How to detect a leak using pressure differential tests
Leak detection method without leak detector
The most sensible differentiation between leak detection test methods used is the 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. For example, the quantity of water dripping from a leaking water faucet over a certain period of time can be determined by collecting the water with a measuring vessel. In this case one would hardly refer to this as a leak detector.
In those cases where the leak rate can be determined during leak detection without using a leak detector, this leak rate 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, 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 often play an important part here.
Pressure rise test
This leak testing method capitalizes on the fact that a leak allows a quantity of gas – remaining uniform through a period of time – to enter a sufficiently evacuated device. In contrast, the quantity of gas liberated from the walls and from the seals declines over time.
The valve at the pump end of the evacuated vacuum vessel is closed in preparation for pressure rise measurements. Then the time Δt is measured during which the pressure rises by a certain amount Δp (by one power of ten, for example). The valve is opened again and the pump is run again for some time, following which the measurement of the pressure rise will be repeated. If the time Δt for the amount of pressure rise Δp remains constant, then a leak is present, assuming that the waiting period between the two pressure rise measurements was long enough. The appropriate length of the waiting period depends on the nature and size of the device. If the time for the pressure rise Δp increases, this effect is most likely caused by a reduced gas liberation on the inside of the apparatus.
One may also attempt to differentiate between leaks and contamination by interpreting the curve depicting the rise in pressure (= pressure as a function of time).
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, 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 schematically below:
Pressure rise in a vacuum container as a function of the time after turning off the vacuum pump
- Leak
- Gas evolved from the container walls
- Leak + gas evolution
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:
qL = V·(Δp/Δt)
With:
- qL = Leak rate in mbar l/s
- V = Volume of the vacuum reservoir in l
- Δp/Δt = Pressure rise in the vacuum reservoir (Δp divided by the measuring time Δt in mbar/s)
Pressure drop test
The thinking here is analogous to that for the pressure rise method. The pressure drop test is, however, only rarely used to check for leaks in vacuum systems. If this is done, however, then gauge pressure should not exceed 1 bar since the flange connectors used in vacuum technology will not tolerate higher pressures.
On the other hand, the pressure drop test is a technique commonly employed in tank engineering. When dealing with large containers and the consequentially long measuring periods required for the pressure drop, 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 so that the measurement is distorted.
Fundamentals of Leak Detection
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