# Sccm, ccm, and accm Leak Rate Units

## What is the relationship of “standard” cubic centimeter per minute (scc/m) to ”actual” cubic centimeters per minute (acc/m)?

“acc/m” defines the volume of air at the unique test pressure and temperature at the point of measurement. Standard cubic centimeters per minute (sccm) defines the volume of air corrected to a standard pressure and temperature. Because air is a compressible fluid, there is a difference between “scc/m” and “acc/m” proportional to the relationship of absolute working (or test pressure) and standard pressure. Based on the Ideal Gas Law, the relationship is defined as:

(PATM) x (VATM/time）=（p在职的) x (V在职的/时间）

(PATM）x（LRscc/m）=（p在职的）x（LRacc/m)

LRscc/m= (P在职的)/ (PATM）x（LRacc/m)

For example when testing at 150 psig, what is the difference between “scc/m” and “acc/m” (or “cc/m”)?

LRscc/m= {（150 +14.7 psia） /（14.7 psia）} x lracc/m

LRscc/m= (164.7 psia/14.7 psia) x LRacc/m

LRscc/m= 11.2 x LRacc/m

So for a 150 psig test, the instrument displaying “scc/m” will show a leak rate that is 11.2 times greater than the leak rate displayed on an “acc/m” instrument. For the example above, 1.0 scc/m is the same leak rate as 0.089 acc/m at 150 psig. This discrepancy makes it look like an actual condition meter is performing a more sensitive test or finding a smaller leak, but it really is not.

For lower pressure tests, the difference is less dramatic. For example, a test at 20 psig would appear as follows.

LRscc/m= {（20 psig + 14.7 psia）/（14.7 psia）}* lracc/m

LRscc/m= 2.36 * LRacc/m

### Advantages of sccm leak rates

Pressure decay tests are usually calibrated using a leak standard with a certified flow at standard conditions. During the calibration procedure the instrument determines the pressure loss to standard leak rate relationship for the test part within the manufacturing plant conditions. Mass flow leak test instruments use a variety of flow meters and calibration methods. Thermal mass flow meters are traditionally calibrated to standard flow conditions. Laminar flow meters measure flow at working (test) conditions and then must correct for temperature and pressure to display standard conditions. Mass flow leak test systems using reference volumes must be calibrated to a leak standard to determine the volumetric relationship between the part and the reference volume. Systems that use a constant pressure sources do not require calibration to a leak standard.

## SCCM单元和ACCM单元的转换

test pressure to sccm

from sccm to accm
at a Test pressure

1 Torr 0.0193 0.0013 761.6580
14 psiv 0.7 0.0476 21.0000
10 psiv 4.7 0.3197 3.1277
5 psiv 9.7 0.6599 1.5155
4 psiv 10.7 0.7279 1.3738
3 psiv 11.7 0.7959 1.2564
2 psiv 12.7 0.8639 1.1575
1 psiv 13.7 0.9320 1.0730
0 psig 14.7 1.0000 1.0000
1 psig 15.7 1.0680 0.9363
2 psig 16.7 1.1361 0.8802
5 psig 19.7 1.3401 0.7462
10 psig 24.7 1.6803 0.5951
20 psig 34.7 2.3605 0.4236
30 psig 44.7 3.0408 0.3289
40 psig 54.7 3.7211 0.2687
50 psig 64.7 4.4014 0.2272
60 psig 74.7 5.0816 0.1968
70 psig 84.7 5.7619 0.1736
80 psig 94.7 6.4422 0.1552
90 psig 104.7 7.1224 0.1404
100 psig 114.7 7.8027 0.1282
120 psig 134.7 9.1633 0.1091
150 psig 164.7 11.2041 0.0893
180 psig 194.7 13.2449 0.0755
200 psig 214.7 14.6054 0.0685

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