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United States Bureau of Naval Personnel. Ahmed F. Tristan Needham. Clymer Publications. John Joseph O'Brien. Bestselling Series. Harry Potter. Popular Features. New Releases. Michael A. Description Capture Pumping Technology: An Introduction is a practical book for students, technicians, scientists and engineers involved in the field. Piezo - Works on the principle that under a mechanical load, Semiconducting materials experience a change in their resistivity. Pressure differences result in a voltage change. Capacitance - A more sophisticated variant of the diaphragm principle in that the deflection of a diaphragm is measured electrically rather than mechanically.
The diaphragm is part of a capacitor and the pressure change results in a capacity change which is measured electronically. This is more complicated but offers much greater accuracy and stability. The diaphragm is commonly made of either Ceramic or metal and designed for extremely long life under harsh conditions. This filament is then kept at a fixed temperature, and the voltage needed to keep it constant during a pressure change can then be converted to a pressure.
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Hot Cathode 'hot ion gauge' - Gas particles are ionized by energized electrons. These ionizing electrons originate from a Hot Filament cathode. They are then accelerated to a grid-shaped anode. The ions are accelerated to a collector and the current can be then converted into a pressure reading. Several variants of this method exist, commonly used is the Bayard-Alpert method and the Extractor is used in UHV applications, due to its ability to measure into the mbar region. Cold Cathode - For this gas particles are ionized by electrons emitted from a cathode at room temperature. Applying a large voltage difference between the Cathode and anode a self supporting discharge can be created, provided that the path between the cathode can be made sufficiently long.
This is achieved by applying a strong magnetic field. Its most common form is the Inverted Magnetron , although Penning type gauges are also widely used in some applications. The reading output can be S-shaped, a linearized analog voltage or follow digital communication protocols with characteristic curves that can be modified by software. Although interference along the cable can lead to pressure errors, these gauges are less sensitive to radiation. However, vacuum measurement is not only about choosing the right gauge; correct maintenance and cleanliness of the gauges is also important.
For example, dirt in a compression vacuum gauge will cause an incorrect and uncontrollable pressure indication. Contaminated Pirani sensors will show a pressure which is too high in the lower measurement range since the surface of the hot wire has changed. In Cold Cathode vacuum gauges, contamination will induce pressure readings which are too low, since the discharge current will become smaller. To this end, most vacuum gauges have easily replaceable or cleanable components so that you can rely on the pressure readings during your process.
What is a leak? A leak is a small hole in the technical system that allows the undesirable entry or exit of fabrics into or out of the system. The leak rate describes the size of the leak in terms of the gas amount that passes through the leak per time at a given pressure difference. When measuring leak rates we differentiate between registering.
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These procedures are run with a helium leak detector using a sector field mass spectrometer. These are:. The sniffer test has the advantage that it shows where leaks actually occur. However, helium concentrations of 5ppm in air, limits the minimum detectable rate, and furthermore ambient background signals can impact the possible detection of minimal leaks. In simple terms, the unit is p l aced inside a vessel and pressurised with helium.
Gas within the vessel is then subjected to a mass spectrometer test, and any helium detected will indicate a leak. The major disadvantage—though not the only one—is that the unit needs to be placed within a vessel of a suitable size.
Because of their mechanical limitations, such pumps can only achieve a low vacuum. To achieve a higher vacuum, other techniques must then be used, typically in series usually following an initial fast pump down with a positive displacement pump. Some examples might be use of an oil sealed rotary vane pump the most common positive displacement pump backing a diffusion pump, or a dry scroll pump backing a turbomolecular pump.
There are other combinations depending on the level of vacuum being sought. Achieving high vacuum is difficult because all of the materials exposed to the vacuum must be carefully evaluated for their outgassing and vapor pressure properties. For example, oils, greases , and rubber or plastic gaskets used as seals for the vacuum chamber must not boil off when exposed to the vacuum, or the gases they produce would prevent the creation of the desired degree of vacuum.
Often, all of the surfaces exposed to the vacuum must be baked at high temperature to drive off adsorbed gases. Outgassing can also be reduced simply by desiccation prior to vacuum pumping. High vacuum systems generally require metal chambers with metal gasket seals such as Klein flanges or ISO flanges, rather than the rubber gaskets more common in low vacuum chamber seals. The system must be clean and free of organic matter to minimize outgassing.
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All materials, solid or liquid, have a small vapour pressure, and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. As a result, many materials that work well in low vacuums, such as epoxy , will become a source of outgassing at higher vacuums. With these standard precautions, vacuums of 1 mPa are easily achieved with an assortment of molecular pumps.