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Vacuum Pumps


Table 1 : Properties of air in a vacuum at 25°C. Pressure P


(Pa) 10 5 10 2 10 1


10 -3 10 -6 10 -8


(Torr)


7.5×10 2 7.5×10 -1 7.5×10 -2 7.5×10 -6 7.5×10 -9 7.5×10 -11


Oil diff usion pumps . For inlet


Molecules per volume (molecules/cm 3 ) 2.5×10 19 2.5×10 16 2.5×10 15 2.5×10 11 2.5×10 8 2.5×10 6


Molecules striking a surface (molecules/sec/mm 2 ) 2.8×10 21 2.8×10 18 2.8×10 17 2.8×10 13 2.8×10 10 2.8×10 8


port of a pump of area A (mm 2 ) is the same as the maximum number that would strike a bed of molecular quicksand with the same area, and since no pump can capture gas molecules more effi ciently than the molecular quicksand, the equation for the theoretically maximum pumping speed is the one just derived:


(5)


Since nearly all vacuum pumps have circular inlet ports, and since it is customary to use the diameter of the inlet port to specify their “size,” it is useful to express S TMax in terms of internal fl ange diameter in mm, thus


(6)


(the constant becomes 58.06 for diameters in inches). Values of S TMax for various types of pumps of diff erent inlet fl ange diameters are listed in Table 2 . Here, pump diameters are those that are common to the International Standards Organization’s (ISO) system.


Actual Pumping Speeds Table 2 also lists the highest pumping speeds currently advertised for oil diff usion, turbomolecular, and ion pumps. T ese pumping speeds are a complex function of the area of a pump’s inlet, the extent to which that area is obstructed by internal structures, and the effi ciency with which the pump’s mechanism captures the gas molecules that do manage to get past such obstructions. Furthermore, pumping speeds are rather diffi cult to measure, and diff erent manufacturers use diff erent methods, so the advertised values listed in Table 2 only show general trends.


S TMax S DifPmp


fl anges of a given size, oil diff usion pumps provide the highest pumping speeds. T e values listed in Table 2 are for pumps of “standard” design, which have barrels of uniform diameter from top to bottom. Interestingly, the VHS series of pumps introduced by the Varian Vacuum Company, which are available with inlet flanges up to 300 mm (12 inches) in diameter, are capable of achieving nearly theoretical maximum speeds. In


these pumps the upper vapor jet structure is relatively small and provides little obstruction to gas molecules entering the pump. T e remaining obstruction is compensated for by a bulge in the upper body of the pump, as shown schematically in Figure 1 . A fractionating tube feeds high-molecular-weight oil molecules from the center of the boiler to the top jet, making it highly eff ective in capturing the gas molecules that reach it. Turbomolecular and ion pumps . T e structures that support and drive the blades of turbomolecular pumps provide substantial obstruction to the entry of gas molecules, and the effi ciency with which gas molecules are captured may not be very high. So these pumps achieve speeds that are generally somewhat less than those of oil diff usion pumps of the same size. Sputter ion pumps are even less effi cient ( Table 2 ) because they have very complex internal structures that make it diffi cult for gas molecules to reach the surfaces on which they are captured. Although turbomolecular and ion pumps cannot provide pumping speeds nearly as great as diff usion pumps of comparable size, they off er the important advantage of being totally oil-free and do not pose the risk of contaminating the interior of a vacuum system with oil molecules, as do oil diff usion pumps. When backed by oil-free backing pumps, turbomolecular pumps provide clean evacuation at very respectable pumping speeds.


Performance Characteristics Oil diff usion pumps and turbomolecular pumps do not start to function until their backing pressure falls below about 10 Pa (10 -1 Torr), as shown in Figure 2 . T ey reach their maximum pumping speed at about 10 -1 Pa (10 -3 Torr), and their pumping speed remains constant for lower pressures. Ion pumps require the pressure in the system to be reduced to about 10 -1 Pa (10 -3 Torr) before they start to function, and they reach their maximum speed at about 10 -4 Pa (10 -6 Torr).


Table 2 : Maximum pumping speeds (liters/sec) for different pump sizes and types. Pump Diameter 63 mm


S TMP


100 160 200 250 320 400


30


357 1/s 900


2304 3600 5625 9216


14499


150 1/s 300 750


1550 2130 3650 4500


75 1/s


700 900


1220 1900


S Ion 35 1/s


300 560 800


S TMax =theoretical maximum speed, S DifPmp =speed of diffusion pump, S TMP =speed of turbomolecular pump, and S Ion =speed of ion pump.


Conductance and Speed of Evacuation T e parameter that is most important in describing the perfor- mance of a vacuum system is the speed of evacuation . T is is defi ned as the rate at which gas is removed from the vacuum chamber, expressed in units of liters per second for operation in the HV and UHV ranges. If a vacuum pump is connected directly to a vacuum chamber through a well-designed valve with essentially no impedance to gas fl ow, then the speed of evacuation will


www.microscopy-today.com • 2017 July


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