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INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
791
when positive ions were read but added to the gas electrons when negative ions were read.
Conclusion
A-
Figure 14. Miniature Alphatron Gage
The final adaptation is a small model of the original design where the vacuum shell is used as an electrode and the ion collector also serves as the radium source (Figure 14). Volumes as small as 0.75 cubic inch were achieved with a maximum sensitivity of 300 microns full scale. This construction exhibited some expected but interesting effects. When positive ion; were collected at the ion collector, a dark current of 1.4 X 10-12 ampere was observed, whereas collection of negative ions a t the same point caused dark currents fifty times higher. This has been ascribed to the production of secondary electrons a t the chamber walls by the alpha-particles. These secondaries were lost to collection
Only gages and their controls of the second classification (pressure-dependent gas properties) have been discussed. It is felt that gages in this grouping offer the most convenient methods of obtaining signals for control purposes at vacuum pressures. Although the idnieation phenomenon may now be utilized for pressure information fSom extremely low to relatively high gas pressures, it is not necessarily the best method to use for every problem. The choice of a particular means of measurement and control should be in0uenced by the required accuracy, reliability, and cost, as well as the pressure interval of operation.
Literature Cited Bateman, Proc. CambridgePhil. Soc., 15,423 (1910). Downing, and Mellen, Rev. Sci. Instruments, 17, 218 (1946). Dushman, S., Instruments, 20, 3, 234 (1947). Found, C. G., and Dushman, S., Phus. Rev., 23, 734 (1924). (5) Penning, F. M., Philips Tech. Rev., 2,207 (1937). ( 6 ) Shepard, Roberts, Rev. Sci. Instruments, 10, 181-3 (1939).
(1) (2) (3) (4)
RECEIVED December 13, 1947
Measurement and Control of Leakage in High Vacuum Systems Robert B. Jacobs STANDARD OIL COMPANY (INDIANA), CHICAGO, ILL.
New techniques which maFe possible the rapid and certain location of leaks in extended vacuum systems and give quick quantitative measurements of inleakage have been developed. These techniques have made possible the systematic planning and construction of high quality vacuum systems on a scale never before attempted. The use of the mass spectrometer forms the basis for the newer techniques which are herein described.
B
EFORE World War 11, high vacuum systems were familiar
principally to university scientists. Those industries which now employ high vacuum for distillation, dehydration, and evaporation have had their greatest periods of growth during and since the war years. The building of two large uranium separation plants, one requiring high vacuum for operation, and the other a high degree of vacuum tightness, greatly advanced vacuum engineering practices during the war period. The present paper deals with those methods for detecting leakage and for the quantitative measurement of leakage which were developed in connection with the gaseous difPusion plant for the separation of uranium isotopes.
Vacuum Testing Techniques , As late as 1942, the attainment and measurement of vacuum tightness were very uncertain and time-consuming operations. The first operation, that of leak location, was usually performed
by watching for a change in the apparent pressure of a vacuum system as measured with a hot wire or ionization gage while suspected leakage areas were sprayed with volatile liquids. The principal shortcoming of this method, especially for large systems, is that the larger leaks must be found and repaired before the smaller leaks can be located. This, of course, often resulted in repeated probings before the desired,tightness was attained. The second operation, that of the measurement of tightness, was usually handled in the following manner: The vessel under consideration was pumped down as far as possible, then isolated from the pumping system and the rate of pressure rise recorded. Although this is extremely simple in principle, it is far from satisfactory. Ita chief disadvantages are that very large errors are likely to occur because of outgassing, and prolonged pumping and heating are often necessary before an accurate leak rate can be obtained. The magnitude of outgassing initially may easily be several times as great as the amount of air inleakage permitted. New vacuum techniques were visualized which would permit a tenfold improvement in sensitivity and a simultaneous improvement in the speed of testing. These new techniques included: Use of selective instruments-that is, instruments which give a nearly null reading for air and residual gases, and respond only to a probe gas. Use of these instruments dynamically-that is, as adjuncts to a high speed evacuating system. This permits their use under optimum conditions.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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ally, however, due to unavoidable physical imperfections in the apparatus, there is a continuous background reading.' The presence of helium then is indicated by an increase over the normal background reading. Figure 2 shows the general arrangement of one type of spectrometer tube. This instrument is known as a 60" spectrometer since it bends the ion beam through 60 '. More compact instruments of the 180 type have been built recently. Leak location proceeds in the folloving manner: The system under test is pumped down as far as possible, due attention being paid to pump and line sizing. The mass spectrometer is connected to the vacuum pump which is used to exhaust the equipment under test (usually between the diffusion and mechanical stages). Areas suspected of leakage are then sprayed with a fine jet of helium gas. If the system has adequate pumping, the helium jet may be moved along a t a rate of 4 or 5 feet per minute, and when the helium hits a leak, the output meter of the spectrometer registers almost immediately. On complex systems, the location of exhausting ports is an important design consideration. However, on simple straight runs of pipe, accurate work has been done in locating leaks as far as 0.5 mile from the leak detector station. O
