Pressure in the "old" metric system was indicated with several different units:
- as kp/cm2 where kp is coming from gravity, e.g.. 1 kp = 9,80665 N.
The gravity varies depending where you are.
- as atm which comes from the atmospheric pressure that was also indicated
as torr and measured as mm of mercury. 1 atm = 760 torr = 101,325
kPa = 14,696 PSI
- as "technical atmospheric pressure" at, this was used to so that the "ambient
air" was considered as 0 at. This helped to distinguish vacuum and pressure. 1 at
= 1 kp/cm2 = 98,0665 kPa = 14,5038 PSI
Common to these units was that they were not very "exact", the way they were
defined.
Pressure in the "new" metric system is indicated in "pascal" Pa.
1 Pa = 1 N/m2
1 N (newton) = 1 kgm/s2, this comes from "a force of 1 N
will accelerate an object of 1 kg at 1 m/s2.
When this SI system was introduced, it was realized that it can be difficult to
"understand" as 1 kp/cm2 is 100 000 Pa, too many zeros and even 100 kPa can be
confusing, so a "transient" unit was developed, a bar.
1 bar = 100 kPa ~ 1 kp/cm2.

Hope that helps

Regards