Basics of Vacuum Leak Detection
Due to the high-spec nature of many vacuum applications, there are several aspects of such systems which are beyond compromise. The requirements for:
- highly-engineered pumping units and vessels;
- ultra-accurate means of measuring and controlling flows and pressures;
- and tightly-closed systems which do not leak.
What is a leak?
The detection of leaks in both pressurized and vacuum systems, as well as their elimination, management and/or accountability, is a serious business but unfortunately is often considered to be a trivial matter—which it most certainly is not.
But what exactly is a leak? A leak is a small hole in one or several parts of the system that allows the uncontrolled entry or exit of gas. As for the leak rate, this is dependent on several factors including: the size of the hole/holes; gas type; and the pressure differential (between the inside of the system and the outside).
"The leak rate describes the magnitude of the leak in terms of the amount of gas that passes out of the system per unit time."
There are several reasons why a system may fail to maintain its vacuum-levels, including outgassing or contamination. Furthermore, different vacuum processes and applications call for...
Overview of vacuum leak detection methods
As with almost every facet of vacuum systems, there is no single method which fulfils every situation and every criterion. This is certainly the case with leak detection, with four main methods being employed: the bubble test; pressure decay test; pressure rise test; and helium sniffer mode/helium vacuum mode tests. These four tests roughly correspond to the “simplistic” bubble test (for low-vacuum pressures), through to the “high-tech” helium tests (for high-vacuum pressures).
The Bubble test is best illustrated by placing a punctured bicycle tube under water and marking where the bubbles come from or placing washing-up liquid around the joint of an active water/gas pipe and observing whether the liquid forms a froth. Both are reliable ways of detecting a low-pressure leak. The bubble test is employed up to vacuums of 10-4 mbar.
The pump-down test is conducted by evacuating a closed vacuum vessel until a certain pressure is obtained, then closing...
Helium leak detection tests
It must be noted that the only credible method to detect leaks smaller than 1x10-6 mbar*l/s is with a helium leak detector. A leak diameter for 1x10-12mbar*l/s (which equates to 1Å) is also the diameter of a helium molecule, which is the smallest detectable leak rate.
N.B. A leak rate of 1 mbar*l/s means a rise of 1 mbar from a 1 liter vessel in a second. To put this into context:
- a leak rate of < 1x 10-2 mbar*l/s would be classified as “water tight”;
- < 1x 10-3 mbar*l/s “vapor tight”;
- < 1x 10-5 mbar*l/s “oil tight”;
- < 1x 10-6 mbar*l/s “virus tight”;
- < 1x 10-7 mbar*l/s “gas tight”;
- whilst < 1x 10-10 mbar*l/s would be classified as “absolute tight”.
Fig 1: Leak rate of 1 mbar l/s
Other than diameter, there are other reasons why helium is employed in leak detection:
- it constitutes only about 5 ppm in air, so background levels are very low
- its relatively low mass means that it is very “mobile” (i.e. it mixes very quickly with other gasses)
- it is completely inert/non-reactive, non-flammable and harmless
- and is widely available at relatively low cost.
There are several ways to leak-test vacuum vessels and components using helium, but all employ the same principle. The unit being checked is either helium-pressurized from within or helium-pressured from without. The gas from any potential leaks are collected and ‘pumped’ into a mass spectrometer for analyzing, and any value above the background level is evidence of a leak.
The spectrometer itself works in the following way: any helium molecules flowing into the spectrometer will be ionized, and these helium ions will then “fly” into the ion detector, where the ion current is analyzed and recorded. Before reaching the detector, the ions have to pass a magnetic field which deflects all ions other than helium ones. Based on the ionization current, the leak rate can then be calculated.
These helium tests, known as “vacuum” and “sniffer” tests, can detect leaks with both precision and certainty. Here, the term “certainty” means that there is no other method with which one can, with greater reliability and better stability, locate leaks (even small ones) and measure them quantitatively. For this reason, helium leak detectors, even though relatively costly, are often far more economical in the long run since considerably less time is required for the actual leak detection procedure to be concluded.
