# Glossary

**C _{v} - 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

**Frequently Requested Downloads:
**Fundamentals of Flow Measurement

Choked Flow of Gases

Technical Considerations

Archived Catalogs