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Vacuum Pumps


on so many diff erent factors. Many of these are discussed in my book on Vacuum Methods in Electron Microscopy [ 2 ]. Qualitatively, however, its eff ect is to add a positive term to the equation for the pump down rate, which then becomes the following [ 1 ]:


(9)


Figure 3 : Effect of pumping line conductance C in reducing evacuation speed S e relative to pump speed S p .


where q = desorption rate, V s = system volume, and S evac = speed of evacuation. T us, the performance of a vacuum system depends critically on both the speed of evacuation and the characteristics of its interior surfaces. While a very small pump could handle the free gas molecules in a very large system, it is desirable to have a large, high-speed pumping system to handle this gas-desorption problem. However, large pumps, valves, and other fi xtures are very expensive, and so cost may become an overriding factor. In the end designers of vacuum systems have to compromise between producing an expensive unit with a very large pumping system that provides a short pump down time and one with a more modest, but less expensive, pumping system that may pump down somewhat more slowly. It should also be apparent that ultimately the performance of a vacuum system can be strongly aff ected by what is put inside it and how well its interior is kept clean and dry.


Ultimate Pressure


this point the pump’s speed is about one-half the rated value, or about 75 l/sec for our pump. T us, our eff ective initial speed of evacuation would be about S evac ~ (52 × 75)/(52 + 75) = 31 l/sec. Now one characteristic of a vacuum system that is of prime importance to the operator is the rate at which it pumps down. For a system with a volume V s and a speed of evacuation S evac , the rate at which the pressure in the system P s decreases is given by R PmpDn = -( P s /V s ) S evac. Using this equation and these assump- tions, the initial pump down rate for this bell jar system would be about R PmpDn . = -(1 / 125)31 = -0.5 Pa/sec. Since the pressure in the bell jar is only 1 Pa, this suggests that this unrealistically small pumping system should be able to pump this rather large vacuum chamber down in few seconds.


Anyone who has ever operated a vacuum system knows that this is not the way it goes. What actually happens is that there is an initial rapid decrease in P s as the high-vacuum pump starts to function and remove the free gas molecules from the system. T en the rate at which P s decreases slows down markedly. T is is because gas molecules (principally water molecules) desorb from surfaces inside the vacuum system causing a pressure increase that opposes the pump-out process and prolongs the evacuation time considerably [ 1 ]. T is is particularly true if the system contains fi xtures that have a lot of screws and close- fi tting parts that release trapped gas molecules very slowly. T e very short pump down time suggested by the above calculations is that which would be required to remove the initially free gas molecules. T e actual pump down rate must take into account the rate of pressure increase produced by this gas desorption process. T is is given by the rate of desorption q (Pa-liters/sec) divided by the volume of the system (that is, q / V s l/sec). T is rate of gas desorption is almost impossible to measure accurately, or even estimate for any given vacuum system, because it depends


32


From the above discussion it should be apparent that both the speed of evacuation and the rate of gas desorption are important parameters in determining the performance of a vacuum system. Although this discussion has focused on the pump down rate, it is also interesting to note that these two parameters critically determine the ultimate pressure attainable in a vacuum system. T is is so because the evacuation process causes the pressure P s in the system to decrease so that ultimately the value of the second term in the above rate equation decreases to the level of the fi rst, whereupon R PmpDn = 0, and q = P s S evac . T en no further decrease in pressure will occur. T e pressure that has been reached at this point is called the ultimate pressure. It is given by the simple equation P ult = q / S evac that involves only these two critical parameters. From this last equation, it is easy to see why ultra-high vacuum, say 10 -8 Pa, requires baking of the entire vacuum chamber to reduce q to a low value.


Conclusion


Vacuum systems are critical components of many diff erent types of apparatus commonly found in today’s scientifi c labora- tories. It is therefore helpful for scientists in a wide variety of disciplines to have a basic understanding of the factors that determine their performance characteristics. In this brief article, I have reviewed the characteristics of the common types of high-vacuum pumps, showed the importance that the design of the external vacuum line has on the speed of evacuation and the pump down rate, and pointed out how the desorption of gas molecules from surfaces inside a vacuum system is perhaps the most important factor in determining its performance characteristics.


References (1) WC Bigelow , Microscopy Today 21 ( 5 ) 2013 ) 28 – 33 . (2) WC Bigelow , Vacuum Methods In Electron Microscopy . Portland Press , London , 1994 .


www.microscopy-today.com • 2017 July


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