Feature Article

Sensing extremely low differential pressure
Measuring airflow in and out of human lungs requires careful attention to potential contamination from humidity and infectious particles. Whether such measurements involve ventilation equipment, spirometers, sleep-apnea treatment systems or other devices, new technologies are being developed that advance the accuracy and performance of these machines.

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 flow-induced differential pressure at two ports positioned along the side of the tube, as shown in Figure 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 flow resistance during normal breathing.

Figure 1: A typical differential pressure flow measurement setup.

With air flows in the range of ~0.1 L/sec for spontaneous respiration and up to ~7 L/sec for forced expiration, the differential pressures sensed in a shunt configuration are still very low—in the range of less than 100 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 nonlinearity places extreme demands on the ΔP sensor.

To accurately measure low flows within ~1% accuracy, the pressure sensor must be able to overcome this nonlinearity whilst measuring ΔP over a dynamic range of ~104× or greater. Also, resistance to contamination from humidity and infectious particles must be considered. A thermal mass flow–based device is able to meet these needs in a mass-producible, cost-effective solution.

Figure 2: The thermal mass flow measuring principle.

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.

In the flow-based device, air travels through a flow channel and is guided 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. In turn, this unbalances the temperatures in a pair of temperature sensors, which are positioned symmetrically on each side of the heating element as shown in Figure 2.

Figure 3: The principle of the LBA silicon sensor chip is shown here in a cross section.

Air flow 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/sec).

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 sufficiently consistent from unit to unit so as not to overly affect ambient pressures (p1 and p2) at the two ends of the channel and high enough to minimise air leakage through the channel.

MEMS to the rescue
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. The new LBA style sensor removes these inadequacies from the equation by controlling the minimum air-flow channel dimensions through the use of MEMS technology at the silicon die level, as seen in Figure 3.

Figure 4: A simplified comparison of air flow using LBA style and conventional sensors in respiration applications.

Using MEMS technology, the air-flow channel’s critical dimension is etched into the sensor die during the silicon wafer manufacturing process, affording the device repeatability, immunity to manufacturing and assembly variations, and controllability of pneumatic impedance up to 200 KPa/(ml/sec). These high flow impedances, which allow extremely low (effectively no) air flow through the sensor in operation, make LBA pressure-from-flow sensors nearly equivalent to piezoresistive pressure sensors in terms of contamination resistance. During respiration, air flow in the LBA, which is typically a few nanolitres per second, oscillates within the sensor and bypass hoses, preventing humidity and infectious particles from reaching the sensor element. Additionally, long hose connections and filters between the breathing tube and ΔP sensor can be used without affecting calibration, even when the hose connections have different lengths/diameters, because the overall flow impedance is dominated by the sensor flow channel geometry, not the hose connections.

The thermal mass flow sensing principle, combined with a flow channel that has a 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 and resulting in a small, low-cost component. The high flow impedance also improves performance that otherwise might become impaired due to variability of connection hoses, changing gas filter properties and humidified air. The high flow impedance makes the flow-based LBA pressure sensor, and any hose-connections to and from the sensor, easier to protect from contaminants. The sensor’s output voltage versus 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 thousand Pa pressure.

Dr. Adriano Pittarelli
is Senior Product Manager at Sensortechnics GmbH; tel. +49 8980 0830
E-mail: info@sensortechnics.com
www.sensortechnics.com