Real leaks

There are two ways to demonstrate that a leak in a vacuum system really exists Either you are unable to obtain a dynamic+ vacuum that previously could be obtained with no problem, or the system is not maintaining a static vacuum that it previously could maintain. 

This side shows the problem of this virtual leak. (a) 

Verifying that a real leak exists only tells you part of the story because there are four types of leaks. You must eliminate them by analysis of the symptoms, experience, the history of the vacuum system, and/or trial and error. The following are the four types of real leaks  

Real leaks through the walls of the vacuum system 

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In practice, it is often necessary to take readings from hot-filament ionization gauges or other devices. Figure 5 gives pump-down curves for six different types of pumping equipment on the same vacuum chamber (23). The shape of curve 1 indicates that a real leak could be responsible for the zero slope demonstrated by the Bayard-Alpert gauge (BAG). The shape of the other curves could result from a combination of real and virtual leaks. 

When a suspect section is located, pump it out, isolate it from the pump and measure the pressure rise at regular time increments. A steady pressure rise indicates a real leak. A leveling off of pressure rise with time indicates a virtual leak. 

Jennings, T. A. A model for the effect of real leaks on the transport of microorganisms into a vacuum freeze-dryer. J Parenter Sci Technol 44 22-25 (1990). 

Leaks in System Walls. Real leaks in the walls of your system can occur along three different passageways ... 

Fig. 7.55 A chart of a real leak versus outgassing as a matter of time.

When a vacuum system is suspected of having a leak, one of the first tests is to determine whether the leak may be caused by outgassing. An easy way to determine this is to chart the rate of pressure loss versus time. To chart this rate, obtain the lowest vacuum you can in a reasonable amount of time, then close the section in question from the pumping section by a stopcock or valve. Next, periodically over a few minutes, or an hour or two (or three), note the pressure and elapsed time. As seen in Fig. 7.55, a real leak will indicate a constant rate of pressure rise over time while an outgassing problem will indicate a decreasing rate of pressure rise over time. 

Once you have determined that you have a leak (as opposed to outgassing), you must then decide if the leak is virtual or real. If you know your system (and its history), you should be able to review your own operations, procedures, and activities to make this determination. On the other hand, if this system is new (to you) or there are a variety of people who work on the same system, then you may have to assume that there is a real leak and prove that it does or does not exist. Once you have proved that there is no real leak, and all other indications lead you to believe that a leak exists, you can assume that you have a virtual leak. 

Real leaks connect the vacuum volume to the outside ambient through a low-conductance path. Real leaks may be due to ... 

Real leaks are minimized by proper vacuum engineering, fabrication and assembly. 

A common area for a virtual leak is the mechanical mounting of a part on a surface the virtual leak is from the entrapped volume between the part and the sixrface. Virtual leaks are minimized by proper design and construction. The evacuation of virtual leaks is aided by heating. The determination of whether a leak is real or virtual may take appreciable detective work. One technique is to backfill with an uncommon gas such as neon. On pumpdown, if the neon peak in a mass spectrometer spectrum disappears rapidly, the leak is probably a real leak, but if it decreases slowly it is probably a virtual leak. 

The leak rate is the amount of gas passing through a leak in a period of time and depends on the pressure differential as well as the size and geometry of the leak path. Leak rates are given in units of pressure-volume/time such as Torr-liters/sec. Real leaks can be determined by using a calibrated helium leak detector. Helium should be applied to local areas and used from the top down since helium is lighter than air. The speed of movement of the helium probe is important since small leaks can be missed by a fast-moving probe. A coaxial helium jet surrounded by a vacuum tube has been used with success to isolate leak locations. 

Leak-up rate (vacuum technology) The time for the pressure in a system to rise a specified amount with no vacuum pumping taking place. Generally, the leak-up pressure range is specified, i.e. from 10 to lO Torr. The leak-up rate is an indication of the presence of outgassing, desorption, virtual leaks, and real leaks. Also called Rate-of-rise. 

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