Vacuum - Expansion Air Ratio

Standard air volume flow - SCFM - can be converted to actual air volume flow - ACFM - with the expanded air ratio equation

EAR = (30 / P) ((T + 460) / 520)                                            (1)

where

EAR = ACFM / SCFM = expanded air ratio - ratio between actual air volume flow - ACFM - and standard air volume flow - SCFM

P = absolute vacuum pressure (inches Hg) where 29.92 inches Hg is normal atmospheric pressure and 0 inches Hg is absolute vacuum

T = temperature (^oF)

Actual air volume flow can be calculated by multiplying standard air volume with the expanded air ratio.

Expanded air ratio for temperatures 60^oF (15.5^oC) are indicated in the diagram below:

The diagram above can be used to calculate air flow in SI-units where

• actual air flow - al/s
• standard air flow - sl/s

Example -  Vacuum Expansion of Air 

From the table above the expansion ratio of air from atmospheric pressure to 50% vacuum is 2. 

The relation between vacuum in % and other pressure units are indicated in

• Vacuum in %, torr, mm Mercury and psi

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