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The use of a quartz crystal microbalance for measuring vapour backstreaming from mechanical pumps
The use of a quartz crystal microbalance for measuring vapour backstreaming from mechanical pumps M Baker and L Laurenson,
CentralResearch Laboratory,EdwardsHigh VacuumInternationalLtd,ManorRoyal,Crawley,Sussex
The backstreaming of oil vapour from mechanical rotary pumps has been measured by condensing the vapours at 87°K on an hf crystal vibrator with a frequency of oscillation that depends on the condensate mass. Backstreaming rates were obtained for rotary pumps charged with lubricating oils with and without inhibitors. Experiments were made with both single and two-stage rotary pumps and the rate for a two-stage pump was half that of the equivalent single stage pump. A typical rate for a single stage pump at 15 millitorr gas pressure was 31~gcm 2 min l rising to 61~gcm "2 min ~ for an inhibited oil. Several methods of reducing backstreaming were assessed. These were as follows: enhancement of gas pressure to reduce oil vapour diffusion from the pump; condensation on a liquid nitrogen trap; adsorption on zeolites, alumina and charcoal; and the decomposition of the organic vapours in a glow discharge device. The greatest reduction in backstreaming with a moderate effect on the pumping speed was obtained using an adsorption trap charged with activated alumina. The trap was operated at room temperature and the backstreaming vapour penetrating the trap gave a mass flow rate of < 1 per cent of the unrestricted value. 1. Introduction It has long been known that oil-filled mechanical rotary pumps can act as a source of organic contaminants. The contaminants occur in the form of vapour which diffuses or backstreams into the pump system. Two approaches to this problem can be made, either to replace the rotary pump by an alternative such as a zeolite sorption pump, or prevent the contaminating vapours reaching the vacuum vessel. Both methods have already been extensively employed. The present work studies the extent of this contamination problem and the efficiency of various preventive methods. For gross backstreaming the vapours after collection on a cooled surface can be removed from the vacuum system and directly weighed. However, for low backstreaming rates a period of even weeks might be required to collect a sensible amount of oil and a more sensitive method must be devised. Such a method is the use of vibrating quartz crystal to collect the backstreaming vapours. The use of a crystal vibrating in the thickness shear mode was described by Sauerbrey 1 to measure the weight of thin super-imposed films, and an apparatus using a 6 Mc/s crystal to measure vacuum evaporated films has been designed by Steckelmacher et al 2. This instrument was further modified by Deville et al 3, s o as to run the crystal at liquid nitrogen temperature and thus assess diffusion pump oils. In the present work the same apparatus as used by Deville and his co-workers has been employed to study the backstreaming rates of mechanical Gaede type pumps for different oils and to assess the efficiency of various preventative methods.
2. Experimental system A 12 in. diameterx 15 in. high stainless steel vessel was exhausted through a smaller mild steel vessel, 5 in. diameter × 7 in. long, via a 1 in. pumping line by one or other of the mechanical pumps to be assessed. The small vessel was to house trapping media when they were used. A 1 in. quarter-swing valve was mounted directly above the pump to isolate the pump from the vessel when required. A 6 Mc/s AT-cut quartz crystal Vacuum/volume 161number 11.
with a sensitivity of 3pgrams for l kc change in frequency was attached to a cold finger mounted in the top of the 12 in. diameter vacuum vessel. One side of the plate only was exposed to receive a deposit on an effective area of 0.2 cm 2. The cold finger was a hollow copper rod 1¼ in. diameter which could be filled with liquid nitrogen. The final solid portion of the finger and crystal had a temperature rise of 10°C and as a result the crystal face during the experiments was at 87°K. Throughout the experiments the finger was mounted directly above the pumping orifice. Pressures were measured using Pirani gauges mounted above the pump and in the vacuum vessel. The experimental system is shown in Figure 1.
