Vacuum and Flow Measurement

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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

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:

q_a = q_s (29.92/p) ((t + 460)/520) (1)

where

q_a = actual volume flow (acfm)

q_s = 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

T(^oC) = 5/9[T(^oF) - 32]
The calculation above is available as an excel file - vacuum.xls
Altitude and pressure

Measuring volume flow in vacuum

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