Friday, May 27, 2016
VOLUME -25 NUMBER 12
Publication Date: 12/1/2010
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ARCHIVE >  December 2010 Issue >  Special Features: Test and Measure > 

The Leak Test "Bubble" — Choosing the Right Technology
Sentinel I-24 precision leak test instrument.

It is the simplest and sometimes the cheapest form of leak testing: seal your part, attach a shop air hose to the part and hold it in a tub for a few seconds looking for bubbles. We all have done it and we all have gotten wet when a major leak erupts in the water. However the time comes when it is not very efficient to give our operators rubber gloves to hold parts underwater all day long. This impasse is when improvement decisions are made to finally invest in leak testing technology that will test production parts without depending on an operator's vision and attentiveness.

To choose the right technology, it is important to understand the choices available on the market, the cost of each, their complexity and most importantly the type of leak the part can have while continuing to function correctly. In the leak testing industry we have a saying "Everything leaks, so how much leakage is acceptable?" Acceptable leak rates are identified to set limits for evaluating production parts' leakage. Bubbles that are emitted by a dunked part can be correlated to a leak rate as a volumetric measurement of air loss over a set amount of time. The volumetric measurement is identified in standard cubic centimeters per second or per minute — depending on the level of allowable leak. The level of allowable leak also dictates which technology will be selected for measuring leaks.

The technologies available for measuring leaks range from simplistic to complex. Getting past the simplistic approach of operator dependency and dunk testing, Used for approaching the moderate scale for operator-independence are pressure measurements of pressure decay, vacuum decay and mass flow. And there are more complex operator independent techniques: tracer gas measurement technologies using sniffing, accumulation and hard vacuum mass spectrometry. The main factor leading us to choose a technology for repeatable leak test measurement is the acceptable leak rate. Setting an acceptable leak rate can include many factors, but mainly depends on what material shouldn't leak into or out of the part. When testing for liquid or gaseous leakage, the viscosity of the material can be taken into account. For example, air will travel through a leak path 80-300 times faster than water will travel through the same path (flow is dependent upon the length and internal contours of the path). Eventually a leak path will become small enough that water molecules will clog the leak path but air will still continue to flow through. Non-water leak rates are in the range of 1-5 SCC/Min (standard cubic cm/minute) air flow and sometimes as high as 10 SCC/Min. These values depend on the characteristics of the leak path and pressure. As the leak path becomes smaller, air flow is reduced and will transition from viscous flow to molecular flow.

Another factor for choosing the right technology is selecting a measurement device that will supply 100 times greater resolution than the leak rate being measured.

Pressure Based Technologies
The three different test methods utilized for pressure based technologies are Pressure Decay, Vacuum Decay and Mass Flow. Each of these techniques utilizes pressurized air as their media and conducts measurements that are correlated to a volumetric flow of air as the part leaks.

Pressure Decay and Vacuum Decay measurements are based on the same technique. The part is pressurized/evacuated to pressure by a regulator. Once test pressure is achieved, the regulated air is stopped and pressure is isolated in the part and then a stabilization time allows for pressurization characteristics to equalize. Next a pressure drop over a set amount of time is measured. To achieve a volumetric flow measurement, this technology is calibrated to a certified flow rate orifice.

Mass Flow measurements are based on pressurizing a part with the use of a regulated source, maintaining the pressure on the part, then measuring the amount of air flowing through a mass flow sensor into the part as it loses air volume.

In these technologies, the resolution and repeatability are based on a pressure differential occurring due to the leak. This measurement dictates the minimum resolution for this technology because the biggest culprit for noise is due to temperature change either in the part or the pneumatic measuring circuit. Other factors that affect resolution for measuring flow are the volume in the part and the pneumatic circuit, sensor repeatability, regulation repeatability and the calibration method of the system.

Repeatable Flow Rate
With these technologies, a repeatable flow rate measurement depends on test pressure and part characteristics. But in general terms, testing capabilities for Mass Flow technology is as high as hundreds of SL/Min to as low as 0.5 SCC/Min, depending on the range of flow meter selected for the test. Pressure and vacuum decay technology can be in the thousands of SCC/Min to as low as 0.5 SCC/Min. Utilizing either technology might go as low as 0.1 SCC/Min but is highly dependent on the temperature stability of the test, the part characteristics, its size (smaller volume size is better), the type of test system regulation, and flow rate calibration.