Figure 1. Schematic Diagram of Mass Spectrometer Showing Path of Ion Beam
Use of the selectivity of the indicating instruments to permit measurement of the amount of leakage even in the presence of outgassing and unrepaired leaks. It was therefore decided to investigate and develop, if practical, a selective, continuously sampling leak detector, and to determine the optimum conditions for its use. After investigation of a number of possibilities, the mass spectrometer was selected as the basic testing instrument. This instrument works in the following manner: With reference to Figure 1, the gaseous mixtwe to be analyzed is pumped into that part of the spectrometer which includes the source. The pressure is kept somewhat less than 10-4 mm. by continuous pumping. I n the sowee of the spectrometer, molecules of the different elements present are ionized by an electron beam. The number of ions of any particular element so produced is a function of the concentration of that element in the gaseous mixture fed into the spectrometer. The ions thus formed are collimated and given a certain momentum toward the analyzer by suitable electrical potentials. On passing through the wedge-shaped magnetic field, the ions are deflected through certain angles which depend upon their respective masses. In Figure 1, the electrical fields are adjusted so that helium ions enter the collector where they are measured. Ions of greater or of smaller mass fall on either side of the collector slit and are not measured. Ions of any given mass may be brought into the collector by properly adjusting the electrical fields. For leak detecting purposes, the instrument is adjusted for He+ ions because helium is a most satisfactory probe gas. Theoretically, the instrument, when thus adjusted, will show a zero reading unless helium is present in the gas stream under analysis. Actu-
Dynamic Requirements for Rapid High Sensitivity Vacuum Testing Use of a high sensitivity, dynamic, leak detector such as the mass spectrometer is of little avail if the system under test does not possess the required pumping characteristics. It is shown below that the ratio of S / V (pumping speed of the system with respect to the volume pumped) is of greatest importance, with respect to both dynamic sensitivity and speed of leak hunting. I n Figure 3, the response (at the leak detector) after exposure ot the leak to probe gas for 1 second is plotted for different pumping speeds. The sharpness of the response to probing will, in a large measurc', determine the ability of any leak detector t o function efficiently. Figure 3 illustrates the impossibility of attaining a sharp response without a sufficiently high S / V ratio-that is, about 6 or mort. reciprocal minutes. When a large leak is probed, the system becomes temporarily flooded with a probe gas and it is impossible to continue leak hunting until this gas is removed. I n Figure 3, probing is discontinued a t the end of 1 second. Those parts of the curves for time greatel than 1 second represent the cleanup period. The length of time required for cleanup is a function of the ratio S / V . Thus for example, if a large leak is probed until a probe response 257' of maximum for that leak is obtained, and pumping is then con-
I
INPUT
OUTPUT S/V= 600
OUTPUT S/V=3OO
OUTPUT S / V = 3 0
Figure 2.
-
-Mass Spectrometer Leak Detector Assembly
0
I
2
3
4
TIME (SECONDS)
Figure 3. Response of Leak Detector as a Function of Ratio of Pumping Speed to Volume of System
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tinued until all but 0.5% of the probe gas is removed, the cleanup times indicated in Table I are obtained. These calculations have been confirmed experimentally.
FAN
0
CRANE
HOOD VALVE
Table I.
,TANK
Evaluation of Cleanup Time as Function of S / V
S / V minutes-1 CleLnup time, seconds
60 5.8
300 1.7
30 11.1
6 139
3
CALIBRATED LEAK
546
Loss of time by slow cleanup can prolong the leak hunting period considerably, perhaps much more than is indicated in Table I. The reason for this is as follows: The use of too small a pump (10x7S / V ) for leak location gives a low dynamic sensitivity, and as a result, leaks must be carefully and repeatedly probed to establish their exact location. Consequently, by the time the leak is located, a considerable quantity of probe material will be introduced into the system. This must be removed by the leakdetecting crew before probing can continue and its removal a t a slow rate is doubly time-conpuming. When used under the proper conditions, the mass spectrometer will perform in an almost unbelievable manner.
ON T E S T
HELIUM
(EXHAUST
HOOD T E S T
Figure 4.
Helium Hood Test for Quantitative Measurement of Inlealcage / / I
TESTING OUTLETS \ \
Relation between Volume Pumped, Pumping Speed, and Response to Probe Assumed that L standard cubic feet per minute are leaking into a volume of V cubic feet which is being pumped at a rate of S cubic feet per minute a t the total pressure existing in V . Then, if a t time t = 0 probing of the leak is initiated, the volume V will gain: Ldt (standard cubic feet of probe gas per differential of time dt), where p is the partial pressure of probe gas, expressed in atmospheres, in V , The net gain of probe gas in volume V is:
d ( p V ) = (L
- pS)dt U
v -dP = (L - p S )
LEAK DETECTOR
at
< T'
If, at time T , the probing is discontinued, a similar analysis will show that the partial pressure of the probe gas remaining in the system will be given by the equation:
L
pt =
-
S
[l
- exp( - ST/V)I exp [
- s(t - T ) / V l