3. Direct backstreaming Two pumps were examined for backstreaming, a 150 1/min single stage pump (Edwards 1SC150) and a 150 l/min twostage pump (Edwards ED150). Both pumps had seen some service and could be considered representative of pumps in general use. The single stage pump was operated both with charges of uninhibited and inhibited pump oil (Edwards 16 and 18) and the two-stage pump only with uninhibited oil. The actual formulae of the inhibitors is not disclosed by the oil manufacturers, but they are for the normal purposes, ie to limit oxidation, rust, foam, corrosion and wear. Both pumps were operated at their normal operating temperatures (45 to 50°C) without air ballast. In the experiments the pump exhausted the vessel to an ultimate pressure as measured on a Pirani gauge: 15 millitorr for the single and 9 millitorr for the two-stage pump. There was an initial high backstreaming rate when fresh oil was used in the pump, but this settled down to a steady value after about the first half-hour of operation. The steady backstreaming rate from the single stage pump with uninhibited oil was 3/~g cm-2 min-1 on the crystal. This rate persisted for several hundreds of hours of operation of the pump. The rate from the same pump with inhibited oil was 6pg cm-2 min-1 and from the two-stage pump with uninhibited oil, 1.5/~gcm-2min- 1 (see Table 1). The lower backstreaming rate from the two-stage
PergamonPress Ltd/Printedin GreatBritain
633
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pump is no doubt due to stripping of the oil before it enters the high vacuum stage, which is carried out by an interstage degasser working at the low vacuum stage pressure. The higher rate with inhibited oil indicates that additives have an adverse effect on backstreaming, either by having high vapour pressures themselves or by aiding in the cracking process. A check was made during each experiment to ensure that the vapour reaching the cold finger had originated from the pump and was not desorbed from the vessel. This check consisted of isolating the pump and continuing to measure the condensation rate on the crystal. A very low rate of condensation was invariably obtained which signified that the bulk of the vapour came directly
Table 1. Backstreaming rates from single and two stage pumps. Single Stage Pump (ISC 150) Pump oil Uninhibited (Edwards 16 oil) Inhibited (Edwards 18)
Rate # g c m -2, min -1 > 10 3 > 10 6
Initial rate (first half hour)* Steady rate Initial rate (first half hour)* Steady rate Two Stage Pump (ED 150) Uninhibited Initial rate (Edwards 16 oil) Steady rate *Initial rate from a new charge of unused oil.
034
from the pump. It was observed however, that if the valve was kept shut for several days with the pump running then the initial backstreaming rate was considerably higher than previously recorded. For example, a single stage pump operating on uninhibited oil after 32 hours of isolation gave an initial rate of 6/~g cm-2 rain-1 against the steady rate of 3/~g cm -2 min-1. The possible reason for this is discussed in the next paragraph. Discussion. The vapour which backstreams from a rotary pump may be present in the new oil, or it may be produced by the action of the pump. The work reported here indicates the latter source to be more likely since the steady rate of backstreaming indicates that the light fractions are formed continuously whilst the pump is running. The increased rate observed after isolation of the pump further supports this view since this would not be expected if the light fractions involved were originally present in the oil. Evidence provided by Hockly and Bull4 for the occurrence of boundary lubrication at the tips of the pump blades would also support this view since this condition leads to the formation of frictional spots at which a small amount of cracking occurs. Chromatographic Analysis. It was found that after a day or so of backstreaming sufficient oil could be collected from the finger for gas-liquid chromatography. The cold finger was allowed to warm up to ambient temperature and removed from the vessel. The visible oil film, which had the distinctive odour of cracked mineral oil, was washed off with petroleum ether preparatory to insertion in the column. Twelve components were separated from such a sample with molecular weights varying from 130 to 260.
3 1½
A preliminary examination of two weU-tried methods for the reduction of backstreaming was carried out in a 12 in. UHV coating plant equipped with diffusion pump, which was not used in these experiments, and two liquid nitrogen traps. The traps were positioned one above the diffusion pump and one in the backing line. The apparatus is shown in Figure 2. The first method was the use of an air purge by bleeding air into the backing line. For this test the traps were not cooled. The second method consisted of cooling the backing line liquid nitrogen trap. The efficiencies are expressed as percentage reductions in measured backstreaming. The results are given in this way as the remaining quantity which is measured contains the background desorption of the vessel and thus the mass backstreaming is less than the measured value. In the case of the air bleed at 100 millitorr the reduction was 95 per cent and for the backing line liquid nitrogen trap better than 97 per cent. Figure 3 shows these results in a graphical form. The figure also shows the high emission of vapours when the liquid nitrogen trap is allowed to warm up to room temperature. In fact, if the warming up is accelerated, eg by the insertion of an air line in the trap the evolution rate is high enough to give a pressure in the pump above that at which an oil duffusion pump can be backed. Sorbents. For a more detailed study of other methods of reducing backstreaming the apparatus shown in Figure 1 was used. The baekstreaming vapour was provided by the 150 1/min single stage rotary pump charged with uninhibited oil. The first series of tests was carried out with the open tray trap shown in Figure 4. Three materials were tested in this trap, namely phosphorus pentoxide, 13X zeolite and 10X zeolite. The zeolites and also
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Figure 2. UHV system used for preliminary examination of preventative techniques the activated alumina and charcoal used in later experiments were pre-dried before use in the traps. The drying process consisted of baking the sorbents to 350°C for four hours at a vacuum of around 10-1 torr. The phosphorus pentoxide and 13X zeolite each reduced backstreaming by 50 per cent and the 10X zeolite by 30 per cent. It was considered from these figures that the open tray design was inefficient and an alternative design consisting of a 5 in. high x 2 in. diameter column was constructed (Figure 5). Initially three grades of zeolite were tested in the column trap, namely 3A, 10X and 13X. The 3A reduced hackstreaming by 65 per cent, the 10X by 70 per cent, and the 13X by 90 per cent. The considerable backstreaming reduction (65 per cen0 by 3A xeolite is surprising when it is remembered that this material has a pore size of only 3A, and will therefore only accept molecules of 3A diameter or less, ie very low molecular weight. The above result indicated that either the backstreaming vapour was of a very low molecular weight or that it was adsorbed on the outer zeolite surface. To elucidate this point untrapped backstreaming vapour was captured on the cooled crystal and the crystal then allowed to return to room temperature. In this way the proportion of condensate on the cold finger which re-evaporated when raised to ambient temperature whilst still under vacuum, could be determined. An initial loss of only 25 per cent was measured.
