Leak Testing Essentials

Leak testing succeeds—or fails—long before the instrument is switched on. Excerpted from the recently released ASNT Level III Study Guide: Leak Testing (LT), second edition, this tutorial walks through the practical decisions that drive accurate, efficient results: designing (or evaluating) the system so the test is even possible, planning the sequence to avoid false outcomes, and defining “leaktight” in measurable, defensible terms rather than the unhelpful promise of “no leak.” From there, it connects method selection to real-world trade-offs in sensitivity, cost, and test time—highlighting where helium mass spectrometry can simplify workflows and where lower-cost approaches like bubble, pressure-change, or specialized techniques make sense. The result is a technician-focused roadmap for planning the test, controlling the variables that distort readings, and choosing equipment and techniques that match the application.

Planning and Performing Leak Tests

Physical Considerations

Leak tests, as with all types of NDT methods, may be expensive and time-consuming operations. They require skilled technicians and need to be performed in an efficient, competent, and cost-effective manner. In many applications, the technician should be brought in during the design phase so that adequate provision can be made for performing a leak test. Without proper planning, it may not be possible to perform the desired test or maintain costs at a reasonable level. The tubulation for evacuating a chamber should be large enough that long pumpdown times of several hours are not encountered, or that the system response time constant is more than a few minutes’ duration. The shorter these times are, the better, because they directly contribute to the time and cost of performing a test.

Some test specifications require the leak standard be at the most remote point in the system from the leak test instrument. During design is the time to provide plumbing for such arrangements; otherwise, this condition cannot be met. In other cases, the equipment may be complex and the most effective procedure is to leak test each module before the final assembly. Then, at final assembly, the only untested boundaries are the main couplings between modules. This greatly reduces the amount of disassembly and reassembly that could be encountered. Such provisions for testing can only be arranged during the design stage of a product.

The application of LT is vast and there are many LT techniques to go along with the various industries.

Measurements

If a helium mass spectrometer leak tester is already owned, it has the flexibility to perform many of the simpler tests quite economically because it can measure large leaks as readily as small leaks. Hood and bell jar methods do not usually require thermal stabilization times that can greatly affect the performance of pressure change or flow measurement tests. Often a helium leak test can save time and money.

Depending on the industry, the term “leaktight” has a variety of meanings. The degree of leaktightness depends on the application, whether a simple liquid such as water or a radioactive gas is being contained in a stainless steel container. An engineering decision must be made to set a specification that will be acceptable in the industry. The specification may range from a gross amount to an extremely small amount. The specification of “no leak” is not acceptable because all systems have some degree of leakage. Nothing is totally leaktight.

Detail design of a chamber must be consistent with the intended vacuum application. Possible welds should be full penetration to minimize the possibility of having trapped volumes in the weld itself. Such volumes could leak into a vacuum and cause a virtual leak that would lead to rejection of the part.

The fabrication sequence, or testing sequence, can be very important in performing the leak test of a high-vacuum chamber. To prevent a temporary plug in the leak path, the chamber should not be penetrant tested, magnetic particle tested, or hydrostatic tested before helium leak testing. A temporarily plugged leak path could lead to an invalid leak test.

Application of helium tracer gas to detect a leak requires considerable care because elevated levels of helium in the atmosphere at the test site can saturate the helium mass spectrometer. It takes hours to days of continuous operation of the leak tester to bring helium background down to acceptable levels. Another cause of high background is outgassing of helium trapped in the vacuum system or in the test equipment. The system should have a minimum of rubber or plastics because they can become saturated with helium and require extensive pumpdown times to remove the helium contamination.

Before helium leak testing, the component must be cleaned to remove liquid or other contamination. Such contamination will outgas, create a partial pressure, require additional test time, and possibly contaminate the oil in the mechanical pumps, disabling their pumping ability.

