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Monday - January 9, 2023

Cost Effective Flow Sensing Solution for Positive Airway Pressure Devices

Obstructive Sleep Apnea (OSA) is a common breathing disorder that occurs while individuals are . While we’re sleeping, our airwards are typically more relaxed. Sleep apnea can cause narrowing and collapsing of the airways during this relaxed state interrupting normal breathing. There are many people living with OSA – about 5-10% of people worldwide. Many more go undiagnosed since diagnosis typically involves participating in a sleep study which can be an uncomfortable experience. Routine treatment for obstructive sleep apnea employs a positive airway pressure (PAP) device that prevents a user’s airway from collapsing while sleeping.

There are various types of PAP machines: continuous (CPAP), bi-level (BiPAP) and automatic (APAP). CPAP machines deliver a constant level of air pressure while a BiPAP varies the pressure for inhalation and exhalation. APAP devices are able to deliver different air pressure rates based on exhalation levels. Positive airway pressure devices are used by patients at home, so they need to be easy to use and extremely reliable. In order to deliver the appropriate level of pressure and air flow, PAP equipment uses highly sensitive differential and gauge pressure sensors.

Traditional CPAP Sensors Setup

In a typical setup, a gauge pressure sensor is used to monitor system pressure while a differential pressure sensor measures airflow. The CPAP controller calculates air flow by correlating the drop in pressure across a flow restrictor. Both CPAP sensors are installed onto a PCB and a custom gasket is inserted over the sensor ports which seals the air pathway. As the blower generates air flow and pressure, a pressure drop occurs when the air is constrained by the flow restrictor and upstream pressure becomes greater than downstream pressure. The pressure drop value is used to calculate flow rate within the PAP device.

Gauge Pressure Sensors Versus Differential Pressure Sensors for CPAP Sensors

The main difference between a gauge pressure sensor and a differential pressure sensor is the reference pressure source. A gauge pressure sensor has an external vent that uses atmospheric pressure as a reference point, while DP sensors offer a second pressure port as a reference.

The structure of the DP sensor is more complex which results in higher costs and a larger footprint. By modifying the custom gasket, PAP manufacturers could substitute a more cost-effective gauge pressure sensor within the same pressure range as a DP sensor. Instead of sealing the gasket to the low pressure port of a DP sensor, the gasket would be designed to create a seal around the gauge pressure vent opening.

Reduce Costs With an Alternative Design for CPAP Sensors

In this recommended cost-saving design, two gauge pressure sensors are installed on the PCB of a PAP device. One has a standard 40cm of water rating, while the second is rated for ultra low pressure of less than 2.5cm of water. The custom gasket seals the port on each sensor to the same pressure aperture located upstream of the flow restrictor. The standard sensor is vented to the atmosphere while the ultra low pressure sensor vent is sealed onto a downstream aperture. When the blower is turned on, the two gauge pressure sensors operate in the same manner as a traditional setup. Please also watch our video for an in-depth explanation.

In summary, using a gauge pressure sensor in place of a differential pressure sensor can help reduce system costs since gauge pressure sensors are typically less expensive. MinebeaMitsumi offers the standard MMR920C04 sensor for upstream pressure measurement and is currently developing the MMR921C25, a new ultra low pressure sensor ideal for PAP downstream monitoring.

Ohlan Silpachai
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