From this it is seen that only 25 per cent of the backstrcamed vapour could be light enough to enter the pores of the 3A zeolite. The remaining 40 per cent must therefore reside on the surface. The value of 90 per cent reduction for 13X zeolite is in fair agreement with Hennings and SchfitzcS who measured a value of 98 per cent with a commercial trap, Varian Model 956-5001, using a mass spectrometer technique. A surface adsorbent was now chosen for the next investigation, ie a grade of activated alumina with an effective surface area of 210 m2g-1. (13X zeolite 1030 m2g-1). The reduction in backstreaming of this material was 99 per cent or better. This figure was approaching the limit of detection due to the background of vapour desorbed from the vacuum chamber wall. The insertion of the sorbent although reducing the backstreaming rate also reduced the pump speed. For example, the 5 in. alumina column reduced speed by 20 per cent over an untrapped pump. An extended trial of over 100 hours was carried out with half a column (2½ in.) of alumina and at the end of this period the backstreaming rate was still at the same low level. During this trial the column was exposed to the atmosphere for short periods to check that atmospheric humidity had no adverse effect on trapping efficiency. On completion of the test the alumina was removed from the trap when the bottom granules were seen to be bright yellow with the adsorbed vapours. Although light colourless fractions may have travelled further up the column the colouring indicated the extent of gross contamination.
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ROTARY PUMP Figure 4. Open Tray Sorbent Trap Activated charcoal also reduced backstreaming by 99 per cent but because of the small particle size in which this material was available the p u m p performance was reduced by 95 per cent. Activated charcoal in a better form, however, might well be as effective as the alumina. The results for sorbent materials are shown in Table 2a. Electrical Discharge. Two forms of electrical discharge traps were studied, namely a cold cathode discharge and a Penning discharge.
Cold cathode glow discharges were run in a trap container with the trap container as one of the electrodes, the other electrode consisted of various assemblies suspended down the centre of a container. Discharge currents at the ultimate pressure (15 millitorr) of about 1 milliamp were obtained with a 3kv discharge potential. N o detectable reduction in backstreaming was observed with any of the electrode structures irrespective of whether the central electrode was made an anode or a cathode. Open ended Penning discharge devices of two sizes were tested by m o u n t i n g them in a trap container. The smaller of the two, m o u n t e d in an 1¼ in. diameter tube, had stainless steel cathodes and a simple ring anode. A discharge was sustained with a 600 gauss magnet. The larger assembly had two titanium cathodes, 3½ in. x 2¼ in. with a multi-cell stainless steel anode consisting of b in. square boxes ½ in. deep. The magnetic field in this case was only 300 gauss as the outer trap body was close
Table 2a. Reduction in backstreaming by sorbents.