Two basic methods of helium leak detection: “integral” testing and “local” testing
The choice of which method to use depends on the application, as well as what the final product will be used for. The “integral” method shows if there is a leak (but not how many different leaks), the “local” method shows where there is a leak (but exact determination of the leak rate or the leak size is difficult). Both of these detection methods can each be sub-divided into two further parts: “sample under pressure”, and “sample under vacuum”.
(i) Integral testing, occurs where the sample is either under pressure or under vacuum, and is contained in a vessel. These two “integral” methods are frequently referred to as the “helium vacuum tests” since the sample is either itself evacuated or placed in a vacuum, with helium gases leaking in or out of the sample, which is then detected as it flows through a mass spectrometer. The major disadvantage—though not the only one—is that the unit needs to be placed within a vessel of a suitable size. Furthermore, the helium “vacuum” test is usually only employed on units subjected to high or ultra-high vacuums
Fig 2: Integral testing with helium (sample under pressure).
- Vacuum chamber
- Test sample under pressure
- Leak detector
- Test gas (helium)
- Pumping stage (*only needed for big chamber volumes)
Fig 3: Integral testing with helium (sample under vacuum).
- Pressure chamber
- Test sample under pressure
- Leak detector
- Test gas (helium)
- Pumping stage (only needed for big chamber volumes)
(ii) Local testing occurs, where (again) the sample itself is under pressure or under vacuum. These two “local” methods are frequently referred to as the “sniffer” test, since it uses a “sniffer” probe.
In the “local-spraying (sample under pressure)” method, the chamber is pressured up with helium and a sniffer device passed around the chamber’s likely leak points (i.e. welds, flanges, portals, instrument ducts etc.) to capture any escaping gas. This “sniffed” gas is passed to a mass spectrometer to record any elevated (i.e. above background) helium levels.
Fig 4: Local testing with helium (sample under pressure).
- Sniffer
- Test sample under pressure
- Leak detector
- Test gas (helium)
In the “local-spraying (sample under vacuum)” method, the chamber is vacuum pumped and helium gas is liberally sprayed/directed towards likely leak points, with the intention that some of this pure helium will be plumped into the chamber. The gas, from within the chamber, is then passed into a spectrometer to record any elevated helium levels.
Fig 5: Local testing with helium (sample under vacuum).
- Test gas sprayer
- Test sample under vacuum
- Leak detector
- Test gas (helium)
- Pumping stage (only needed for big test sample volumes)
The sniffer test has the advantage that it shows where leaks actually occur. However, helium concentrations of 5 ppm in air, limits the minimum detectable leakage rate, and furthermore ambient background signals can also impact the possible detection of minor leaks.
However, before a helium reading is accepted as “fact”, reference (or background) readings for helium—which are an important part of the process—must be taken and accounted for. Such reference readings provide the “background noise” for helium, which can be thought of as the ambient level of helium.
The majority of background helium is contained in between 100 and 150 mono-layers of surface gas molecules and is permanently contained in the air that is found in the leak detector, pumps, valves, flanges, pipework etc. The removal of this surface helium is called “degassing” and starts when all of the gas has been pumped out, causing the molecules to...
Vacuum Leak Measurement
As gas is compressible, the pressure (or vacuum) influences the extent of the leak, so leak-rates are quoted in mbar*l/s, with the “leak rate” being the amount of gas that flows through a leak at a given pressure differential per time.
The basis of leak rate calculations are: the diameter of the leak is circular; and the leak channel is equivalent to the thickness of the material that the leak “passes” through.
There are several standards relating to leak detectors and leak detection. One of these, DIN EN 1330-8, designates the “helium standard leak-rate” for use where a leak-test is carried out with helium at a pressure differential of 1 bar external atmospheric pressure to < 1 mbar internal pressure (which in practice are common conditions).
Environmental and safety standards require manufacturers to guarantee leak tightness of their products by carrying out leak testing as part of the...
Fundamentals of Leak Detection
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