Pressure Measurments

Scuba tank pressure reading

A full tank reads ~3000 psi (≈207 atm). Air escapes until internal pressure equals atmospheric pressure, at which point the gauge reads zero, though true pressure is still 1 atm.

Gauge pressure

Pressure measured relative to atmospheric pressure. Positive when greater than atmospheric, negative when less.

Absolute pressure

Total pressure, equal to gauge pressure plus atmospheric pressure. Minimum possible is zero.

Minimum gauge pressure

pg=−patm ​, which makes absolute pressure zero.

Pressure measurement principle

Any property that changes with pressure in a known way can be used to construct a gauge.

Strain gauge pressure sensor

Measures pressure by detecting shape changes in a material under stress.

Capacitance pressure gauge

Measures pressure via changes in electric capacitance caused by shape deformation.

Piezoelectric material

A material that generates an electric voltage when mechanically stressed, due to the alignment of internal electric dipoles.

Piezoelectric pressure gauge

Generates a voltage across a piezoelectric material under a pressure difference.

Ionization gauge

High-sensitivity device that ionizes gas molecules in a vacuum; the resulting current indicates pressure.

Manometer principle

Uses the relation p=hρgp = h \rho gp=hρg. With both ends open, atmospheric effects cancel.

One-sided manometer

Measures gauge pressure p_g=hρg by comparing fluid level difference to atmospheric pressure.

Manometer with higher pressure source

When pabs>patm, fluid is pushed down on the source side; gauge pressure is positive

Manometer with vacuum or lower pressure source

When pabs<patm, atmospheric pressure pushes fluid higher on the source side; gauge pressure is negative.

Barometer

Device using a mercury column to measure atmospheric (not gauge) pressure. Height h satisfies hρg=patm

1643

Year when the mercury barometer was invented

Weather forecasting with barometers

Rising mercury signals improving weather; falling mercury indicates worsening conditions.

Barometer as altimeter

Measures altitude, since average atmospheric pressure decreases with height.

mm Hg unit

Common unit for atmospheric and blood pressures, based on the height of mercury columns in barometers/manometers.

1psi = 6.895×10^3Pa

1psi = ?

1mm Hg = 133.3Pa

1mm Hg = ?