Method of reduction
Pump: 150 l/min (ISC 150) Oil: Uninhibited 30 SAE (Edwards 16) Effect on pump performance Efficiency (percentage decrease in b a c k s t r e a m i n g )
Open Tray Trap
P205
Zeolite 13X ( ~ pellets)
Zeolite 10X ( ~ pellets)
Slow pump down required None None
50 % 50 % 30 70
"~ Approximately f ~ 40 70 reduction t in speed 20 % reduction in speed 10 % reduction in speed 95 % reduction in speed
65 % 70 % 90 % 99 % 99 % 99 70
Packed Column Trap (h = 5")
Zeolite 3A (t" pellets) Zeolite 10X (t x pellets) Zeolite 13X (t" pellets) Activated a l u m i n a (.t"-¼" granules) Activated a l u m i n a (t column) Activated c h a r c o a l (~"-~" granules)
636
M Baker and L Laurenson : The use of a quartz crystal microbalance for measuring vapour backstreaming
Table 2b. Reductions in backstreaming by other methods. Pump: 150 l/min (ISC 150) Oil: Uninhibited 30 SAE (Edwards 16) Method of reduction Effect on pump performance Efficiency (percentage decrease in backstreaming) *Liquid nitrogen trap (high conductance container type) None ~97 % *Air bleed Affects ultimate and speed 95 % (at 100 millitorr) Air ballast: Single stage pump Affects ultimate 95 ~ (ultimate 100 miUitorr) Two stage pump Affects ultimate No reduction Glow discharge None No reduction Penning discharge None No reduction *Rate through a diffusion pump stack. e n o u g h to a m a g n e t to p r o v i d e a n alternative field path. I n b o t h cases n o r e d u c t i o n in b a c k s t r e a m i n g was observed. However, with the larger device p o i s o n i n g o c c u r r e d after a few m i n u t e s of o p e r a t i o n a n d the discharge blacked out. T h e a b o v e tests were of a p r e l i m i n a r y n a t u r e a n d with the t r a p a r r a n g e m e n t used it was possible for gas to e n t e r the vessel w i t h o u t i n t e r a c t i o n with the discharge. It is h o p e d to carry o u t f u r t h e r w o r k o n s u c h traps to ensure t h a t b a c k s t r e a m ing molecules m u s t traverse the discharge region a n d to investigate the p o i s o n i n g o f t h e P e n n i n g discharge. W o r k in this l a b o r a t o r y by L a u r e n s o n 6 o n a n electron b o m b a r d m e n t t r a p used a b o v e a diffusion p u m p s h o w e d a r e d u c t i o n of cont a m i n a t i o n r e a c h i n g the vessel, b u t such a t r a p c a n n o t b e used a b o v e a r o t a r y p u m p as t h e r e is n o k n o w n t h e r m a l electron e m i t t e r to w i t h s t a n d such conditions. G a s Ballast. O n e f u r t h e r test w h i c h s h o u l d b e m e n t i o n e d was carried o u t o n a gas ballast p u m p since such p u m p s are fairly c o m m o n . W h e n a single stage p u m p is o p e r a t e d with air ballast the b a c k s t r e a m i n g was reduced b y 95 p e r cent b u t the p u m p u l t i m a t e for this level was only 100 millitorr. A i r ballasting produced n o i m p r o v e m e n t in the case o f the two stage p u m p s . T h e results o b t a i n e d with m e t h o d s of reducing b a c k s t r e a m i n g o t h e r t h a n by the use of s o r b e n t s are s u m m a r i s e d in T a b l e 2b. 5. Conclusions T h e q u a r t z crystal m i c r o b a l a n c e t e c h n i q u e h a s p r o v e d to be a versatile a n d relatively simple m e t h o d of assessing b a c k s t r e a m -
ing f r o m r o t a r y p u m p s . W i t h t h e system described here a n e r r o r of 1 p a r t in 100 was likely in the m e a s u r e m e n t s . However, by using a smaller vessel (1 litre capacity) it has been possible to assess a level o f 0.1 p a r t s in 100, where 100 is representative o f the u n t r a p p e d b a c k s t r e a m i n g rate. T h e i n h i b i t e d oil used here was s h o w n to h a v e a m u c h higher b a c k s t r e a m i n g rate t h a n the p u r e m i n e r a l oil, so t h a t if a n u n t r a p p e d r o t a r y p u m p is used, a straight m i n e r a l oil s h o u l d be e m p l o y e d to give m i n i m u m b a c k s t r e a m i n g . O f the t r a p p i n g m e t h o d s examined, a n in-line c o l u m n t r a p c o n t a i n i n g dry activated a l u m i n a h a s p r o v e d to be the m o s t efficient a n d p r o b a b l y the m o s t e c o n o m i c a l if a small r e d u c t i o n in p u m p p e r f o r m a n c e c a n b e tolerated. Acknowledgements
O u r t h a n k s are due to M r L H o l l a n d for suggesting this work a n d his e n c o u r a g e m e n t d u r i n g its progress.
References I G Sauerbrey, Z. f. Phys, 155, 1959, 206. 2 W Steckelmacher, J English, H H A Bath, D Haynes, J Holden and L Holland, Electronic Components, Parts I - I V Vol 5, 1964. 3 j p Deville, L Holland and L Laurenson, 3rd Internat Vacuum Congress, Stuttgart, 1965, (to be published). 4 D A Hockly and C S Bull, Vacuum, 4, 1954, 40. 5 K E Hennings and H J Schfitze, Vakuum Technik, 15 (2), 1966, 35. 6 L Laurenson, Electron Bombardment Trap, 1966, (private communication).
