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We are committed to providing advice to help you detect costly leaks. When testing for leaks, there are four main nondestructive test (NDT) methods to consider:

Bubble Testing

Pressure Leak Testing

Pressure Change Measurement Testing

Mass Spectrometry Testing

Testing Methods

Bubble Testing is a fast, simple, and inexpensive test that can be done using the immersion or film solution techniques.

Methodology

The unit-under-test is pressurized to create a pressure differential.

Leak Rate

As low as 1 x 10-5 std.cc/sec

Advantages

• Simple, fast, inexpensive
• Fairly sensitive
• Especially useful for leak location
• Entire component can be evaluated at once
• Allows the observer to distinguish between real and virtual leaks

Limitations

• Cannot be used to provide a specific leak rate
• Operator-dependent
• Small leaks take longer to detect
• Unit-under-test must be cleaned or dried following testing
• Restricted to tests with internal pressures 1000 PSI and below

Testing Methods

Pressure Leak Testing is an excellent proof test that is best for leak location. The two techniques for best results are hydrostatic and pneumatic tests.

Methodology

The unit under test is gradually pressurized with water or air to a specified mark and held for a predetermined length of time. Pressure is then reduced to design pressure and the unit is observed for leakage.

Leak Rate

As low as 1 x 10-2 std.cc/sec or less, if additives to enhance leak detection are used.

Advantages

• Excellent proof test
• Inexpensive, simple, clean
• Good for leak location
• Entire assembly can be evaluated at once
• Generally safe

Limitations

• Dangerous if air is not completely evacuated • Slow
• Water can temporarily seal small leaks
• Water is not very sensitive
• Cannot be used to provide a specific leak rate
• Assemblies must be cleaned and dried following testing

Testing Methods

Pressure Change Measurement Testing determines total leakage in a simple, inexpensive way. The four common techniques are pressure decay, pressure change absolute, pressure change reference, and volume or flow measurement.

Methodology

Pressure Change Measurement Testing is based on the measurement of change across a pressure boundary caused by leakage.

Leak Rate

A decrease in pressure indicates leakage. The leak rate can then be calculated and evaluated to ensure the amount of leakage is within acceptable limits.

Pressure decay is the most commonly used technique, but it is optimal for small systems with volumes less than 7.5 cubic feet.

Advantages

• Determines total leakage
• No special tracer gas
• Inexpensive
• Simple
• Largely operator-independent
• Increased sensitivity in small-volume applications

Limitations

• Many factors affect sensitivity (especially in larger-volume applications)
• Internal volume must be known
• Cannot locate leaks

Testing Methods

Mass Spectrometry Testing is a versatile, reliable method of measuring leakage and locating leaks. Five techniques of Mass Spectrometry Testing are hood, tracer probe, detector probe, accumulation, and bell jar.

Methodology

In Mass Spectrometry Testing, a mass spectrometer is used to measure the amount of tracer gas, usually helium, present in the unit-under-test. To accomplish this, a pressure differential is created between the unit-under-test and the mass spectrometer. The presence of the tracer gas inside the mass spectrometer is indicative of a leak.

Leak Rate

Mass spectrometry Testing is typically used to measure specific leak rates between 1 x 10-4 and 1 x 10-10 std.cc/sec.

It is not used to detect leak rates larger than 1 x 10-4.

Advantages

• A great degree of reliability and sensitivity can be measured
• Ability to measure leakage and locate leaks
• Clean

Limitations

• High initial costs and equipment is costly to repair
• High helium cost
• Requires a skilled operator
• Sensitive to background helium levels and outgassing
• Requires a comprehensive test plan when used on large or complex systems

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