Test Equipment and Techniques

Helium Mass Spectrometer

A spectrometer tube is an electronic device that separates molecules on the basis of their mass-to-charge ratio. It requires ionization, separation, and measurement of the separated ions. For leak detection, the spectrometer is typically limited to detection of helium, although it is possible to detect other tracer gases.

Usually, a deflection-type spectrometer accelerates the ions and performs a separation by passing the ions through a magnetic field. Helium mass spectrometers are available commercially with high sensitivity, ruggedness, and ease and range of operation. They are most often calibrated in standard (std) cm³/s, even though the actual factor being measured is helium partial pressure.

Counterflow and Older Direct Flow Spectrometers

In counterflow units, the test gas is introduced into the foreline of the high-vacuum pump rather than directly into the spectrometer tube. The helium tracer gas backflows against the pumping action of the diffusion or turbomolecular pump into the spectrometer tube and achieves suitable sensitivity. The high-vacuum pumps retain high efficiency for pumping heavier gases, such as air, oil, or water vapors.

Advantages of counterflow units include reduced test time (because testing can be performed at higher pressure) and a significant reduction in effort (because liquid nitrogen is not needed). A disadvantage is that it cannot test a system that is at a high-vacuum. If total oil-free testing is needed, a dry forepump and oil-free turbomolecular pump can be used.

Bubble Leak Testing

Gas leakage from a pressure chamber can be readily detected by formation of bubbles. If the chamber is immersed, bubbles pinpointing the location of a leak are readily evident. Where immersion is not possible, a thin layer of liquid film may be flowed over the test object to act as a test medium. If the leak is large, a layer of foam may be used so the escaping gas blows a hole through the foam. The liquid film must wet the test surface so that leaking gas will be trapped and cannot escape from forming bubbles.

Sometimes bubble testing is used to test boundaries that cannot be pressurized, such as welds in the side or floor of a tank. A vacuum box is then used to provide a pressure differential across the boundary, and a liquid film is used to detect the leaks.

Many factors influence the sensitivity of a bubble test. These include pressure differential, surface tension, viscosity of gas, weather conditions, and personnel technique. Solutions with low surface tension form and reform many small bubbles. Higher surface tension solutions form larger bubbles.

Pressure Change Leak Testing

LT using the pressure change technique is performed by observing the change in pressure in a vessel as a function of time; it may be either an increase or a decrease in pressure. This technique is only suitable for leakage measurement and can be used on small or large systems. A vessel may either be pressurized or evacuated to create a pressure drop across the containing surface.

As with any pressure measurement, the volume of the system must be known or calculated to determine the leak rate. Sensitivity of the technique is important and is determined by several factors, such as the ability to measure small changes in pressure or to extend the duration of the test for a longer period of time. Also, the sensitivity is inversely related to volume; greater sensitivity can be obtained with a smaller volume. More accurate pressure gauges or longer test times can both be used to increase test sensitivity.

When tests are made on evacuated systems, outgassing may occur. To minimize outgassing, the system must be kept clean and dry. To reduce outgassing, heat the system and/or introduce a dry bleed gas and remove the contaminant from the system. When tests are made on pressurized systems, the temperature of the test gas must become stabilized before the test begins or a large measurement error may be introduced.

If a system has an unacceptable leak rate, some leak-location technique, such as bubble testing, would be used to find the leak path. After the leak has been fixed, the original test would be repeated to demonstrate that the vessel has adequate leaktightness.

Other Techniques

There are many other techniques for LT, such as the halogen diode leak test, sonic, infrared, chemical, hydrostatic, radioactive, tesla coil, and others. Many areas of technology have developed very specific techniques that testers should follow to ensure product reliability. The halogen diode leak test is very important in the area of refrigeration. Several instruments, such as the heated anode halogen detector, have been specifically developed to locate leaks and have high sensitivity for halogen leaks—even more sensitive than the helium mass spectrometer detector probe technique.

Editor’s note: This article is excerpted from ASNT Level III Study Guide: Leak Testing, second edition (2025), now available for purchase in print and ebook at ASNT’s store at source.asnt.org.