637
CentralResearch Laboratory,EdwardsHigh VacuumInternationalLtd,ManorRoyal,Crawley,Sussex
The backstreaming of oil vapour from mechanical rotary pumps has been measured by condensing the vapours at 87°K on an hf crystal vibrator with a frequency of oscillation that depends on the condensate mass. Backstreaming rates were obtained for rotary pumps charged with lubricating oils with and without inhibitors. Experiments were made with both single and two-stage rotary pumps and the rate for a two-stage pump was half that of the equivalent single stage pump. A typical rate for a single stage pump at 15 millitorr gas pressure was 31~gcm 2 min l rising to 61~gcm "2 min ~ for an inhibited oil. Several methods of reducing backstreaming were assessed. These were as follows: enhancement of gas pressure to reduce oil vapour diffusion from the pump; condensation on a liquid nitrogen trap; adsorption on zeolites, alumina and charcoal; and the decomposition of the organic vapours in a glow discharge device. The greatest reduction in backstreaming with a moderate effect on the pumping speed was obtained using an adsorption trap charged with activated alumina. The trap was operated at room temperature and the backstreaming vapour penetrating the trap gave a mass flow rate of < 1 per cent of the unrestricted value. 1. Introduction It has long been known that oil-filled mechanical rotary pumps can act as a source of organic contaminants. The contaminants occur in the form of vapour which diffuses or backstreams into the pump system. Two approaches to this problem can be made, either to replace the rotary pump by an alternative such as a zeolite sorption pump, or prevent the contaminating vapours reaching the vacuum vessel. Both methods have already been extensively employed. The present work studies the extent of this contamination problem and the efficiency of various preventive methods. For gross backstreaming the vapours after collection on a cooled surface can be removed from the vacuum system and directly weighed. However, for low backstreaming rates a period of even weeks might be required to collect a sensible amount of oil and a more sensitive method must be devised. Such a method is the use of vibrating quartz crystal to collect the backstreaming vapours. The use of a crystal vibrating in the thickness shear mode was described by Sauerbrey 1 to measure the weight of thin super-imposed films, and an apparatus using a 6 Mc/s crystal to measure vacuum evaporated films has been designed by Steckelmacher et al 2. This instrument was further modified by Deville et al 3, s o as to run the crystal at liquid nitrogen temperature and thus assess diffusion pump oils. In the present work the same apparatus as used by Deville and his co-workers has been employed to study the backstreaming rates of mechanical Gaede type pumps for different oils and to assess the efficiency of various preventative methods.
2. Experimental system A 12 in. diameterx 15 in. high stainless steel vessel was exhausted through a smaller mild steel vessel, 5 in. diameter × 7 in. long, via a 1 in. pumping line by one or other of the mechanical pumps to be assessed. The small vessel was to house trapping media when they were used. A 1 in. quarter-swing valve was mounted directly above the pump to isolate the pump from the vessel when required. A 6 Mc/s AT-cut quartz crystal Vacuum/volume 161number 11.
with a sensitivity of 3pgrams for l kc change in frequency was attached to a cold finger mounted in the top of the 12 in. diameter vacuum vessel. One side of the plate only was exposed to receive a deposit on an effective area of 0.2 cm 2. The cold finger was a hollow copper rod 1¼ in. diameter which could be filled with liquid nitrogen. The final solid portion of the finger and crystal had a temperature rise of 10°C and as a result the crystal face during the experiments was at 87°K. Throughout the experiments the finger was mounted directly above the pumping orifice. Pressures were measured using Pirani gauges mounted above the pump and in the vacuum vessel. The experimental system is shown in Figure 1.
