By Bryce Beamer
Pneumatic sensing is an essential enabling technology for ensuring precise control and monitoring for various medical applications. Sleep apnea equipment, ventilators, and even pneumatic beds use different types of pressure sensors. Understanding how these sensor systems operate — and defining the required performance for an application — is the first step in navigating the vast array of pneumatic sensors available. This article will delve into the applications of different sensing technologies to better equip product developers with the knowledge they need to streamline their development.
How do pressure sensors work?
The most common type of pressure sensors for most applications are piezoresistive strain gauges, composed of a semiconductor that exhibits a change in electrical resistance when subjected to mechanical strain. When you apply pressure to a diaphragm or membrane, it changes the electrical resistance of the strain gauge. The system converts this resistance into a precise measurement of the applied pressure.
Piezoresistive pressure sensors are perfect for many medical applications due to their small size, variety of sensing ranges, and low power requirements. Although the four primary types of piezoresistive strain gauge sensors share the same core mechanical functionality, their varied configurations make them suitable for different applications.
Gauge pressure sensors
Gauge sensors measure system pressure (p1) relative to the ambient atmospheric pressure (p2). Because atmospheric pressure is dynamic, gauge sensors are the right solution for systems where atmospheric pressure could impact the system pressure, such as in a soft bladder. Devices such as sphygmomanometers for measuring blood pressure, continuous positive airway pressure machines (CPAP), intermittent pneumatic compression sleeves, and pneumatic alternating pressure hospital beds commonly use these sensors.
Absolute pressure sensors
Absolute sensors measure system pressure (p1) relative to a vacuum-sealed reference chamber (p2), making them independent of atmospheric conditions. These sensors are ideal for applications that require vacuum conditions or absolute pressure measurements, such as specific surgical procedures.
Differential pressure sensors
Differential sensors measure the difference in pressure between two distinct environments (p1 & p2). BiPAP machines and ventilator pressure sensors are typically differential pressure sensors. Maintaining the correct pressure differential is crucial for effective respiratory support with these devices.
Sealed gauge sensors
Sealed gauge sensors measure system pressure (p1) relative to a fixed, sealed reference pressure (p2). Unlike absolute sensors, sealed gauge sensors are not referenced to a vacuum but are sealed at a pre-defined pressure. This type is helpful in applications where you want to measure pressure relative to a standard pressure not subject to environmental changes.
Illustrations by Studio KMD- High resolution and vector available.
Medical applications of flow sensors
Air flow sensors are used extensively in medical devices for monitoring and controlling gas flow. These sensors measure the velocity or volume of a gas that passes through a specific point within a given period. Like pressure sensors, air flow sensors use different core-enabling technologies to cater to various medical applications to measure volumetric or mass flow.
Volumetric flow
Volumetric flow sensors measure the volume of air that passes through a point per unit of time. Turbines or multiple pressure sensors are two standard methods for determining volumetric flow.
A turbine-based volumetric flow sensor measures the turbine's rotational speed in a pipe or duct. Typically, these sensors use an optical or magnetic transducer to detect the speed of the turbine and correlate that to the volumetric flow of the system. These turbine-based systems are versatile, but the mechanical rotation and mass of the turbine introduce inertia as well as potential wear and durability challenges are important drawbacks that should be considered before use. These sensors also present resistance in airflow that can cause pressure drops (from P1 to P2) in the pneumatic system. Early-stage detections of ventilation abnormalities frequently utilize turbine-based spirometers.
Pressure/orifice-based volumetric flow sensors operate differently; they introduce a restriction in airflow to generate a pressure differential that can be measured and correlated to the volumetric flow. As air travels from one pressure system through the orifice to another (P1 to P3), the air pressure drops as the air accelerates through the orifice (P2). Pressure changes are typically measured using differential or absolute piezo resistive sensors tuned for accurate flow monitoring. These systems tend to have fewer mechanical parts for improved durability. However, flow restriction and pressure drop might make it unsuitable for some applications.
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Mass flow
Mass flow sensors measure the mass of gas passing through a point per unit of time. The simplest form of thermal mass sensor is a hot wire anemometer. It measures the amount of current applied to a resistive heater to maintain a constant temperature in a probe. As air passes over the probe, more current must be applied to maintain the temperature. This configuration is adequate for analyzing a known gas's flow but cannot determine the flow direction.
Thermal mass flow sensors work on a similar principle but instead measure the change in temperature before and after (T1 & T2) the heating element at a constant current. This method provides directional flow that can target a variety of flow rates. These sensors are advantageous in applications that require a precise mixture of gasses, such as anesthesia delivery systems.
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Development path
Flow and pressure sensors are indispensable in the medical field, providing the critical data required for the safe and effective operation of various medical devices. Whether your application requires meticulous control of gas flow or the ability to withstand dynamic atmospheric conditions, the available sensor technologies offer a comprehensive array of options to meet these needs. By comprehending the underlying mechanisms and applications, product developers are better positioned to make informed decisions, thus streamlining the development process and ultimately delivering more efficient, reliable, and safe medical devices.
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