New technologies are advancing the accuracy and performance of ventilation equipment, spirometers, sleep-apnea treatment apparatus, and other devices that measure airflow in and out of human lungs and require careful attention to potential contamination from humidity and infectious particles.
Typical measurement techniques involve sensing a flow-induced differential pressure, detected in a shunt attached to the breathing tube. With the assistance of a small baffle, a small portion of the air flowing in a breathing tube enters a bypass hose that is connected to the sensor, causing a flow-induced differential pressure at two ports positioned along the side of the tube, as shown in Fig. 1.
Since it is important not to interfere with lung function, it is critical that the breathing tube, and especially the baffle, do not increase the flow-resistance during normal breathing. It is common practice that the overall breathing tube and baffle are designed to offer low flow-resistances.
Sensing of extremely low differential pressure
With airflows in the range of ~0.1 l/s for spontaneous respiration, up to ~7 l/s for forced-expiration, the differential pressures sensed in a shunt configuration are still very low—in the range of under one-hundred Pascals to a few thousand Pascals. The pressure differential at the two pressure ports increases roughly as the square of the main flow in the breathing tube. This severe non-linearity places extreme demands on the ΔP sensor. To accurately measure low flows to within ~1% accuracy, the pressure sensor must be able to overcome this nonlinearity while measuring ΔP over a dynamic range of ~104x or greater. Additionally, tough standards regarding resistance to contamination from humidity and infectious particles must be considered.
A new pressure sensor, the LBA from Sensortechnics, Walpole, MA, is a thermal-mass flow-based pressure sensor that is able to meet all of these needs in a mass-producible, cost-effective solution.
Pressure-from-flow sensors have been in use for some time, but their limitations have given many a designer cause for concern. In particular, dust and humidity can wreak havoc on the accuracy of the device, ultimately making the results unusable.
Unlike piezoresistive sensors, in the flow based device, air travels through a flow-channel that guides the air over a central heating element, which locally heats a small volume of gas. The heated volume is displaced by the flow in one direction or the other, which in turn unbalances the temperatures in a pair of temperature-sensors, positioned symmetrically on each side of the heating element as shown in Fig 2.
The airflow through the flow channel is determined by the difference in pressure between the two ends of the flow channel, and by the flow-impedance of the flow channel, measured in (pressure-difference) per (flow rate in ml/s).
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Therefore, flow-based pressure sensors can be used as sensors for differential air pressure, as long as the pneumatic impedance of the flow channel is consistent enough unit-to-unit, and high enough to not overly affect the ambient pressures, P1 and P2, at the two ends of the channel and high enough to minimize air leakage through the channel.
Until now, controlling pneumatic impedance in flow-based pressure sensors has been problematic because the critical dimensions determining the volume of air flowing through the sensor have been unnecessarily large and subject to wide manufacturing variations; primarily, the tolerance of plastic injection molded parts and their assembly tolerances. An LBA style sensor removes these inadequacies from the equation by controlling the minimum air-flow channel dimensions through the use of microelectromechanical systems (MEMS) technology at the silicon die level, as seen in Fig 3.
Using MEMS technology, the airflow channel’s critical dimension is etched into the sensor die during the silicon wafer manufacturing process, affording the device excellent repeatability, immunity to manufacturing and assembly variations, and controllability of the pneumatic impedance up to 200KPa/(ml/s). These high flow-impedances, which allow extremely low or no airflow through the sensor in operation, make LBA pressure-from-flow sensors nearly equivalent to piezoresistive pressure sensors in terms of resistance to contamination. During respiration, airflow in the LBA, typically a few nl/s, oscillates within the sensor and bypass hoses, preventing humidity and infectious particles from reaching the sensor element.
Additionally, long hose connections (longer than those conceptually shown in Fig. 1) and filters between the breathing tube and the ΔP sensor can be used without affecting calibration, even when the hose-connections have differing lengths/diameters, because the overall flow impedance is still dominated by the sensor flow channel geometry, not the hose-connections.
For the record
In summary, there are several reasons why the sensing technology described in this article will improve medical respiration measurements:
• Thermal mass flow sensing principle, combined with a flow channel having very high flow-impedance, allows accurate sensing of low differential pressures, over a wide dynamic range.
• Flow-impedance is predefined at the die-level, dramatically relaxing demands on subsequent packaging operations, resulting in a smaller, lower-cost solution.
• High flow-impedance also improves performance that otherwise might become impaired due to variability of connection hoses, changing gas filter properties, and humidified air.
• High flow-impedance makes the flow-based LBA pressure sensor, and any hose-connections to and from the sensor, easier to protect from contaminants.
• Sensor’s output voltage vs. ΔP curve is typical for pressure sensors based on thermal mass flow measurement, and does not vary significantly with minor manufacturing variations, up to approximately several thousands of Pa pressure.