3. Direct backstreaming Two pumps were examined for backstreaming, a 150 1/min single stage pump (Edwards 1SC150) and a 150 l/min twostage pump (Edwards ED150). Both pumps had seen some service and could be considered representative of pumps in general use. The single stage pump was operated both with charges of uninhibited and inhibited pump oil (Edwards 16 and 18) and the two-stage pump only with uninhibited oil. The actual formulae of the inhibitors is not disclosed by the oil manufacturers, but they are for the normal purposes, ie to limit oxidation, rust, foam, corrosion and wear. Both pumps were operated at their normal operating temperatures (45 to 50°C) without air ballast. In the experiments the pump exhausted the vessel to an ultimate pressure as measured on a Pirani gauge: 15 millitorr for the single and 9 millitorr for the two-stage pump. There was an initial high backstreaming rate when fresh oil was used in the pump, but this settled down to a steady value after about the first half-hour of operation. The steady backstreaming rate from the single stage pump with uninhibited oil was 3/~g cm-2 min-1 on the crystal. This rate persisted for several hundreds of hours of operation of the pump. The rate from the same pump with inhibited oil was 6pg cm-2 min-1 and from the two-stage pump with uninhibited oil, 1.5/~gcm-2min- 1 (see Table 1). The lower backstreaming rate from the two-stage
PergamonPress Ltd/Printedin GreatBritain
633
M Baker and L Laurenson: The use of a quartz crystal mierobalance for measuring vapour backstreaming
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pump is no doubt due to stripping of the oil before it enters the high vacuum stage, which is carried out by an interstage degasser working at the low vacuum stage pressure. The higher rate with inhibited oil indicates that additives have an adverse effect on backstreaming, either by having high vapour pressures themselves or by aiding in the cracking process. A check was made during each experiment to ensure that the vapour reaching the cold finger had originated from the pump and was not desorbed from the vessel. This check consisted of isolating the pump and continuing to measure the condensation rate on the crystal. A very low rate of condensation was invariably obtained which signified that the bulk of the vapour came directly
Table 1. Backstreaming rates from single and two stage pumps. Single Stage Pump (ISC 150) Pump oil Uninhibited (Edwards 16 oil) Inhibited (Edwards 18)
Rate # g c m -2, min -1 > 10 3 > 10 6
Initial rate (first half hour)* Steady rate Initial rate (first half hour)* Steady rate Two Stage Pump (ED 150) Uninhibited Initial rate (Edwards 16 oil) Steady rate *Initial rate from a new charge of unused oil.
034
from the pump. It was observed however, that if the valve was kept shut for several days with the pump running then the initial backstreaming rate was considerably higher than previously recorded. For example, a single stage pump operating on uninhibited oil after 32 hours of isolation gave an initial rate of 6/~g cm-2 rain-1 against the steady rate of 3/~g cm -2 min-1. The possible reason for this is discussed in the next paragraph. Discussion. The vapour which backstreams from a rotary pump may be present in the new oil, or it may be produced by the action of the pump. The work reported here indicates the latter source to be more likely since the steady rate of backstreaming indicates that the light fractions are formed continuously whilst the pump is running. The increased rate observed after isolation of the pump further supports this view since this would not be expected if the light fractions involved were originally present in the oil. Evidence provided by Hockly and Bull4 for the occurrence of boundary lubrication at the tips of the pump blades would also support this view since this condition leads to the formation of frictional spots at which a small amount of cracking occurs. Chromatographic Analysis. It was found that after a day or so of backstreaming sufficient oil could be collected from the finger for gas-liquid chromatography. The cold finger was allowed to warm up to ambient temperature and removed from the vessel. The visible oil film, which had the distinctive odour of cracked mineral oil, was washed off with petroleum ether preparatory to insertion in the column. Twelve components were separated from such a sample with molecular weights varying from 130 to 260.
3 1½
A preliminary examination of two weU-tried methods for the reduction of backstreaming was carried out in a 12 in. UHV coating plant equipped with diffusion pump, which was not used in these experiments, and two liquid nitrogen traps. The traps were positioned one above the diffusion pump and one in the backing line. The apparatus is shown in Figure 2. The first method was the use of an air purge by bleeding air into the backing line. For this test the traps were not cooled. The second method consisted of cooling the backing line liquid nitrogen trap. The efficiencies are expressed as percentage reductions in measured backstreaming. The results are given in this way as the remaining quantity which is measured contains the background desorption of the vessel and thus the mass backstreaming is less than the measured value. In the case of the air bleed at 100 millitorr the reduction was 95 per cent and for the backing line liquid nitrogen trap better than 97 per cent. Figure 3 shows these results in a graphical form. The figure also shows the high emission of vapours when the liquid nitrogen trap is allowed to warm up to room temperature. In fact, if the warming up is accelerated, eg by the insertion of an air line in the trap the evolution rate is high enough to give a pressure in the pump above that at which an oil duffusion pump can be backed. Sorbents. For a more detailed study of other methods of reducing backstreaming the apparatus shown in Figure 1 was used. The baekstreaming vapour was provided by the 150 1/min single stage rotary pump charged with uninhibited oil. The first series of tests was carried out with the open tray trap shown in Figure 4. Three materials were tested in this trap, namely phosphorus pentoxide, 13X zeolite and 10X zeolite. The zeolites and also
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Figure 3. Graphlcal representation of backstreamlng attenuation by liquid nitrogen trap and air purge
Figure 2. UHV system used for preliminary examination of preventative techniques the activated alumina and charcoal used in later experiments were pre-dried before use in the traps. The drying process consisted of baking the sorbents to 350°C for four hours at a vacuum of around 10-1 torr. The phosphorus pentoxide and 13X zeolite each reduced backstreaming by 50 per cent and the 10X zeolite by 30 per cent. It was considered from these figures that the open tray design was inefficient and an alternative design consisting of a 5 in. high x 2 in. diameter column was constructed (Figure 5). Initially three grades of zeolite were tested in the column trap, namely 3A, 10X and 13X. The 3A reduced hackstreaming by 65 per cent, the 10X by 70 per cent, and the 13X by 90 per cent. The considerable backstreaming reduction (65 per cen0 by 3A xeolite is surprising when it is remembered that this material has a pore size of only 3A, and will therefore only accept molecules of 3A diameter or less, ie very low molecular weight. The above result indicated that either the backstreaming vapour was of a very low molecular weight or that it was adsorbed on the outer zeolite surface. To elucidate this point untrapped backstreaming vapour was captured on the cooled crystal and the crystal then allowed to return to room temperature. In this way the proportion of condensate on the cold finger which re-evaporated when raised to ambient temperature whilst still under vacuum, could be determined. An initial loss of only 25 per cent was measured.
