Difference between sccm, ccm, and accm in leak measurement

From the Handbook

“Expressing leakage rates in the SI units of Pa m3 /s (i.e., at standard conditions) provides a leakage rate valid at any pressure.” The reason that standard units are used in science is to eliminate ambiguity in reporting results and defining specifications. Because the world uses these SI units like “scc/m” or “scc/s” as the standard units for measuring leak rates, it is occasionally shortened to “cc/m” assuming “scc/m.” Recently some companies are now stating their leak rates in “cc/m” but meaning “acc/m,” actual cubic centimeters per minute (which is volume of flow at the measurement pressure.)

The use of “cc/m” is now confusing. It is like stating “psi” versus “psig” or “psia” or “psiv.” “cc/m” does not define the conditions under which the volumetric flow is measured. At first glance “cc/m” is assumed to be the same measurement as “scc/m,” but depending on the test pressure there is a significant difference between “cc/m” when it means “acc/m” versus “scc/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.  
 
LR scc/m = {(20 psig + 14.7 psia)/(14.7 psia)}* LRacc/m 
 
LRscc/m = 2.36 * LRacc/m 
 
So a 1 scc/m leak rate would look like 0.42 acc/m at 20 psig on an actual “cc/m” instrument.

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.