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Air Pressure

Summary

Introduction

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.

Human Breath Pressure

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.

Maximum Respiratory Pressures

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.

Devices

Fujikura XFPN-025 Sources
Description -1.1 to 2.5 PSI Pressure Sensor Jameco US$ 24.95
Datasheet pdf
Resources

Notes

Variants Fujikura XFPN-050 (0 to 5 PSI Pressure Sensor)
Servoflo FPM07PG (0 to 7 PSI Pressure Sensor)
Motorola MPX10GS (0 to 1.45 PSI Pressure Sensor)

SenSym/Honeywell SX01GD2 Sources
Description 0 to 1 PSI Absolute Pressure Sensor Digikey US$26.82
Datasheet pdf
Resources

Notes

Variants SenSym/Honeywell SX05GD2 (0 to 5 PSI Absolute Pressure Sensor)
SenSym/Honeywell SX15GD2 (0 to 15 PSI Absolute Pressure Sensor)
SenSym/Honeywell SX01DD4 (0 to 1 PSI Differential Pressure Sensor)
SenSym/Honeywell SX05DD4 (0 to 5 PSI Differential Pressure Sensor)
SenSym/Honeywell SX15DD4 (0 to 15 PSI Differential Pressure Sensor)

Measurement Specialties 1451 Sources
Description 0-5,50,500 (depending on configuration) PSI Pressure Sensor Measurement Specialties US$ 28.33
Datasheet pdf
Resources Digikey part MSP6800-ND
Notes Piezoresistive silicon
Variants Wide Hole
Narrow Hole
Gel Fill
Tube

Media

Air Pressure sensor on a breadboard