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.

Fig. 1: Typical set-up of differential pressure flow measurement.

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.

Fig. 2: Thermal mass flow measuring principle.

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).

Continue on next page