Vacuum - Flow Measurement

Measure air 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:

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)

For full table - rotate the screen!

Vacuum - Flow Measurement Conversion Factor
Pressure			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