What is a pressure gauge?
A pressure gauge is an instrument that measures the pressure in
a vessel, a line, or whatever the pressure gauge is connected to.
Pressure gauges come in at least two different types: differential
pressure gauges, and absolute pressure gauges.
Differential pressure gauges measure - surprise - DIFFERENCES in
pressure. Pressure gauges that read "zero" when not attached to
anything would actually be differential gauges that measure the
difference between atmospheric pressure and the pressure of
whatever they are attached to. Vacuum gauges are differential
gauges that measure how far BELOW atmospheric pressure the pressure
is in a vessel or pipe. Gauges that measure "gauge pressure" are
just differential gauges that are calibrated to measure zero at
atmospheric pressure.
Absolute pressure gauges would only read "zero" if they were
attached to an absolute vacuum. A common type of absolute pressure
gauge is a barometer. Strictly speaking, a mercury barometer is
really a differential gauge that measures the difference between
the vapor pressure of the mercury and the surrounding atmosphere,
but the vapor pressure of the mercury is so low that the error in
treating it as an absolute pressure gauge is generally
negligible.
There are many different kinds of gauges used to measure
pressure including:
Instruments
hydrostatic - These measure pressure according the height of a
liquid in a column. The height of the liquid is proportional to the
pressure. Common types of hydrostatic gauges include: manometers,
McLeod gauges, and piston gauges.
aneroid - The pressure sensing element may be a Bourdon, a
diaphragm, a capsule, or a set of bellows, which will change shape
with changes in the pressure of whatever the gauge is attached to.
The deflection of the pressure sensing element is read by a linkage
connected to a needle or by a secondary transducer. The most common
secondary transducers in modern vacuum gauges measure a change in
capacitance due to the mechanical deflection. Gauges that rely on a
change in capacitance are often referred to as Baratron gauges.
Electronic sensors
thermal conductivity - including two wire and one wire gauges
that measure pressure via changes in the thermal conductivity of
the wires as they are placed under strain. Something like this is
used in a lot of electronic bathroom scales.
ionization gauges - These are primarily used for measurements of
low-pressure gasses. They sense pressure indirectly by measuring
the electrical ions produced when the gas is bombarded with
electrons. Fewer ions will be produced by lower density gases. they
have to be calibrated against another type of pressure gauge and
depend on the gas being measured.
