Vacuum and Flow Measurement
Measuring volume flow in vacuum
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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
At 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 flow is greater than standard flow for the same gas flow.
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 | TemperatureoF | |||||||
| 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 |
- T(oC) = 5/9[T(oF) - 32]
- The calculation above is available as an excel file - vacuum.xls
- Altitude and pressure
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