Units for pressure, length and viscosity

The common pressure unit of HPLC is bar, but the SI unit is pascal (Pa) 1 Pa = 1 N m. The atmosphere (atm or at, respectively) should no longer be used. The unit psi (pounds per square inch) is American and is still in use. Note the difference between psia = psi absolute and psig = psi gauge (manometer), the latter meaning psi in excess of atmospheric pressure. 

English units are often used in HPLC to indicate tube or capillary diameters, the unit being the inch (in or ). Smaller units are not expressed in tenths but as 1/2,1/4,1/8, or 1/16 in, or multiples of these. 

The SI unit of the dynamic viscosity is the pascal second 1 Pas= 1 kgm s Solvents have viscosities around 110 Pas= 1 mPas. The old unit was the centipoise (cP) 1 mPa s = 1 cP. 

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