Leak Detection: Trace Gas Detection Method

Gas leak detection systems are used to “sniff” for a test gas which has been earlier introduced into the package.

Pira uses a Qualitek leakmeter 120 electron capture detector. The leakmeter 120 responds to SF6 and other electron capturing compounds and can detect leak rates as low as 1×10-7 ml/sec SF6.

The gas is injected into the test package and the injection point sealed with tape. The leakmeter fitted with a miniature “sniffer” probe is used to locate any leakage routes.
The technique is easily capable of detecting microleaks in non-porous medical device packaging and modified atmosphere (MAP) food packaging. In the latter application it is often used to identify leakage routes after a high oxygen content has been determined.
The technique is similar to helium leak detection which uses a mass spectrometer detector. One advantage of sulphur hexafluoride as a test gas is that it has a higher molecular weight that helium and less likely to give a false “leak” result from gas permeation through the packaging material.

There are several types of trace gas sensing systems on the market today. This diagram illustrates a simplified trace gas leak detector. The closed testpart is pressurized with a tracer gas, and the sensor is moved around the part to determine if and where there is a detectable leakage of the tracer gas. A constant flow pump allows measurement of varying concentrations of gas. Of course, using this detectoras a leak locator is easy, but using it as a quantitative leak detector requires skill! This instrument is also available for hazardous gases and work areas (intrinsically safe). This method gives quantitative measurements so standards can be precisely described.It is straightforward and reasonably easy to use, and the cost is low. However, because it is dependent on sensing a gas other than air, gas sensing is not useful for sealed packages that cannot be pressurized with a trace gas.

-The Mass Flow test method uses a small sensor that heats air and then measures the change in temperature with regard to mass flow. Since mass flow isa function of the density of air, care must be taken to check that the calibration is for sea level pressure, otherwise small differences may occur from place to place. The mass flow detector is less sensitive to temperature changes than pressure decay. Massflow meters vary in their response time depending on the level of leak rate; low rates can be longer especially if maximum values are exceeded.

-Mass Spectrometry is the most sensitive of all test methods, being capable of detecting 10-11 sccs using helium. The helium-pressurized test part isplaced in a chamber which is evacuated to a vacuum of 10-5 millibars, and any helium leakage is drawn into the mass spectrometer tube. The advantages to mass spectrometry include a very high level of sensitivity and a high degree of quantitative accuracy. However,mass spectrometry is extremely expensive, with equipment costs in the range of $25,000 – $100,000. The method is slow, and the cost to run and maintain the equipment is significant.

  1. Gas obtained from storage tanks or air fractioning may be used to displace headspace gases within food packages before closure.
  2. Concentration of gas within package must be greater than the concentration of that gas in the atmosphere where packages are tested.
  3. There are three modes for detection: ASTM E493, inside-out tracer mode (6); ASTM E498, tracer probe testing mode (7); and ASTM E499, detector probe testing mode (8).
  4. Slight compression of a package may assist the movement of gas molecules through microleaks.

POSITIVE

Detection of gas concentrations greater than the normal atmospheric concentration indicates break in hermetic barrier
of sample package. Confirm with dye testing to locate hole in sample package.

NEGATIVE

No detection of test gas concentration greater than the normal atmospheric concentration indicates hermetically
sealed container.

FALSE POSITIVE

Detection of gas concentrations in excess of the normal background level may result from increase in test gas
concentration in the testing area. Test background concentration before and after testing sample. Packages with high
permeability may lose gas.

FALSE NEGATIVE

Internal gas concentration may be reduced through absorption by the product, reaction with a component
inside the package, or permeability if over an extended storage period.

Sources:

  • VISCIANO s.a.s. Technoparc – 490 rue Appert, 01630 Saint Genis Pouilly, France. TEL +33 (0)4 50 20 87 78 (http://www.vigaz-analyser-detector.com/)TM Electronics, Inc. 45 Main Street , Boylston, MA, 01505. TEL (508)-869-6400 (http://www.tmelectronics.com/)ATC, Inc. 4037 Guion Lane, Indianapolis, IN 46268. TEL (317)-328-8492 (http://www.atcinc.net/)ARNDT., G.W. JR. 1998. Chapter 22C Examination of Flexible and Semirigid Food Containers for Integrity. FDA Bacteriological Analytical Manual (8th Ed)
  • ASTM F2338 – 09 Standard Test Method for Nondestructive Detection of Leaks in Packages by Vacuum Decay Method
  • ASTM F2227 – 02(2007) Standard Test Method for Non-Destructive Detection of Leaks in Non-sealed and Empty Medical Packaging Trays by CO2 Tracer Gas Method
  • Illinois Institute of Technology –┬áTrace Gas Detection Method