Frequently Asked Questions: Leak Testing

• "Actual cc/m" or "accm" 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" is the volumetric flow of gas corrected to standard atmospheric conditions (14.696 psia, 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 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.

The relationship between "actual cc/m" and "std cc/m is calculated as follows:

LRsccm = {(150 psig +14.7 psia)/14.7 psia} x LRaccm LRsccm = 11.2 x LRaccm

LRaccm = {14.7/(150 psig + 14.7 psia)} x LRsccm LRaccm = 0.089 LRsccm

• The industry recognized unit of measurement for “gas leakage rate” per the Handbook for Nondestructive Testing, Second Edition, Volume One Leak Testing is flow at standard conditions (14.696 psia, 293 K). This means that scc/m, Pa m3/s, or mbar l/s are the internationally recognized leak rate units.
• Use of “actual cc/m” which may be stated as “cc/m” by some individuals requires a qualifier for the pressure and temperature under which it was measured.
• Download application bulletin, AB141 Leak Rate Units

• There are several manufacturing methods to produce a hole. The important criteria is a long smooth entry, through hole and exit path. This will create repeatable, laminar flow through the hole. The method for manufacturing the hole greatly affects the cost.
   
• Because these 15 micron leak standards are used to calibrate leak test systems, the flow result of the leak standard (leak rate verses pressure) is the most important characteristic. Cincinnati Test System’s calibrated leak standards match the flow characteristics of the cylinder hole style equivalent channel leak standards.
   
• Cincinnati Test Systems has produced calibrated leak standards for the leak test market for over 18 years. These leak standards cover a wide range of leak rates and pressures and are available in a wide variety of compact mounting assemblies.
   
• Our A2LA certified calibration lab has perfected the manufacturing and calibration process to cost effectively produce leak standards to a very consistent specification (+/-1%) of specified leak rate value. Leak standards have a standard price of $475 that includes the calibrated standard, mounting assembly, and calibration certificate.
   
• Re-certification is available for $275 that includes free replacement of the leak standard if it does not flow within the allowable tolerance range.
   
• Download application bulletin, AB142 Equivalent Leak Standards, and specification sheet, Calibrated Leak/Flow Standards .

• The “Quik Test” function is a standard feature in the Sentinel C-20 and Sentinel M24 instruments.
• “Quik Test” evaluates the pressure decay curve within the test cycle and compares it to the original calibration curve. If it is within a specified +/-band of the original calibration curve at a defined time into the test cycle, the instrument will decide whether to accept, reject, or continue the test cycle.
• Since most production parts will test consistently close to the same near-zero leak rate, these parts will be quickly accepted. Since most reject parts are major leakers, they will be quickly rejected. Only the few parts that test as marginal accept or marginal reject will actually use the full test cycle time.
• Complete tests are performed on the marginal parts. Therefore, a quality test is performed on the critical parts that have leak rates close to the reject value. The obvious accept and obvious reject parts are quickly passed or failed.
• Download AB122 C-20 Quik Test.

• “Auto Cal” is a standard feature in the CTS products. It establishes the “No-leak” value and volume of the test part.
• For pressure decay instruments two automatic sequential tests teach the instrument how a “No-leak” part tests and determine the pressure loss to leak rate relationship (based on volume and test time) for the part.
• By testing a master good part the instrument detects the mass flow or pressure loss that occur during a test (using the timer settings) due to adiabatic temperature effects, test volume expansion, or virtual leaks.
• For the mass flow instruments no second test is required when a constant pressure source (regulator) is used as the reference pressure to the flow meter. If a constant pressure volume is used as the reference pressure, a second test is required to determine the ratio of the part volume to the total system volume that includes the constant pressure volume. When a constant pressure volume (no active regulator in line during test) is used, the measured flow is a fraction of the actual leak rate from the part.
  
  FLOWorifice = (FLOWmeas – FLOWno-leak) x (Total system volume/constant pressure volume)
   
• Download application bulletin, AB121 Automatic calibration procedure (AUTOCAL) and Process Drift Correction (Zero shift).

