Unfortunately the multiplicity of units available in pressure and vacuum metrology causes considerable problems, both to newcomers and experienced practitioners alike. Fortunately, though, life is getting easier as the obsolete and ill-defined units disappear in favour of the pascal.

Many old pressure units have obvious practical and historical origins; for example, inches of water was the unit used where pressures were measured with a water column whose top surface was sighted against an inch scale. Initially the measurement accuracies required of such systems were consistent with fairly crude measuring techniques and no one bothered too much whether the water was hot or cold. Pressure is defined as force-per-unit-area and all manner of 'force' and 'area' (or rather 'length-squared') units were invented and adopted to suit local convenience and fashion. As technological demands increased, the need for more consistent units emerged; definitions were refined to take account of variations in fluid density due to temperature and purity, variations in gravitational acceleration etcetera, and the mathematical models of the measuring instruments were refined considerably. For example, in one traditional design of mercury barometer allowance was (and still is) made for the differential expansions between the mercury in the column, the glass from which the column is made, the brass from which the scale is made and a steel reservoir.

The mathematics used to calculate more accurate values of pressure from instrument readings often used arbitrary datum values but unfortunately manufacturers often picked alternative ones. For temperature it might have been 0 ºC or 68 ºF; for gravitational acceleration it might have been the value associated with standard conditions or a value 'helpfully' modified to take account of the location, such as London laboratory conditions. Some barometers even used different conditions for adjacent scales, making it impossible to compare one with the other properly!

Even with refined definitions and associated mathematics, however, many of the traditional units cannot be used at the limits of modern technology - their definitions are simply not adequate and cannot be made so.