# Vacuum - Evacuation Time

## Vacuum pumps - calculating flow rate and required evacuation time

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The evacuation time for a vacuum pump can be calculated as

t = V / q ln(p_{0 }/ p_{1}) (1)

where

t = evacuation time (s)

V = enclosed evacuated volume (m^{3}, cu ft)

q = volume flow rate capacity of the vacuum pump (m^{3}/s, cu ft/s)

p_{0}= initialization pressure - normal atmospheric pressure (mbar, mmHg)

p_{1}= end vacuum pressure (mbar, mmHg)

Note! Leakage through sealing's etc. should be compensated by adjusting the volume flow rate of the vacuum pump.

(1) can be simplified to

t = V / q N (2)

where

N = natural log constant for the actual vacuum

N = 1 for vacuum up to 15 in Hg gauge

N = 2 for vacuum up to 22.5 in Hg gauge

N = 3 for vacuum up to 26 in Hg gauge

N = 4 for vacuum up to 28 in Hg gauge

### Example - Vacuum Evacuation Time

Evacuation time of *1 m ^{3}* volume, evacuated to

*500 mbar abs*- vacuum pump volume flow capacity

*0.1 m*- can be calculated as:

^{3}/s

t = (1 m^{3}) / (0.1 m^{3}/s) ln((1000 mbar) / (500 mbar))

= 6.9 (s)

Evacuation to *100 mbar abs:*

t = (1 m^{3}) / (0.1 m^{3}/s) ln((1000 mbar) / (100 mbar))

= 23 (s)

Evacuation to *10 mbar abs*:

t = (1 m^{3}) / (0.1 m^{3}/s) ln((1000 mbar) / (10 mbar))

= 46 (s)

*Vacuum *Evacuation Time Calculator

The calculator below can be used to calculate the vacuum evacuation time.

### Estimated Evacuation Time to Reach a 500-Micron Vacuum - 6 feet pipe

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