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Kind of leak testing

For leak test we mean a proceeding that can verify the pneumatic tightness of a piece. Unlike most of the types of measurement, such as the one dimensional or weight, the leakage detection almost necessarily a leak tester device.

We recognize above all two kinds of equipments for leak testing:

       A) Verification system, generally controlled by an operator, with location of the leak point:
  • Test in water with the piece under pressure (visual inspection)
  • Test with soap with the piece under pressure (visual inspection)
  • Test with reagents under pressure and UVA lamps (visual inspection)
  • Test with gas (Helium) under pressure (control with a mass spectrometer)
  • Test with hot air (visual inspection at infrared)
  • Test with variation of dielectric on plastic pieces (ion high-voltage system)
  • Test with hydrogen gas (localization inspection)
       B) Automatic systems with indication of Good, Waste and value of the leakage:
  • Measure by the flow measurement of the piece under pressure.
  • Measure by the differential decay of pressure between the piece under test and the sample of reference
  • Measure by the pressure decay of the piece under pressure
  • "Interception" measure of pressure increasing under bell

If on one side the first class of equipments (A) represents an irreplaceable area test in statistical control and off-line, allowing to find very small leaks and analyzing the fault directly in a visual way, the equipment of kind (B) represents the effective "barrier" or filter of end of line concerning the non-conforming production.

The equipment of kind (B) allows also to display during the time any drift of quality, if applied on all the production.
Considering the evidence of working principles of measuring systems of kind (A), we shall not remain on a detailed technical description.

We have only to consider that the systems based on mass spectrometers (helium "sniffer"), even if expensive both in terms of plant and of management, they are the top for sensitivity in terms of determinable leak compared to any other system described in the present document.

On the contrary, the systems with water, soap or reagents, if on one side they allow to detect very small leaks with cheap operating cost, they cannot be automated and then they need necessarily a visual inspection, and in this way an evaluation by an operator.

Unlike most system of units, as for example that measurable or that dimensional, the loss control requires almost necessarily a machinery. The right test method selection depends generally on the following parameters:

  • Admissible loss rate value
  • Test type: loss location or loss measurement
  • Specific list of the tested detail: dimensions, pressure and empty resistance limit, packaging materials, surface finishes etc...
  • Utilization and test conditions
  • Security and environment parameters
Some of the applicable methods are quoted in the following schedule:
Method Gas Test type Sensitivity
[ Pa m³/Sec]
Tracer gases and Spectrometer Helium Local./ Pass-No Pass 10-11 ... 10-6
"In bell" compliance interception test Air Pass-No Pass 10-6
Loss pressure test Air Pass-No Pass 10-5
Volume flow rate or mass flow test Air Pass-No Pass 10-4
Visual inspection in water tank and pressure air Air Visual 10-4

The Helium method is not tested, as it is not contemplated in our production. It is considered in fact that the system is placed at the height of the sensitivity and the equipment and management costs make it applicable only where it is really necessary, that is to say on the limits of components for refrigerant gases, microelectronics, pace-maker, etc.

The immersion test is not mentioned here too, because it has none of the technical abilities, but for the fact that it can really find and identify the loss point. The short sensitivity level according to the rule must, in fact, be interpreted as method impossibility to give a measurement, and that means a total uncertainty if applied on product line, and a high management cost due to the impossibility to be automated.

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