What is a leak and how to measure the leak rate in vacuum systems
In addition to the actual vacuum systems and their individual components (vacuum vessel, lines, valves, measuring devices, etc.) there are numerous other systems and products in the fields of industry and research with high requirements regarding tightness or so-called “hermetic sealing”. These include, in particular, assemblies for the automotive and refrigeration industry.
Generalized statements often made, such as “no detectable leaks” or “leak rate zero”, do not represent an adequate basis for acceptance testing.
Every experienced engineer knows that properly formulated acceptance specifications will indicate a certain leak rate under defined conditions. Which leak rate is acceptable is also determined by the application itself.
Types of leaks
The simplest definition for the term “leak” is: A leak is an “opening” in a (separating) wall or barrier through which solids, liquids or gases can undesirably enter or exit.
Depending on the type of material or joining fault, the following leak types are differentiated:
- Leaks in detachable connections: Flanges, ground mating surfaces, covers
- Leaks in permanent connections: Solder and welding seams, glued joints
- Leaks due to porosity: particularly following mechanical deformation (bending!) or thermal processing of polycrystalline materials and cast
components - Thermal leaks: opening up at extreme temperature loading (heat/ cold), above all at solder joints
- Apparent (virtual) leaks: leaks: quantities of gas will be liberated from hollows and cavities inside cast parts, blind holes and joints (also due to the evaporation of liquids).
- Indirect leaks: leaking supply lines in vacuum systems or furnaces (water, compressed air, brine)
- “Serial leaks”: this is the leak at the end of several “spaces connected in series”, e.g. a leak in the oil-filled section of the oil pan in a rotary vane pump
- “One-way leaks”: these will allow gas to pass in one direction but are tight in the other direction (very seldom)
An area which is not gas-tight but which is not leaky in the sense that a defect is present would be the:
- Permeation: natural permeability of gas through materials such as rubber hoses, elastomer seals, etc. (unless these parts have become brittle and thus “leaky”).
Leak rate, leak size (gas) mass flow
No vacuum device or system can ever be absolutely vacuum-tight and it does not actually need to be. The simple essential is that the leak rate must be low enough that the required operating pressure, gas balance and ultimate pressure in the vacuum container are not influenced. It follows that the requirements in regard to the gas-tightness of an apparatus are the more stringent the lower the required pressure level is.
In order to be able to register leaks quantitatively, the concept of the “leak rate” with the symbol qL and the unit mbar·l/s was introduced.
A leak rate of qL = 1 mbar·l/s is present when in an enclosed, evacuated vessel with a volume of 1 l the pressure rises by 1 mbar per second or, where there is positive pressure in the container, pressure drops by 1 mbar per second.
The leak rate of a vessel indicates the amount of gas flow which escapes through the walls of the vessel. It must be noted, however, that the leak rate for a leak depends on the type of gas.
If the gas temperature T and the molar mass M of a gas G is known, the gas mass flow can be calculated from the leak rate qL using the equation of state for ideal gases via the relationship
Δm/Δt = (qL·M)/(R·T)
Unit: g/s
with:
- R = 83.14 (mbar·l) / (mol·K)
- T = Gas temperature in K
- M = Molar mass in g/mol
- Δm = Mass in g
- Δt = Time span in s
The relationship is used:
a) to determine the mass flow Δm/Δt at a known leak rate of qL or
b) to determine the leak rate qL at a known gas mass flow Δm/Δt
For high-vacuum systems, the following rule of thumb applies:
- qL(air) < 10-6 mbar·l/s = System is “very tight”
- qL(air) < 10-5 mbar·l/s = System is “sufficiently tight”
- qL(air) > 10-4 mbar·l/s = System is “leaky”
A leak can in fact be compensated by a vacuum pump of sufficient capacity since the following applies to the reachable ultimate (operating) pressure Pult:
Pult = qL/Seff
with:
- QL = Leak rate in mbar·l/s
- Seff = Effective pumping speed of the vacuum pump at the vacuum vessel in l/s
If Seff is increased sufficiently, it is therefore always possible to reach a specified ultimate (operating) pressure pult independent of the leak rate qL.
In practice, however, a desired increase of Seff may not be realizable due to economic and design reasons (high investment costs, high space requirement).
If the desired ultimate pressure is not reached in a vacuum system, there are usually two causes for this:
1. the presence of leaks and/or
2. the gas liberation from the vessel walls and seal outgassing.
