Understanding absolute pressure sensors

Absolute pressure sensors are most commonly used to
measure changes in barometric pressure or as altimeters.
These applications require reference to a fixed pressure as
they cannot be simply referenced to the surrounding ambient
pressure.
Absolute pressure is defined as the pressure measured relative
to a perfect vacuum. For example, 10 pounds per square inch
absolute (psia) would be 10 psi above a perfect vacuum. This
is roughly 4.7 psi below the standard atmospheric pressure
at sea level of 14.7 psia. 0 psia is then the pressure of a perfect
vacuum.*
Sensym’s absolute pressure sensors are made by hermetically
sealing a vacuum reference chamber on one side of the integrated
circuit sensing element (see figure I). Pressures to be
measured are then measured relative to this vacuum reference.
The actual ,,vacuum”, which is sealed into the sensor
is approximately 0.0005 psia (25 millitorr). Using this near
vacuum as a reference eliminates any potential thermal errors
which would occur if any gas was trapped in the reference
chamber as it would exert a pressure during expansion and
contraction with temperature in accordance with Boyles law.
One of the advantages of integrated circuit sensors is the small
volume of trapped vacuum reference which, in conjunction
with a reliable silicon-to-silicon hermetic seal, makes these
devices time and temperature stable.

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