Isolation to Find Leaks

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. 

Regardless of whether a leak is real or virtual, the first step after a leak is verified is to isolate the area or section of the vacuum system that is leaking. This iso- 

In the stylized vacuum system shown in Fig. 7.56, one would go through the following process to locate a leak  

Close Stopcock 1. If the vacuum gauge shows a drop in the vacuum, the leak is somewhere within this area. If there is no leak, continue. 

---