In order to differentiate between the two causes, a partial pressure analysis with a mass spectrometer or the time-related pressure rise test may be used. Since it is only possible to determine the existence of a leak and not its position in the system when using the pressure rise test, it is recommended to use a helium leak detector with which the leaks may also be localized significantly faster.
In order to achieve an overview of the correlation between the geometric size of the hole and the associated leak rate it is possible to operate on the basis of the following, rough estimate:
A circular hole with a diameter D = 1 cm in the wall of a vacuum vessel is closed with a valve. Atmospheric pressure (p = 1013 mbar) prevails outside, a vacuum inside. When the valve is opened, the air flows at the speed of sound (vs = 330 m/s) through the opening cross section of A = π·(D2/4) ~ 0.79 cm2 into the vessel. The air quantity flowing into the vessel amounts to qL(air) = p·vs·A ~ 2.6·104 mbar·l/s.
If all other conditions are kept identical and helium is allowed to flow into the hole at its speed of sound of 970 m/s, then the helium leak rate qL (helium) is ~ 7.7·104 mbar·l/s, so the leak rate is significantly higher.
This greater “sensitivity” for helium is used in leak detection and has resulted in the development and mass production of highly sensitive helium-based leak detectors.
Shown in Fig. 1 is the correlation between the hole size and leak rate for air, with the approximate value of qL (air) = 104 mbar·l/s for the “1 cm hole”.
The table shows that when the hole diameter D is reduced to 1 µm = 0.001 mm (= reduction of D by the factor 10000) the leak rate will amount to 1.0·10-4 mbar·l/s, a value which in vacuum technology already represents a major leak (see the rule of thumb above).
A leak rate of 1.0·10-12 mbar·l/s corresponds to hole diameter of 1 angstrom (Å); this is the lower detection limit for modern helium leak detectors.
Since the grid constants for many solids amount to several Å and the diameter of smaller molecules (H2, He) are about 1 Å, inherent permeation through solids can be registered metrologically using helium leak detectors. This has led to the development of calibrated test leaks with very small leak rates. This is a measurable “lack of tightness” but not a “leak” in the sense of being a defect in the material or joint.
Correlation between hole diameter and leak rate, estimation for air
Fig. 1: Correlation between hole diameter and leak rate, estimation for air
Correlation between tightness criteria and leak rates
Estimates or measurements of the sizes of atoms, molecules, viruses, bacteria, etc. have often given rise to everyday terms such as “watertight” or “bacteria-tight”.
| Concept / criterion | Comment | qL (mbar·l/s) | Relevant particle size |
|---|---|---|---|
| Water-tight* | Droplets | < 10–2 | |
| Vapor-tight | “Sweating” | < 10–3 | |
| Bacteria-tight* (cocci) (rod-shaped) |
< 10–4 | ∅ ≈ 1 μm |
|
| Oil-tight | < 10–5 | ||
| Virus-tight* (vaccines e.g. pox) (smallest viruses, bacteriophages) (viroids, RNA) |
< 10–6 < 10–8 < 10–10 |
||
| Gas-tight | < 10–7 | ||
| “Absolutely tight” | Technical | < 10–10 |
* As opposed to vapor, it is necessary to differentiate between hydrophilic and hydrophobic solids. This also applies to bacteria and viruses since they are transported primarily in solutions..
Nature and detection limits of frequently used leak detection methods:
Fig. 2: Nature and detection limits of frequently used leak detection methods.
Helium standard leak rate
Required for unequivocal definition of a leak are the pressures prevailing on either side of the (vessel) wall and the nature of the medium passing through that wall (viscosity, molar mass). For the case where the test is carried out with helium at a pressure difference of 1 bar from the atmosphere pressure (external) to the vacuum (p < 1 mbar, internal) which is frequently found in practice, the designation “helium standard leak rate” has been introduced in the standard DIN EN 1330-8.
In order to indicate the rejection rate for a test using helium under standard helium conditions it is necessary first to convert the real test conditions of use to helium standard conditions. Some examples of such conversions are shown here:
Fig. 3: Examples for the conversion of leak rates to helium standard leak rates
Conversion formulas
Regarding the conversion of pressure and gas type (viscosity, molar mass), it must be noted that different formulas apply to laminar viscous and molecular flow. The boundary between these areas is very difficult to determine. As a guideline, the following can be assumed: at leak rates
qL > 10–4 mbar·l/s laminar viscous flow
and at leak rates
qL < 10–6 mbar·l/s molecular flow
In the intermediate range the manufacturer (who is liable under the guarantee terms) must assume values on the safe side.
