Vacuum and Measurement of Flow

Measuring volume flow in vacuum

For standard or normal pressure it is common to measure flow in

  • standard cubic feet minute - scfm - imperial units
  • normal metric liter per second - nl/s - metric units

In vacuum the air or gas is expanded and it is common to use

  • actual cubic feet minute - acfm - imperial units
  • actual metric liter per second - al/s - metric units

Since the gas is expanded, actual volume flow is greater than standard volume flow for the same amount of gas.

The flow volume can be converted from standard volume flow to actual volume flow by the expression:

qa = qs (29.92 / p) ((t + 460) / 520)         (1)

where

qa = actual volume flow (acfm)

qs = standard volume flow (scfm)

p = actual pressure (in Hg)

t = temperature (oF)

Conversion Factor from scfm to acfm (or nl/s to al/s)
Pressure Temperature (oF)
in Hg absolute in Hg gauge kPa absolute 0 30 60 90 120 150
29.92 0.00 101.42 0.88 0.94 1.00 1.06 1.12 1.17
28.92 1.00 98.03 0.92 0.97 1.03 1.09 1.15 1.21
27.92 2.00 94.64 0.95 1.01 1.07 1.13 1.20 1.26
26.92 3.00 91.25 0.98 1.05 1.11 1.18 1.24 1.30
25.92 4.00 87.86 1.02 1.09 1.15 1.22 1.29 1.35
24.92 5.00 84.47 1.06 1.13 1.20 1.27 1.34 1.41
23.92 6.00 81.08 1.11 1.18 1.25 1.32 1.40 1.47
22.92 7.00 77.69 1.15 1.23 1.31 1.38 1.46 1.53
21.92 8.00 74.31 1.21 1.29 1.36 1.44 1.52 1.60
20.92 9.00 70.92 1.27 1.35 1.43 1.51 1.60 1.68
19.92 10.00 67.53 1.33 1.42 1.50 1.59 1.68 1.76
18.92 11.00 64.14 1.40 1.49 1.58 1.67 1.76 1.86
17.92 12.00 60.75 1.48 1.57 1.67 1.77 1.86 1.96
16.92 13.00 57.36 1.56 1.67 1.77 1.87 1.97 2.07
15.92 14.00 53.97 1.66 1.77 1.88 1.99 2.10 2.20
14.92 15.00 50.58 1.77 1.89 2.01 2.12 2.24 2.35
13.92 16.00 47.19 1.90 2.03 2.15 2.27 2.40 2.52
12.92 17.00 43.80 2.05 2.18 2.32 2.45 2.58 2.72
11.92 18.00 40.41 2.22 2.37 2.51 2.65 2.80 2.94
10.92 19.00 37.02 2.42 2.58 2.74 2.90 3.06 3.21
9.92 20.00 33.63 2.67 2.84 3.02 3.19 3.36 3.54
8.92 21.00 30.24 2.97 3.16 3.35 3.55 3.74 3.93
7.92 22.00 26.85 3.34 3.56 3.78 4.00 4.21 4.43
6.92 23.00 23.46 3.82 4.07 4.32 4.57 4.82 5.07
5.92 24.00 20.07 4.47 4.76 5.05 5.35 5.64 5.93
4.92 25.00 16.68 5.38 5.73 6.08 6.43 6.78 7.13
3.92 26.00 13.29 6.75 7.19 7.63 8.07 8.51 8.95
2.92 27.00 9.90 9.06 9.66 10.25 10.84 11.43 12.02
1.92 28.00 6.51 13.79 14.68 15.58 16.48 17.38 18.28
0.92 29.00 3.12 28.77 30.65 32.52 34.40 36.27 38.15
0.82 29.10 2.78 32.28 34.38 36.49 38.59 40.70 42.80
0.72 29.20 2.44 36.76 39.16 41.56 43.95 46.35 48.75
0.62 29.30 2.10 42.69 45.47 48.26 51.04 53.83 56.61
0.52 29.40 1.76 50.90 54.22 57.54 60.86 64.18 67.50
0.42 29.50 1.42 63.02 67.13 71.24 75.35 79.46 83.57
0.32 29.60 1.08 82.71 88.11 93.50 98.89 104.29 109.68
0.22 29.70 0.75 120.31 128.15 136.00 143.85 151.69 159.54
0.12 29.80 0.41 220.56 234.95 249.33 263.72 278.10 292.49

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