The most sensible differentiation between the test methods used is differentiation as to whether or not special leak detection equipment is used.
In the simplest case a leak can be determined qualitatively and, when using certain test techniques, quantitatively as well (this being the leak rate) without the assistance of a special leak detector. Thus, the quantity of water dripping from a leaking water faucet can be determined, through a certain period of time, using a graduated cylinder but one would hardly refer to this as a leak detector unit. In those cases where the leak rate can be determined during the course of the search for the leak without using a leak detector (see the pressure rise test below), this will often be converted to the helium standard leak rate. This standard leak rate value is frequently needed when issuing acceptance certificates but can also be of service when comparing leak rate values determined by helium leak detector devices.
In spite of careful inspection of the individual engineering components, leaks may also be present in an apparatus following its assembly – be it due to poorly seated seals or damaged sealing surfaces. The processes used to examine an apparatus will depend on the size of the leaks and on the degree of tightness being targeted and also on whether the apparatus is made of metal or glass or other materials. Some leak detection techniques are sketched out below. They will be selected for use in accordance with the particular application situations; economic factors may play an important part here.
This leak testing method capitalizes on the fact that a leak will allow a quantity of gas – remaining uniform through a period of time – to enter a sufficiently evacuated device (impeded gas flow, see Fig. 1.1). In contrast, the quantity of gas liberated from container walls and from the materials used for sealing (if these are not sufficiently free of outgassing) will decline through time since these will practically always be condensable vapors for which an equilibrium pressure is reached at some time (see Fig. 5.5). The valve at the pump end of the evacuated vacuum vessel will be closed in preparation for pressure rise measurements. Then the time is measured during which the pressure rises by a certain amount (by one power of ten, for example). The valve is opened again and the pump is run again for some time, following which the process will be repeated. If the time noted for this same amount of pressure rise remains constant, then a leak is present, assuming that the waiting period between the two pressure rise trials was long enough. The length of an appropriate waiting period will depend on the nature and size of the device. If the pressure rise is more moderate during the second phase, then the rise may be assumed to result from gases liberated from the inner surfaces of the vessel.
1 – Gas flow rate qm choked = constant (maximum value)
2 – Gas flow not impeded, qm drops to Δp = 0
One may also attempt to differentiate between leaks and contamination by interpreting the curve depicting the rise in pressure. Plotted on a graph with linear scales, the curve for the rise in pressure must be a straight line where a leak is present, even at higher pressures. If the pressure rise is due to gas being liberated from the walls (owing ultimately to contamination), then the pressure rise will gradually taper off and will approach a final and stable value. In most cases both phenomena will occur simultaneously so that separating the two causes is often difficult if not impossible. These relationships are shown schematic ally in Figure 5.5. Once it has become clear that the rise in pressure is due solely to a real leak, then the leak rate can be determined quantitatively from the pressure rise, plotted against time, in accordance with the following equation:
Once the vacuum vessel with a volume of 4 gallons (20L) has been isolated from the pump, the pressure in the apparatus rises from 1 · 10-4 mbar to 1 · 10-3 mbar in 300 s. Thus, in accordance with equation 5.2, the leak rate will be
The leak rate, expressed as mass flow Δm / Δt, is derived from equation 5.1 at QL = 6 · 10-5 mbar · l/s, T = 68°F (20°C) and the molar mass for air (M = 29 g/mole) at
If the container is evacuated with a TURBOVAC 50 turbomolecular pump, for example (S = 50 l/s), which is attached to the vacuum vessel by way of a shut-off valve, then one may expect an effective pumping speed of about Seff = 30 l/s. Thus, the ultimate pressure will be
Naturally it is possible to improve this ultimate pressure, should it be insufficient, by using a larger-capacity pump (e.g. the TURBOVAC 151) and at the same time to reduce the pump-down time required to reach ultimate pressure.
Today leak tests for vacuum systems are usually carried out with helium leak detectors and the vacuum method (see page on local vacuum leak detection). The apparatus is evacuated and a test gas is sprayed around the outside. In this case it must be possible to detect (on the basis of samplings inside the apparatus) the test gas which has passed through leaks and into the apparatus. Another option is to use the positive-pressure leak test. A test gas (helium) is used to fill the apparatus being inspected and to build up a slight positive pressure; the test gas will pass to the outside through the leaks and will be detected outside the device. The leaks are located with leak sprays (or soap suds) or – when using He or H2 as the test gas – with a leak detector and sniffer unit.
The thinking here is analogous to that for the pressure rise method shown above. The method is, however, used only rarely to check for leaks in vacuum systems. If this is nonetheless done, then gauge pressure should not exceed 1 bar since the flange connectors used in vacuum technology will, as a rule, not tolerate higher pressures. Positive pressure testing is, on the other hand, a technique commonly employed in tank engineering. When dealing with large containers and the long test periods they require for the pressure drop there, it may under certain circumstances be necessary to consider the effects of temperature changes. As a consequence it may happen, for example, that the system cools to below the saturation pressure for water vapor, causing water to condense; this will have to be taken into account when assessing the pressure decline.