However, in molecular flow, particles impacting with the walls, predominate. In rough vacuums, the collision of gas particles frequently occurs, whereas in the high and ultra-high vacuums, impact of the gas particles with the container walls predominates. All fixtures between intake of pump system and chamber will lead to a reduction of pumping speed. The pV flow through any desired piping element, i.
In the molecular flow range, C is a constant which is independent of pressure; in the transitional and viscous flow range it is, by contrast, dependent on pressure. As a consequence, the calculation of C for the piping elements must be carried out separately for the individual pressure ranges.
This is measured at the pump inlet and depends upon gas species, vapour etc. The pumping capacity throughput for a pump is equal either to the mass flow through the pump intake port:. Here p is the pressure on the intake side of the pump. If p and V are constant at the intake side of the pump, the throughput of this pump can be expressed with the simple equation. Ultimate pressure p ult is the lowest pressure of a blank-flanged vacuum pump under defined conditions without gas inlet.
At ultimate pressure, the usable pumping speed will be zero. It is a theoretical value. The correct choice and usage of a vacuum pump is essential. Kinetic pumps require a supporting primary pump since they are unable to exhaust to atmospheric pressure.
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Capture pumps immobilize gas molecules on special surfaces within the vacuum system. More information about the pump principles can be found in our eBook here. In order to ensure a proper pump selection for a vacuum application several points need to be considered, such as:. Click here to find out more about the criteria you should consider to select the right pump in our eBook. The physical properties of gases change with pressure. For example, the thermal conductivity and the internal friction of gases in the MV range are highly sensitive to pressure.
However, in the HV range, these two properties are virtually independent of pressure. Therefore, different vacuum gauges will be required to accommodate different vacuum ranges with their usage dependent upon a wide range of factors including: the pressure range; what gases are involved which will determine any correction factors, media compatibility and potential for chemical reactions ; the accuracy required; the operating conditions dirty vs.
Indirect measurement is best suited to Medium to Ultra high vacuum. Absolute pressure measurement has vacuum as reference point; relative pressure measurement has ambient pressure as reference point. So you can see there is a variety of methods with which to measure vacuum, so knowing your environment is key to picking the right gauge.
Measuring Pressure in a Vacuum System - Meyer Tool & Mfg.
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.
Hot Cathode 'hot ion gauge' - Gas particles are ionized by energized electrons. Auth with social network: Registration Forgot your password? Download presentation. Cancel Download. Presentation is loading. Please wait. Copy to clipboard.
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The latter are seeing a resurgence in niche tube driven HiFi amplification. These are generally glass vessels that are evacuated to about 1. They are sealed containing a vacuum getter to help maintain the vacuum level. The second type of vessel is usually metal, considerably larger than the sealed units and is continuously pumped.
These pumped systems are tools in which processes are performed. In general, vacuum technology is not that complicated, but is a rather niche discipline, hidden from the layman. The needs of the semiconductor sector has driven the advances in this area to benefit itself, affiliated disciplines and other industries.
The requirement for low defect levels in devices has pushed the development of pumps that provide a low level of vacuum , together with very low levels of contamination. Contamination typically takes the form of particulates and residual vapours, such as hydrocarbons. It is worthwhile to summarise some basic physics relating the behaviour of gases.
These are well covered in very good text books and shall simply be stated here. From the idea gas law above we can quickly see that pressure of a volume of gas is related the number density of molecules. Even if we generate a level of vacuum of 1 x 10 Pa EHV each litre US quart still contains about 3 x 10 4 molecules; that is 30, in a each cubic centimetre.
The lowest vacuum normally achieved in practical process equipment is generally around 1 x 10 -6 Pa. It is also possible to derive these laws using molecular kinetic theory of gases. The kinetic theory, initially developed by Bernoulli, then later expanded upon by Maxwell-Boltzmann and others is covered in several texts. To remove the air from a system we use pumps. These pumps are connected by a series of ports and pipes. Commonly we use two types of pumps; compression pumps and capture pumps.
In either instance a single pump is not capable of evacuating a chamber from atmosphere to the desired level of vacuum. Typically two types of pump are used in a complementary fashion to achieve the whole evacuation process. In order to take advantage of high vacuum pumps a separate means of reducing the pressure from atmosphere 10 5 Pa to 10 Pa, or thereabouts is needed. To achieve this a mechanical displacement pump is used.
For a detailed analysis of the various types of pumps in this category the reader is referred to a text by Bello. Two approaches to achieving high vacuum in process systems are in common usage. The first uses a dry mechanical pump to achieve the initial evacuation. This stage is called roughing. A turbomolecular pump is then used to continue the process.
This second pump cannot exhaust directly to atmosphere and so the mechanical pump is used in series, as a backing pump. Figure 1 illustrates the pumping schematic of two such arrangements. In A, the high vacuum pump is coupled directly to the chamber. Between it and the mechanical pump there is an isolation valve. In this arrangement, to evacuate the chamber from atmosphere, the isolation valve is opened and the two pumps are turned on together. Where the chamber is large and the mechanical pump cannot evacuate it quickly enough to prevent the turbomolecular pump from stalling, then a delay is introduced before it is started.
In B, there are two additional valves. Between the chamber and the turbomolecular pump there is a high vacuum isolation valve. There is also a second pipeline valve. This is a bypass arrangement where the valve may be switched to allow the mechanical pump to back the turbomolecular pump , or directly pump the chamber. The advantage of A is that it provides a higher effective pumping speed for less capital outlay than for B. The advantage of B is that is allows the turbomolecular pump to remain on when the chamber is vented.
An example of a Nordiko pumping system is shown in figure 2.
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In combination they achieve a differential pressure of eleven orders of magnitude, 10 The second approach still requires a mechanical pump, but the high vacuum pumping element is provided by a capture pump. Unlike the case where a compression pump is used and the gas is progressively swept out of the system, the capture pump retains the gas through sorption phenomena. The most common capture pumps used are cryogenic pumps. These are typically two stage pumps, cooled using a Helium compressor. The first stage is refrigerated to about 80 K and the second stage to about 15 K.
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The first stage is used to condense non-permanent gases principally water vapour and the second pumps the permanent gases. While Nitrogen and Oxygen are frozen, Hydrogen and Helium are trapped within a cold molecular matrix normally a carbon absorber. In order to cool a cryogenic pump, it first needs to be evacuated. The thermal conductively of the air within the pump causes too much heat transfer from the outer case. As with a Dewar a vacuum provides very good thermal insulation.