create the functionality of a pressure switch within the sensor itself. Thus, in an oxygen concentrator or conserver, a switch output can be created to increase the flow of oxygen at a certain set point. The pressure sensing output can continuously monitor the flow rate, which can then be reduced when the required threshold is met. The oxygen concentrator designer gets the best of worlds, a switch and a sensor combined in one component.

 

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Small size gives designers flexibility for space constrained areas

The availability of digital outputs such as I2C or SPI in pressure sensors creates for a faster response and more precise control of the oxygen concentrator. Direct interface to the systems microprocessor eliminates the possibility of signal drift over time. On board temperature calibration and amplified signals improves the performance range of the sensor, again assisting with portability requirements. Similarly, field calibration is not necessary, thus reducing service costs and increased functionality for the end user.

Design Implications
The many improvements in sensor functionality offer significantly more flexibility for the medical equipment designer. A fully packaged, signal conditioned sensor can be purchased for immediate installation. Alternatively, the availability of lower cost signal conditioning chips embedded with many features enables the designer to create a custom sensor solution by incorporating an unamplified pressure sensor and integrating a signal conditioning chip to meet a specific sensing requirement. The small size combined with low cost and included calibration allow for use in disposable applications.

In summary, power management, better accuracy, more sensitivity, and fewer components provide the ability to lower operating cost of the medical equipment, improve reliability, and increase functionality. With increasing market pressure to reduce medical costs, cost-effective and creative medical equipment solutions benefit both patients and manufacturers.

David Ezekiel is vice president of Servoflo Corp., a provider of pressure sensors, humidity sensors, and sensor signal conditioning. He can be reached at 781-862-9572 or david@servoflo.com.

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