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Vacuum - Air Expansion Ratio

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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)


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 60oF (15.5oC) 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

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