When it is not possible to accommodate repeatable leak testing utilizing pressure based technologies or the leak rate is well below 1 SCC/Min, Tracer gas technologies are selected to accommodate low level leak measurements.

Tracer gas technologies are utilized to test leak rates well below 1SCC/Min by using a gas that is detected by a device as the gas leaks from the part. The common devices utilized are helium mass spectrometers, and less frequently hydrogen detection devices and Random Gas Analyzer systems. The most common gas utilized for tracer gas detection is Helium. Helium is a non-reactive gas found in our atmosphere at 5.24 ppm. When using Helium as a tracer gas, it is possible to control the sensitivity for measurement by controlling the percentage of Helium mixture to pressurize the part (5 percent up to 100 percent).

Hydrogen, on the other hand, is a reactive gas that occurs 0.55 ppm in our atmosphere. However it is usually used in a 5 percent mixture with Nitrogen due to its flammable nature above that percentage. The percentages of gaseous mixtures are important when addressing the sensitivities of tracer gas technologies and the leak rate levels necessary to detect leaking parts. When higher leak rates are being tested, low percentages of tracer gas mixtures can be used to keep from filling the test area with large amounts of the tracer gas. Then on the other end of the spectrum, when testing for low leak rates, higher percentages of trace gas allow greater sensitivity for the testing process.

Tracer Gas Testing
There are two levels for tracer gas testing. Level one is measuring for a tracer gas leaking into atmosphere and level two is measuring a tracer gas leaking into a vacuum atmosphere. The simplest form of a tracer gas test is to pressurize the part with the gas mixture and use a hand held sniffing wand to pass over the leaking part. As the part leaks, the gas will be pulled into the end of the wand and sensed by the device. This method is an attribute test utilized to find a leak location; its sensitivity is based on the speed that the operator moves the wand across the leak. Typical sensitivity is in the range of 1 x 10-3 SCC/Sec with both Helium and Hydrogen. This type of testing is also highly dependent upon the operator.

To accommodate a more repeatable leak test with tracer gases, the part is placed into a test chamber, sealed and a vacuum is pulled on the inside of the part to remove the atmospheric air. Removing the atmospheric air is important to make sure all portions of the part receive the tracer gas mixture when the part is charged. With the proper evacuation and charge of the gas mixture, a leak anywhere in the part will be leaking the gas mixture verses possibly just atmospheric air.

The closed test chamber is at atmospheric pressure with a baseline gas ppm. As the part leaks, the percentage of trace gas will increase in the chamber over time. The tracer gas rate of rise in the chamber is measured by the tracer gas detection device and correlated to a leak rate. Typical sensitivity for this type of test system is 1 x 10-5 SCC/Sec and is dependent upon the size of the free air space in the test chamber, the type of tracer gas, percentage of tracer gas mixture and the tracer gas background levels. With 5 PPM Helium background in the atmosphere, lower leak rates are only possible by removing the background gases from the chamber.

To accommodate smaller leak rate measurements, the part chamber is evacuated to low vacuum levels and a Helium Mass Spectrometer measures Helium molecules emitted from the part into the test chamber. The low leak rate testing range for this technology is 1 x 10-8 CC/Sec, sometimes 1 x 10-9 CC/Sec.

Transitioning from bubble or dunk testing to measurable leak test techniques provides the ability to test parts within design parameters and to set their own limits. Applying the correct testing technique at 100 times the required leak rate resolution will also allow repeatable and operator-independent test measurements. Performing leak tests are made easier and more accurate by proprietary test algorithms processed on an advanced 32-bit processor from Cincinnati Test Systems called the Sentinel I-24 precision leak test instrument — the fastest, most repeatable and accurate leak test instrument on the market. An operator can program up to five discrete tool motions from the I-24 menu. Also, startup is a snap with time-saving Auto Setup.

Contact: Cincinnati Test Systems, Inc., 5555 Dry Fork Road, Cleves, OH 45002 513-367-6699 fax: 513-367-5426 E-mail: sales@cincinnati-test.com Web:
http://www.cincinnati-test.com

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