atmospheric pressure unit

mb(millibar) is a sub-unit of "bar". One bar equlas to 1000 millibar. Millibar is a non-SI unit, mostly used in meteorology to indicate atmospheric pressure. 1 bar (or 1000 mb) is close to a standard atmospheric pressure.

The conversion between bar/mb and Pa (Pascal) is,
1 bar = 1000 mb = 100 000 Pa
[One standard atmospheric pressure = 101 000 Pa = 1.01 bar, or 1010 mb]

You mat refer to the following web-page for more conversion factors:

http://www.sensorsone.co.uk/pressure-measurement-glossary/bar-pressure-unit.html