FAQ

Question
What is the difference between “actual cc/m”, “std cc/m” and cc/m?

Answer
"Actual cc/m" or "accm" stands for actual cubic centimeters per minute. It is the volumetric flow measured at working conditions (at test pressure). The actual volume of the gas is compressed by the ratio of the absolute test pressure verses standard pressure (14.696 psia). When stating flow using this terminology, it is imperative that the test pressure and temperature be stated with the flow value. In other words, a 5 accm flow varies with every different pressure and temperature when comparing it to standard conditions.

"Std cc/m" or "sccm" stands for standard cubic centimeters per minute. It is the volumetric flow of gas corrected to standard atmospheric conditions. The standard conditions that have generally been agreed upon by the Leak Test Machines industry and most other industry is 14.696 psia and 293 K (20 C). Volumetric flow with this designator "std" is always adjusted to the standard pressure and temperature conditions above. Therefore, a 5 sccm leak rate is the same flow at any pressure.

"cc/m" stands for cubic centimeter per minute. It does not reference standard or actual conditions so it is an abbreviation for either one. Historically the Leak Test Machines industry and most every other industry has universally embraced stating flow in terms of standard conditions because it eliminates the need to stipulate pressure and temperature. Because most everyone refers to leak rates in "sccm", they usually abbreviate it to "ccm". Today some individuals are using "ccm" to refer to "accm". Because of this, it is very important to clarify how this flow is defined - at standard conditions or at a stated pressure and temperature.

Question
What is all the talk about a 15 micron hole? What is a 15 micron hole?

Answer
Some companies in the fuel component portion of the automotive industry have begun specifying leak rates based on a hole size and a leak path length. Through research, companied have determined that a 15 micron hole is a suitable size for proper functioning of components that are in contact with liquid hydrocarbon fuels. A 15 micron hole is a hole with a 15 x 10-6 m (0.000015 meter) diameter. As mentioned above, the leak path length must also be specified (typically 1000 times greater than the hole diameter or 1.5 mm). The area and path length of the hole together control the effective flow rate at different differential pressures across the hole or leak. This may be used as the Leak Test Machines specification.

Because leaks are usually not round holes and cannot be located and measured for diameter or area, flow or leak rate instruments must still be used to gage parts for these effective holes. By using a leak standard with an effective hole size (derived from existing leak rates measured at specific pressures), tests can be conducted at higher or lower pressures to find the same effective hole.

Question
How do you decide what is an acceptable leak rate specification?

Answer
The best way to determine an acceptable leak rate is to define what is required of the product in order to function properly. In some case this is very straight forward; for instance in the medical industry, it is vital that organisms not enter sealed containers housing sterilized equipment. You can use the size of the smallest microorganism as the hole size and conduct an experiment to determine a leak rate for this size hole at a particular pressure.

Question
Is there any way that I can estimate my leak rate if I am currently performing a dunk tank test?

Answer
Yes. However, this method will only give a rough approximation. The conversion of a bubble test to a leak rate involves calculating the amount of air escaping the product. To calculate the volume of air you must estimate the bubble diameter in mm (a sight card with various hole diameters can help) and the bubble frequency in bubbles/minute. With this information you can use the following formula to calculate the leak rate.

Where D is in mm

This formula will produce a result in the units of sccm.

Question
How can I avoid exhausting the dirty air from my part through the Leak Test Machines?

Answer
You can mount a normally closed two-way valve on a tee off the test line or a three-way valve on the test line.