What is a pressure gauge?

Answer 1

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.