# Glossary

Cv – The flow coefficient of a device is a relative measure of its efficiency at allowing fluid flow. It describes the relationship between the differential pressure across an orifice, and the corresponding flow rate.

Cracking Pressure – The differential pressure across a check valve, in the free flow direction, above which greater than 50 SCCM air flow will occur.

1psi = 1 pound-force per square inch = PSIA – Pounds per Square Inch Absolute is used to make it clear that the pressure is relative to an absolute vacuum rather than the ambient atmospheric pressure.

PSIG – Pounds per Square Inch Gauge indicates that the pressure is relative to atmospheric pressure.
At sea level PSIA = PSIG + 14.7 psi.

PSID – Pounds per Square Inch Differential is gauge pressure with the reference to something other than ambient atmospheric pressure. It is frequently used to describe the pressure across an orifice in a piping system or in the wall of a vessel where the upstream pressure and downstream pressures are something other than atmosphere.

PSIV – Pounds per Square Inch Vacuum is gauge pressure relative to atmosphere and is equal to a negative 1 times PSIG

SCFH – Standard Cubic Feet per Hour is a volumetric flow rate of a gas equal to 1 cubic foot of air at 70F and 14.7 psia* flowing past a point per hour.

SLPM – Standard Liters Per Minute is a volumetric flow rate of a gas equal to 1/1000 cubic meter of air at 70F and 14.7 psia* flowing past a point per hour.

*There are other standards used by different industries such as 0C and 1013 mbar which results in a different mass flow rate Common Pressure Unit Conversions:

The following are all equivalent to 1 psi:

 Psi Atms. Inches H2O mm H2O Cm H2O Inches Hg Mm Hg (Torr) mbar bar Pa (N/m2) kPa MPa Kg/cm2 1 .0681 27.71 703.8 70.38 2.036 51.715 68.95 .0689 6895 6.895 .0069 .0704

Common Volumetric Unit Conversions:
1 SLPM = 2.118644 SCFH
1 SCFM = 0.007867 SLPM
1 SCCM = 3.53107E-05 SCFM
1 SCFH = .472 SLPM
1 SCFH= 472 SCCM
1 SLPM = 1000 SCCM