Air Pressure

Air pressure sensors are commonly used to measure breath pressure. They usually incorporate a diaphragm and strain gauge. A pressure chamber contains an inlet port and an outlet port, which are separated by a diaphragm. The diaphragm is mechanically linked to a strain gauge.

Depending on the thickness of the diaphragm, it is classified as either a membrane (thicker) or thin plate (thinner). In either case, the diaphragm will deform in a predictable fashion under pressure. The deformation is mechanically translated through the strain gauge. The resistance of the strain gauge will change depending upon the deformation it undergoes, and this resistance can be translated into an output voltage.

There are three main types of air pressure sensors:

• Absolute pressure sensors. These have an internal vacuum reference and an output voltage proportional to absolute pressure. There is only one inlet which allows air pressure to be applied to one side of the diaphragm.
• Differential pressure sensors. These allow application of pressure to either side of the diaphragm and can be used for differential pressure measurements. They have two inlets, each leading to opposite sides of the diaphragm. The output voltage of the device is proportional to the pressure difference across the diaphragm.
• Gauge pressure sensors. These measure pressure compared to ambient pressure. They function in a similar way to the differential pressure sensors but without a nipple or other attachment for the ambient port.

When measuring human breath pressure, relatively low values of pressure are needed. Two of the citations below show maximum pressure generated by human lungs, with trumpet players generating greater pressure than a general caucasian population. Still, these values are from static pressure generation and values measured from free-flowing air will be significantly lower.

This graph from Wilson et al. (1984) shows a maximum generated pressure for adult males during exhalation (PE) to be 148 cmH20 which is just 2.1 psi.

Another simple way of measuring free-flowing breath is with a simple or cheap microphone and an envelope-following circuit.

• Bicking, Robert E. (Honeywell), 2005. Fundamentals of pressure sensor technology, Sensors Online.
• Manometer
• Fiz JA et al. Maximum respiratory pressures in trumpet players., Chest. 1993 Oct.; 104(4): 1203-4.
• Wilson SH, Cooke NT, Edwards RH, Spiro SG. Predicted normal values for maximal respiratory pressures in caucasian adults and children., Thorax. 1984 Jul; 39(7): 535-8.