Sccm, ccm, and accm Leak Rate Units

泄漏测量中SCCM，CCM和ACCM之间的差异

泄漏测试行业在美国已经标准化d on stating leak rates in units of “scc/m” or “scc/s” (atm cc/s) – standard cubic centimeters per minute or second. European industry uses “Pa-m3 /s” or “mbarl/s.” These are the SI gas leakage rate units defined in the Nondestructive Testing Handbook, Second Edition. These measurement units refer to the volume of gas leaking from the product in terms of standard conditions. Standard is referenced as 101.325 kPa (14.696 psia) and 293 K (20 C) in the “Handbook.” 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” verses “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” verses “scc/m.”

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:

(P_ATM) x (V_ATM/time）=（p_在职的) x (V_在职的/时间）

(P_ATM）x（LR_scc/m）=（p_在职的）x（LR_acc/m)

LR_scc/m= (P_在职的)/ (P_ATM）x（LR_acc/m)

压力表示为绝对压力（PSIA = PSIG + 14.7 PSIA）

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

LR_scc/m= {（150 +14.7 psia） /（14.7 psia）} x lr_acc/m

LR_scc/m= (164.7 psia/14.7 psia) x LR_acc/m

LR_scc/m= 11.2 x LR_acc/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）}* lr_acc/m

LR_scc/m= 2.36 * LR_acc/m

因此，在实际的“ CC/M”仪器上，在20 psig时​​，1 SCC/M泄漏率看起来像0.42 ACC/m。

Conditions for stating ccm units

对于将泄漏率称为“ CC/M”的应用，请检查以确保在什么条件下说明该体积流量，实际（在测试压力下）或标准（在标准大气条件下为14.696 PSIA）。标准条件有一些变化，因为温度是条件的一部分。“手册”指出参考温度为293 K（20 C）。对于其他一些科学情况，0 C和22 C被称为标准温度。

Advantages of sccm leak rates

标准条件是测量的首选单位，因为它们是所有流量测量行业普遍应用的。它将测得的气体流与从零件泄漏到大气条件的气体体积相关联。当一部分泄漏比另一部分泄漏更多时，很容易联系。因此，在10 psig下测试的2 SCC/m泄漏率将显示出比在50 psig测试的1 SCC/m泄漏率泄漏的气泡更多。相反，在10 psig处的2 ACC/M泄漏率将显示出比在50 psig测试的1 ACC/M泄漏率的泄漏少的气泡。使用“ SCC/M”，无论测试压力如何，相同数量的气泡将由1 SCC/m泄漏产生。

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

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