• “Process Drift” will minimize the need to re-calibrate the instrument several times a day. It will maintain the accuracy of the test and eliminate the acceptance of marginally bad products in a plant environment with cyclical changes in the environment of the test.
• Pressure decay and mass flow instruments measure changes in flow or pressure loss caused by leaks relative to the flow or pressure loss reading of a non-leaking part determined in AUTO CAL. Because these test methods are affected by the variables of the Ideal Gas Law (PV=nRT), temperature changes, volume changes, and virtual leaks influence the measurements.
• In the manufacturing environment there will be changes in temperature, part volume, and/or virtual leak characteristics. These changes will cause the baseline (no-leak parameter) of the calibration to be inaccurate. (The flow or pressure loss value associated with no-leak will not be correct.)
• The “Process Drift” function monitors the test results and automatically calculates corrections for the calibration data to adjust the flow or pressure loss value associated with no-leak to track the production no-leak results.
• Download application bulletin, AB121 Automatic calibration procedure (AUTOCAL) and Process Drift Correction (Zero shift).

• 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.
• The Sentinel instruments have a programmable output that activates during the exhaust cycle of the test. This output will active the valve to direct the exhaust out this externally mounted valve.
• Download application bulletin, AB138 External Exhaust Valve.

• containers called the Sentinel MD instrument. Integrated onto a test stand this instrument will detect hole sizes down to 40 microns.
• Download application bulletin, AB124 Contact Lens Packaging and Sentinel MD specification sheet for more information about this process.

• First one must accept that everything leaks something over time. The leakage may be several molecules of gas per year to some higher amount. Once you accept that everything leaks, you must quantify a tolerance amount that meets the part’s functional requirements.
• Download application bulletin, AB120, How to Establish an Acceptable Leak Rate Criteria for Automated Testing.

• 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) and bubble frequency (bubbles/minute). With this information you can use the calculator found here. It uses the following formula to calculate the leak rate.
• Leak rate = (3.14 x (dia)3 x 0.001/6) x frequency

• The conversion for leak rate at a specified pressure to hole size is not a simple correlation. The flow rates through a given hole size vary depending on the regime of flow which is controlled by the viscosity and density of the fluid, differential pressure across the hole and outlet pressure vs. inlet pressure, and the path length through the hole.
• There are several flow regimes (turbulent, laminar, and molecular) that exhibit different flow verses hole diameter relationships. In the laminar flow regime, Poiseuille’s equation can be used to approximate the hole size to leak rate relationship. At higher flow rates, flow transitions toward turbulent flow and Poiseuille’s equation predicts higher flow rates than reality. As flow transitions to molecular flow, Poiseuille’s equation predicts flow rates that are lower than reality.

• Because pressure decay and mass flow technologies work on the Ideal Gas Law (PV=nRT), variation in temperature “T” will affect the relationship of dP/dt V = dn/dt RT. The calibration function of our Sentinel instruments detects the baseline dP/dt due to changes in volume and temperature as a tare factor for future tests. As long as the changes in volume and temperature are consist, the instrument will automatically correct for the changes.
• When there are deviations in the change in volume or temperature during the test time of the production test with a given calibration, there will be repeatability or drift issues that affect the tests results. The percentage affect of these changes depends on how sensitive of a test is being conducted.
• Refer to the calculator, click here, to estimate the tolerable change in the temperature change curve and still meet a gage R&R requirement.

• Because pressure decay and mass flow technologies work on the Ideal Gas Law (PV=nRT), variation in temperature “V” will affect the relationship of dP/dt V = dn/dt RT. The calibration function of our Sentinel instruments detects the baseline dP/dt due to changes in volume and temperature as a tare factor for future tests. As long as the changes in volume and temperature are consist, the instrument will automatically correct for the changes.
• When there are deviations in the change in volume or temperature during the test time of the production test with a given calibration, there will be repeatability or drift issues that affect the tests results. The percentage affect of these changes depends on how sensitive of a test is being conducted.

• Download AB143 Derivation of Pressure Decay to Leak Rate formula.