Here indices “I” and “II” refer to the one or the other pressure ratio and indices “1” and “2” reference the inside and outside of the leak point, respectively. For a sensible use of the formulas, the pressure p1 must always be the higher pressure ( p1 > p2 ).
Table 2: Formulae for the conversion of pressure and gas type
p = pressure, q = gas flow (leak rate), η = viscosity, M = molar mass
| Flow | Laminar viscous | Molecular |
| Pressure | qI · (p12− p22)II = qII · (p12−p22)I |
qI · (p1−p2)II = qII · (p1−p2)I |
| Gas type | q GasA · η GasA = q GasB · η GasB | q GasA·(M GasA)1/2 = q GasB·(M GasB)1/2 |
Terms and definitions
When searching for leaks one will generally have to distinguish between two tasks: (1) locating leaks and (2) measuring the leak rate.
In addition, we distinguish, based on the direction of flow for the fluid, between the:
a. vacuum method (sometimes known as an “outside-in leak”), where the direction of flow is into the test object; the pressure inside the test object is less than ambient pressure and the
b. positive pressure method (often referred to as the “inside-out leak”), where the flow takes place from inside the test object outward; the pressure inside the test object is higher than the ambient pressure.
The test objects should wherever possible be examined in a configuration corresponding to their later application, i.e. components for vacuum applications using the vacuum method and using the positive pressure method for parts which will be pressurized on the inside.
When measuring leak rates, we differentiate between registering:
a. individual leaks (local measurement), Fig. 4b and 4d below,
and registering
b. the total of all leaks in the test object (integral measurement), Fig. 4a and 4c below.
The smallest leak rate which is no longer tolerable in accordance with the acceptance specifications is known as the rejection leak rate. Its calculation is based on the condition that the test object may not fail during its planned utilization period due to faults caused by leaks, and this to a certain degree of certainty.
Often it is not the leak rate for the test object under normal operating conditions which is determined, but rather the throughput rate of a test gas under similar conditions. The achieved measuring values have to be converted to correspond to the actual application situation in regard to the pressures inside and outside the test object and the type of gas (or liquid) being handled.
Where a vacuum is present inside the test object (p < 1 mbar), atmospheric pressure outside, and helium is used at the test gas, one refers to standard helium conditions. Standard helium conditions are always present during helium leak detection for a vacuum system when the system is connected to a leak detector, if the system is pumped down to p less than 1 mbar and if it is sprayed with helium (spray technique) (see Fig. 4b).
If the test object is evacuated solely by the leak detector, then one would say that the leak detector is operating in the direct-flow mode of the leak detector (LD). If the test object is itself a complete vacuum system with its own vacuum pump and if the leak detector is operated in parallel to the system’s pumps, then one refers to partial-flow mode of the leak detector. One also refers to partial-flow mode when a separate auxiliary pump is used parallel to the leak detector.
When using the positive pressure method it is sometimes either impractical or in fact impossible to measure the leakage rate directly while it could certainly be sensed in an envelope which encloses the test specimen. The measurement can be made by connecting that envelope to the leak detector or by accumulation (= increasing the concentration) of the test gas inside the envelope (see Fig. 4c). The bombing test is a special version of the accumulation test.
In the so-called sniffer technique, another variation of the positive pressure technique, the (test) gas issuing from leaks is collected (extracted) by a special apparatus and fed to the leak detector (see Fig. 4d). This procedure can be carried out using either helium, hydrogen, refrigerants or SF6 as the test gas.
Usage options for a vacuum leak detector based on the vacuum method (a, b) and based on the positive pressure method (c, d)
Fig. 4: Usage options for a vacuum leak detector based on the vacuum method (a, b) and based on the positive pressure method (c, d)
| Vacuum method = Vacuum inside specimen | Positive pressure method = Pressurized test gas inside specimen |
| a: Enclosure test (integral leak detection) | c: Enclosure test (integral leak detection) |
| b: Spray technique (local leak detection) | d: Sniffer technique (local leak detection) |
Fundamentals of Leak Detection
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