From this it is seen that only 25 per cent of the backstrcamed vapour could be light enough to enter the pores of the 3A zeolite. The remaining 40 per cent must therefore reside on the surface. The value of 90 per cent reduction for 13X zeolite is in fair agreement with Hennings and SchfitzcS who measured a value of 98 per cent with a commercial trap, Varian Model 956-5001, using a mass spectrometer technique. A surface adsorbent was now chosen for the next investigation, ie a grade of activated alumina with an effective surface area of 210 m2g-1. (13X zeolite 1030 m2g-1). The reduction in backstreaming of this material was 99 per cent or better. This figure was approaching the limit of detection due to the background of vapour desorbed from the vacuum chamber wall. The insertion of the sorbent although reducing the backstreaming rate also reduced the pump speed. For example, the 5 in. alumina column reduced speed by 20 per cent over an untrapped pump. An extended trial of over 100 hours was carried out with half a column (2½ in.) of alumina and at the end of this period the backstreaming rate was still at the same low level. During this trial the column was exposed to the atmosphere for short periods to check that atmospheric humidity had no adverse effect on trapping efficiency. On completion of the test the alumina was removed from the trap when the bottom granules were seen to be bright yellow with the adsorbed vapours. Although light colourless fractions may have travelled further up the column the colouring indicated the extent of gross contamination.
M Baker and L Laurenson : The use of a quartz crystal microbalance for measuring vapour backstreaming
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BACK STREAMING OIL FROM
ROTARY PUMP Figure 4. Open Tray Sorbent Trap Activated charcoal also reduced backstreaming by 99 per cent but because of the small particle size in which this material was available the p u m p performance was reduced by 95 per cent. Activated charcoal in a better form, however, might well be as effective as the alumina. The results for sorbent materials are shown in Table 2a. Electrical Discharge. Two forms of electrical discharge traps were studied, namely a cold cathode discharge and a Penning discharge.
Cold cathode glow discharges were run in a trap container with the trap container as one of the electrodes, the other electrode consisted of various assemblies suspended down the centre of a container. Discharge currents at the ultimate pressure (15 millitorr) of about 1 milliamp were obtained with a 3kv discharge potential. N o detectable reduction in backstreaming was observed with any of the electrode structures irrespective of whether the central electrode was made an anode or a cathode. Open ended Penning discharge devices of two sizes were tested by m o u n t i n g them in a trap container. The smaller of the two, m o u n t e d in an 1¼ in. diameter tube, had stainless steel cathodes and a simple ring anode. A discharge was sustained with a 600 gauss magnet. The larger assembly had two titanium cathodes, 3½ in. x 2¼ in. with a multi-cell stainless steel anode consisting of b in. square boxes ½ in. deep. The magnetic field in this case was only 300 gauss as the outer trap body was close
Table 2a. Reduction in backstreaming by sorbents.
