False Leaks

You should not assume that you have a leak just because the vacuum in your system is poor. Reasons for inadequate vacuum can be carelessness (stopcock left open), anxiousness (insufficient pump-down time), or neglect (the diffusion pump was never turned on). You can save a lot of time by eliminating the various reasons why your vacuum is not performing up to expectations before you look for leaks. Factors that can prevent a system from reaching a desired low pressure include  

If such a thing existed, this vacuum system would have no imperfections (holes) in the walls to the atmosphere. 

Pumps that are incapable of pulling a greater vacuum because they are too small (or slow) for the given system or need maintenance. 

System components that cannot be baked (to facilitate outgassing). 

Outgassing of high vapor pressure materials from the system. 

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The manufacturer of the monitoring system must also take out a suitable liability insurance policy (operational liability 2.5m Euro cover, product liability a minimum of 2.5m Euro cover). This should explicitly include liability for follow-up costs that emerge from false leak alarms in intact liners during acceptance measurement or the period of guarantee. The basic rule is that a leak alarm requires compulsory trial pitting, inspection and repair. 

Automatic tank gauging equipment may be used if it can detect a leak of two-tenths (0.2) of a gallon per hour or larger with a probability of detection of 95% and probability of false alarm of 5% or less. Monitoring must be carried out once per week or... 

A thermocouple gauge should not be placed on any system with mercury unless there is strict controls set to trap and prevent the mercury from reaching the gauge. The reason for this is that the mercury can contaminate the wires of the thermocouple gauge and create false readings of a (virtual) leak (see Sec. 7.6.4). 

If you spray a probe gas lighter than air (such as helium) on a vacuum system, start at the top of the system. Start at the bottom with gases heavier than air. Otherwise, the drifting of the respective gases may provide false or inconclusive leak identification. It may be necessary to close windows and doors and might even be necessary to set up baffles to minimize drifting gases to prevent false or inconclusive leak identification. 

Probably the biggest problem with helium leak detector use comes from the construction material of the vacuum system or apparatus being tested. If the materials within the tested piece readily absorb helium, a false and/or confusing reading will exist for a considerable time afterward and will remain until the helium has left the piece. 

Serological tests (Em-2-ELISA, indirect haemagglutina-tion, indirect immunofluorescence) are positive in some 85%-95% of cases. All tests may yield false-positive or false-negative results. The tests are negative if the hydatid shows no leak or contains no or only dead scolices. Differentiation of the species is effected successfully by Western blot. (147)... 

When an apparatus of this type is used, it is important to be sure that there are no leaks in the system which could cause a false value of hydrogen uptake. It is well to maintain the system under a small positive pressure without shaking for about fifteen minutes and to note if there has been a change in pressure as with the micro apparatus. This becomes particularly important if the compound is not easily reduced. 

This system is ideal for thermoformed trays with porous lids and flexible pouches with one porous side. The test has been shown to detect leaks as small as 0.002 in. Guidmt and Medtronic have demonstrated the reliability of detecting leaks in blind tests and have quantified 100 percent of the purposely manufactured leaks in thermoformed trays (Hacked, 1996). In addition, there were no false positive readings in any of the unaltered packages. 

If a leak is suspected, it should first be determined whether a leak really exists or if the vacuum gauges have become dirty or are malfunctioning in some way and are giving false readings. If the vacuum measurements are... 

One disadvantage is that the pressurization gas leaking into the process may be deleterious to the process, so it should be chosen accordingly. A second potential problem would be caused by a leak in the coaxial jacket giving a false indication of a process tube leak. 

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