nary vacuum apparatus is different from the discharge tuhe in that (1) the applied voltage and pressure can vary greatly. (2) The Tesla coil discharge, in leak testing, is a through-glass discharge, whereas a discharge tube is a metal-electrode dis-
charge. (3) the distance between electrodes in a discharge tube is fixed as opposed to the distance between the Tesla coil and ground (this can vary constantlyfrom as little as several centimeters to many meters during leak detection),and (4) the colors indicated in the tahles are for pure gases. In a working vacuum system there will always he some air and moisture mixed with the probe gas. There will also be a trace amount of hydrocarbon vapors (from the mechanical pump) as well as other gases and vapors that incidentally may be in the system. The studv resented here was carried out to determine (experimentaily) the appearance of the discharge formed by various probe gases under actual leak-testing conditions. Experimental The apparatus used to simulateavscuum system with two leaks is shown in the fieure. ,. The prohegases were introdwed hy two methods. The first was to simulate the hayging irf the leak, while the second simulated spray. ing of the probe gas on a system. For first method a hag containing the prahe gas "E"was attached An enclosed leak to the system via the tapered joint at point "H." "D" was used to simulate a bagged leak. Stopcock 5 allowed the chamber around "Dmto be evacuated so that when Stopcock 6 was opened the primary gas around "D" was the probe gas. The discharm was initiated from a Tesla coil at "F". Anv color chanee in the discharge was then ohaerved. I n the second method the gases were placed in a balloon "I" and. controllpd hy a necdlp valve "J", sprayed on the top leak "C". l'he vacuum was rnearured hy a thermocouple gauge. 'l'he pressure with t h e stopcocks to both leaks turned off was -LO-' Turr. When the top leak war ouened, the vacuum was-11.5 X 10-?Torr. When both were openedithe vacuum was -18.5 X 10W Torr ~~~
~~~
~
7
~
~
The slmuiatedleak test apparatus used for studying the discharge colors of probe gases used In leak detection.
Table 2. Appearance ol Discharges In Gases and Vapors of Volatile Llqulds at Low Pressures When Exclted by a Tesla Coll Gas
or Volatile Linuid
Color Observed In Discharoe son violet pale magenta greenish white
Air (rwrn)
Argon
coz
Helium Nitrogen
son pink
(too pale for magenta, not quite purple) white
Oxygen Acetone Oichloramelhane
turquoise (then quickly to purple blulsh white
Methanol
voidD(man slowly
'void:
to son violet)
G s Mixed w/Air
(same) pale magenta washed out magenta purplish magenta purple pale msgenta voida(thenquickly to purple) void' (then quickly to bluish white) voids (then slowly to son violet)
a lack of dlsoharge due to a l b s of vacuum.
The gases were selected on the basis of their safety and availahility in a typical laboratory. The various volatile liquids were selected for their relative safety for both the equipment and the user. Results The results of this study are given in Table 2. I t is clear from Table 2 that the colors observed under actual leak-testing conditions differ from the colors of the same gases observed in a discharge tube (see Tahle 1). Since the system was provided with two leaks, it could also he used to imitate a multiple leak situation. The leak on the bottom (D in the figure) could he used tointroduce the probe gas while the second leak on top (C in the figure) pulled in air. This was used to investigate the effect of air on the color of the probe gas and is identified on Tahle 2 in the column "Gas Mixed w1Air." The sprayed probe-gas method was tested with helium and oxygen only. These were chosen because they displayed the colors most easily distinguishable from air. Both gases had to be sprayed directly over the leak before any significant color change could be noted. The characteristic discharge was observable only in the section of the testing apparatus within the vertical tuhe of leak C (refer to the fiaure). Farther awav from thevertical tube of leak C. within the main horizontal;ube, the color had changed so that only the tmical soft violet discharre of air was observed. This depended on the distance from-the Tesla coil, not on time. Conclusion Despite several limitations, a Tesla coil can be used effectively-~ detect leaks that are in the vicinity of metal parts that would normally preclude its use. However, there is a poor correlation between the colors observed in Table 2 and those that are reported for standard discharge tuhes as in Tahle 1.Therefore, it is inadvisable touse tables of discharge tuhes as a guide to the color expected during leak detection with a Tesla coil. There was limited, or no, change in the observed color of the discharge when the various gases were used as a spray. Therefore, the recommendation here is that the various probe aases he used in sealed environments, such as a baa aroundthe suspected area. This would allow the user a t lea; to verify whether there were a leak present. However, when it is necessary to find the specific location of a leak, i t seems best to use either acetone or dichloromethane on a cotton applicator. Slowly wipe the area until a color change is evident. The liquid should not be squirted or applied with a saturated cotton applicator around greased joints andlor stopcocks, O-rings, or other membranes that could be dissolved, or otherwise effected, hy the solvent. Volume 68
Number 6 June 1991
527
It should also be pointed out that the best observations will be obtained in a very dark room. Literature Clted
do. 1985; p 217. 4. McKeeuer,M. R.;Sur,A.: Hui,A.;Tdiinguisne,J. Reu.Sci.lnst 1979,50.1136-1140. 5. Wheeler. E. L.Scienfi/ie Glasablowing: Inferscience: New York, 1968; p 349.
528
Journal of Chemical Education
6. Martin. L. H.; Hill, R.D. Monvoi of Vacuum Procfiee: Melbourne University: Melbourne, 1946: p 16. 7. Martin. L. H.; Hill, R. D. M a n u a l of Voevum Plaetice: Melbourne University: Melbourne. 1 9 4 6 : ~112. 8. Guthrie, A. Vacuum T Z C L O I O ~Wiley: ~ ; New York, 1965; p 514. 9. Wheeler, E. L. Scientific Glorsblowing; Inferscience: New York. 1958; p 348. 10. Espe, W. M o t e r i d s f o r H i g h Vocvvm Technology; Pergamon: NewYork. 1968; Val. 3, p 393. 11. O'Hanlon, J.F.A User's G u i d e l o Vacuum Technology: Wiley: NewYork. 1980; p365. 12. Reddy, S. P.: Prassd, C. V. V. J.Phys., E . 1989.22,306-308. 13. Chapman. B. Glow DirchorgePloces~ss,SpufL~lingondPlosmoEtching: Wiley: New
.".",.""".
" . b
7oan
14. Weichman, R. L. J. Chem. Educ. 1385.62.340. 15. Gro8s.F. P. J. C h a m E d u c . 1941.13.585-539.