Method of reduction
Pump: 150 l/min (ISC 150) Oil: Uninhibited 30 SAE (Edwards 16) Effect on pump performance Efficiency (percentage decrease in b a c k s t r e a m i n g )
Open Tray Trap
P205
Zeolite 13X ( ~ pellets)
Zeolite 10X ( ~ pellets)
Slow pump down required None None
50 % 50 % 30 70
"~ Approximately f ~ 40 70 reduction t in speed 20 % reduction in speed 10 % reduction in speed 95 % reduction in speed
65 % 70 % 90 % 99 % 99 % 99 70
Packed Column Trap (h = 5")
Zeolite 3A (t" pellets) Zeolite 10X (t x pellets) Zeolite 13X (t" pellets) Activated a l u m i n a (.t"-¼" granules) Activated a l u m i n a (t column) Activated c h a r c o a l (~"-~" granules)
636
M Baker and L Laurenson : The use of a quartz crystal microbalance for measuring vapour backstreaming
Table 2b. Reductions in backstreaming by other methods. Pump: 150 l/min (ISC 150) Oil: Uninhibited 30 SAE (Edwards 16) Method of reduction Effect on pump performance Efficiency (percentage decrease in backstreaming) *Liquid nitrogen trap (high conductance container type) None ~97 % *Air bleed Affects ultimate and speed 95 % (at 100 millitorr) Air ballast: Single stage pump Affects ultimate 95 ~ (ultimate 100 miUitorr) Two stage pump Affects ultimate No reduction Glow discharge None No reduction Penning discharge None No reduction *Rate through a diffusion pump stack. e n o u g h to a m a g n e t to p r o v i d e a n alternative field path. I n b o t h cases n o r e d u c t i o n in b a c k s t r e a m i n g was observed. However, with the larger device p o i s o n i n g o c c u r r e d after a few m i n u t e s of o p e r a t i o n a n d the discharge blacked out. T h e a b o v e tests were of a p r e l i m i n a r y n a t u r e a n d with the t r a p a r r a n g e m e n t used it was possible for gas to e n t e r the vessel w i t h o u t i n t e r a c t i o n with the discharge. It is h o p e d to carry o u t f u r t h e r w o r k o n s u c h traps to ensure t h a t b a c k s t r e a m ing molecules m u s t traverse the discharge region a n d to investigate the p o i s o n i n g o f t h e P e n n i n g discharge. W o r k in this l a b o r a t o r y by L a u r e n s o n 6 o n a n electron b o m b a r d m e n t t r a p used a b o v e a diffusion p u m p s h o w e d a r e d u c t i o n of cont a m i n a t i o n r e a c h i n g the vessel, b u t such a t r a p c a n n o t b e used a b o v e a r o t a r y p u m p as t h e r e is n o k n o w n t h e r m a l electron e m i t t e r to w i t h s t a n d such conditions. G a s Ballast. O n e f u r t h e r test w h i c h s h o u l d b e m e n t i o n e d was carried o u t o n a gas ballast p u m p since such p u m p s are fairly c o m m o n . W h e n a single stage p u m p is o p e r a t e d with air ballast the b a c k s t r e a m i n g was reduced b y 95 p e r cent b u t the p u m p u l t i m a t e for this level was only 100 millitorr. A i r ballasting produced n o i m p r o v e m e n t in the case o f the two stage p u m p s . T h e results o b t a i n e d with m e t h o d s of reducing b a c k s t r e a m i n g o t h e r t h a n by the use of s o r b e n t s are s u m m a r i s e d in T a b l e 2b. 5. Conclusions T h e q u a r t z crystal m i c r o b a l a n c e t e c h n i q u e h a s p r o v e d to be a versatile a n d relatively simple m e t h o d of assessing b a c k s t r e a m -
ing f r o m r o t a r y p u m p s . W i t h t h e system described here a n e r r o r of 1 p a r t in 100 was likely in the m e a s u r e m e n t s . However, by using a smaller vessel (1 litre capacity) it has been possible to assess a level o f 0.1 p a r t s in 100, where 100 is representative o f the u n t r a p p e d b a c k s t r e a m i n g rate. T h e i n h i b i t e d oil used here was s h o w n to h a v e a m u c h higher b a c k s t r e a m i n g rate t h a n the p u r e m i n e r a l oil, so t h a t if a n u n t r a p p e d r o t a r y p u m p is used, a straight m i n e r a l oil s h o u l d be e m p l o y e d to give m i n i m u m b a c k s t r e a m i n g . O f the t r a p p i n g m e t h o d s examined, a n in-line c o l u m n t r a p c o n t a i n i n g dry activated a l u m i n a h a s p r o v e d to be the m o s t efficient a n d p r o b a b l y the m o s t e c o n o m i c a l if a small r e d u c t i o n in p u m p p e r f o r m a n c e c a n b e tolerated. Acknowledgements
O u r t h a n k s are due to M r L H o l l a n d for suggesting this work a n d his e n c o u r a g e m e n t d u r i n g its progress.
References I G Sauerbrey, Z. f. Phys, 155, 1959, 206. 2 W Steckelmacher, J English, H H A Bath, D Haynes, J Holden and L Holland, Electronic Components, Parts I - I V Vol 5, 1964. 3 j p Deville, L Holland and L Laurenson, 3rd Internat Vacuum Congress, Stuttgart, 1965, (to be published). 4 D A Hockly and C S Bull, Vacuum, 4, 1954, 40. 5 K E Hennings and H J Schfitze, Vakuum Technik, 15 (2), 1966, 35. 6 L Laurenson, Electron Bombardment Trap, 1966, (private communication).
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