Leak detection virtual leaks

Virtual Leaks. A virtual leak is an honest-to-goodness leak, but because it is inside the vacuum system, it cannot be detected from the outside. A formal definition of a virtual leak is a self-contained gas supply within a vacuum system. There are two types of virtual leaks ... 

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. 

Here the points suspected of leaking at the pressurized test specimen (see Fig. 5.4, d) are carefully traced with a test gas probe which is connected with the leak detector by way of a hose. Either helium or hydrogen can be detected with the INFICON helium leak detectors. The sensitivity of the method and the accuracy of locating leaky points will depend on the nature of the sniffer used and the response time for the leak detector to which it is connected. In addition, it will depend on the speed at which the probe is passed by the leak points and the distance between the tip of the probe and the surface of the test specimen. The many parameters which play a part here make it more difficult to determine the leak rates quantitatively. Using sniffer processes it is possible, virtually independent of the type of gas, to detect leak rates of about 10 mbar l/s. The limitation of sensitivity in the detection of helium is due primarily to the helium in the atmosphere (see Chapter 9, Table VIII). In regard to quantitative measurements, the leak detector and sniffer unit will have to be calibrated together. Here the distance from the specimen and the tracing speed will have to be included in calibration, too. 

It is important to be sure that all fuel and oxidant lines and their associated connectors within the instrument are in good condition, because virtually no instrument detects automatically slow fuel leaks which could cause a build up of an explosive gas mixture within the instrument casing. Obviously piping and connectors external to the instrument must also be in sound condition. 

The phenol ArOH is always a side product, resulting from some ArO that leaks from the solvent cage and abstracts a hydrogen atom from a neighboring molecule. When the reaction was performed on phenyl acetate in the gas phase, where there are no solvent molecules to form a cage (but in the presence of isobutane as a source of abstractable hydrogens), phenol was the chief product and virtually no o- or p-hydroxyacetophenone was found. Other evidence for the mechanism is that CIDNP has been observed during the course of the reaction and that the ArO radical has been detected by flash photolysis and by nanosecond time-resolved Raman spectroscopy. ... 

Systemic exposure to denileukin diftitox is variable but proportional to dose. It has a distribution t 2 of 2-5 minutes with a terminal t of 70 minutes. Immunologic reactivity to denileukin diftitox can be detected in virtually all patients after treatment but does not preclude clinical benefit with continued treatment. Denileukin diftitox clearance in later cycles of treatment is accelerated by two- to threefold as a result of development of antibodies, but serum levels are greater than required to produce cell death in IL-2R-expressing cell lines (1-10 ng/mLfor more than 90 minutes). Patients with a history of hypersensitivity reactions to diphtheria toxin or IL-2 should not be treated. Significant toxicities associated with denileukin diftitox are acute hypersensitivity reactions, a vascular leak syndrome, and constitutional toxicities glucocorticoid premedication significantly decreases toxicity. 

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