A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Iwancio
Date: 2020 Sep 24, 11:18 -0700
If you can forgive the pun, the "bar" wasn't defined in a vacuum. In the deprecated centimeter-gram-second system of measurement units, it's a nice, round 1 megadyne per square centimeter. It was given its unique name because 1 Mdyn/cm2 is "close enough" to atmospheric pressure for a lot of applications. For example, if a pressure gauge on your steam engine reads 20 bar, adding 1 bar to determine absolute pressure is good enough for a lookup in your steam tables.
Touching on what Geoffrey mentioned, if you're looking through sources older than 50 years or so, 1 bar doesn't necessarily equal 100 kPa. As a practical matter, atmospheric pressure was determined by mercury barometers, and different organizations determined their standard density of mercury under different conditions (typically a variation in temperature and local gravity; see Bowditch tables 32 & 33). CGPM eventually had to weigh (ha!) in with their own standards:
- Standard gravity of 980.665 cm/s2
- Standard fluid density of 13.5951 g/cm3
- Standard fluid column height of 76.0 cm
CGPM eventually got away from manometric units entirely with the current 101,325 Pa standard (producing a difference in column height of less than 200 nanometers).