May 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
sometimes happens that it is within this pressure range that our interest lies. The conductance of long pipes to the flow of gas within this pressure range is given by the semiempirical formula developed by Knudsen. Expressing the conductance for air a t 20 C., this formula may be put in the form
D4 C = 0.25 - p
L
. *
+ 6.5 -
(20)
where C equals conductance in L per second, D equals pipe diameter in inches, L equals pipe length in feet, and equals mean pressure in microns. I t is easy to show that this equation reduces to Knudsen’s equation for molecular flow when ?, is very small, and t o Poisis very large. It is seuille’s equation for viscous flow when from this equation, as a matter of fact, that the pressure limits to which these laws may be extended have been calculated. Use of the general equation in determining the conductance of a given line is tedious, but not difficult; its use in determining the diameter corresponding to a given conductance, however, is difficult. This difficulty may be resolved, for practical purposes, by recalling that the general equation need be used only over the pressure range for which ijD is greater than 7 and less than 220. Within this range the term in parentheses can never be greater than 0.84 or less than 0.81. Assigning to this term a value 0.83 is then a permissible approximation leading to the simple working formula
It is significant that the conductance throughout this transition range is greater than for either viscous or free molecular flow.
787
Thus, use of the simpler formulas up to, or even beyond, the .limits that have been set for them introduces errors that are always on the low side. One final problem of vacuum system design should be mentioned. At some point in the design of any vacuum pumping system a decision must be made as to what reduction in pumping speed due to limited line conductance can be considered reasonable or permissible. I n the case of rough vacuum lines, where pump speeds are not greater than a few hundred liters per second, where flow is viscous, and where pipes a few inches in diameter are adequate, a speed loss of 10% or less may reasonably be expected. I n high vacuum systems, however, much greater speed reduction must often be accepted. Here pump speeds of thousands of liters per second are involved, and line conductance a t free molecular flow is much lower than a t viscous flow. Unless care is taken in the design of the high vacuum system, a speed reduction of 50% or more may easily occur, and even with care it is often impractical to reduce this loss to less than 25%. If so much as a valve or baffle is placed between the pump and the evacuated chamber, it is likely that the speed reduction will exceed 10%.
Literature Cited (1) Clawing, P.,Phusica, 9,65 (1929). (2) Dryer, W.P.,C h m . Eng., 54, 127 (1947). (3) Dushman, S.,“Production and Measurement of High Vacuum,” Schenectsdy, N. Y., General Electric Co., 1922. (4) Loevenger, R., “Fundamental Considerations in Vacuum Practice,” Radiation Laboratory, Univ. of Calif., 1946 (unpublished). ( 5 ) Knudsen, M., Ann. Physik, 28,75 (1909); 35, 389 (1911). (6) Poisseuille, J. M., Compt. rend., 11, 1041 (1840); 12, 112 (1841); 15, 1161 (1842). (7) Smoluchowski, Ann. Physik, 33, 1559 (1910). RECEIVED November 3,.184,.
New Techniques in the Measurement of Pressures below 10 Mrn. Glenn L. Mellen
NATIONAL RESEARCH CORPORATION, CAMBRIDGE, MASS.
T h e general problem of vacuum measurement and control for pressures below 10 mm. is described. Some indication is given of the limitations of conventional type gages and a discussion presented of representative techniques for utilizing intelligence from gages for control purposes. The radium source type of gage in various forms is mentioned with indications of its uses and limitations.
T
HE common units of pressure measurement in a vacuum are the millimeter and micron of mercury (1 mm. = 1000 microns). Pressure is defined as the height of a column of mercury that can be supported by the unknown pressure if a zero pressure exists above the column. It may be measured either by. some gage that indicates the time rate of molecular momentum transfer directly (McLeod gage, Knudsen manometer) or one that measures a pressure-dependent property of the gas (5) (Figure 1). Of these properties, heat conductivity and molecular ionization are the most widely used. Pirani and thermocouple gages depend upon the conductivity phenomenon,
while hot filament and radioactive source gages depend upon the ionization phenomenon, Of the first group (direct indicating) the limit of sensitivity is usually dictated by two natural torces: atmospheric pressure and gravity. These gages have necessary constructional details that cause appreciqble hysteresis effects at low pressures and thus determine their lower limit of usefulness. For example, differential oil manometer can be constructed with immiscible oils of almost the same specific gravity. The calculated length of a 100-micron pressure scale may be several inches, but the adhesion of the oil to the walls of such a gage may well give a 10- to 20-micron hysteresis error. Gages of this group will not be discussed further here. The gages in the second group have different pressure sensitivities for different gases and therefore require a calibration factor for each gas, commonly referred to as air unity. Thus for a radium-source ionization gage, we have a group of curves such as those shown in Figure 2. In general, all ionization gages have linear calibration and the magnitude of their ion currents is a function of the density and the energy of the ionizing agent as
I N D U S T R I A L A N D E N G I N E E R I N G CHEM I S T R Y
788 1
---
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1
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--
PlRANl GAUGE
-------
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FILAMENT TYPE IONIZATION GAUGE
,
l
,
10-7
10-8
10-6
I
, 10-1
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Figure 3. Pirani Isom e t r i c Gage
Low
IMPEDANCE
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, 10-2
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n-ell as the molecular density of the gas in question. To maintain a constant calibration factor, these quantities must be controlled if they are not inherently constant. I n a hot filament ionization gage both the filament emission and the acceleration poteniial must be regulated against fluctuations and the degree of control on these variables determines the accuracy to which the pressure of a pure gas may be measured. Within reasonable limits, the gas-ion current in a particular ionization gage is directly proportional to the product of the acceleration potential, the emission current, and the gas density. In the custor;larg hot filament ion gage, the magnitude of this gas-ion currentLe., the gage sensitivity-is of the order of 0.02 to 0.1 microampere per micron of pressure per milliampere of emission per volt of acceleration potential. The larger of these
VOLTAGE SOURCE
I
I-----
Figure 1. Useful Ranges for Various Vacuum Gages -
I
I
ALPHATRON TYPE GAUGE
KNUDSON GAUGF
-----
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THERMOCOUPLE GAUGE
PHILLIPS GAUGE
I
I
MCLEOD GAUGE
Vol. 40, No. 5
CCLL 919
Figure 2. Pressure Sensitivities
values may be increased by an oom (order of magnitude) through the use of scaled-up physical dimensions, but exceedingly large gages result from increasing the sensitivity in this manner, It has been observed experimentally that the sensitivity is roughly proportional to the plate diameter in a cylindrical, triode ionization gage, but it becomes impractical to use gages Those volume is comparable to the volume of a vacuum system simply to get greater sensitivity. Increasing the emission is limited by filament size and filament evaporation a t the high temperatures required for high emission densities. In this connection, a pure tungsten filament is preferable to other types of emitters, as its emission characteristics do not change as radically as those of other emitters under the wide variety of operating conditions encountered in ion gage service. High acceleration potentials not only are hazardous to personnel but also cause departure from linearity of calibration at low pressure. It appears that we can measure a pressure range, the lower limit of which is determined only by the sensitivity of the ion current meter. This is not entirely true, because with usable gage dimensions, acceleration potentials, and emission densities, greater sensitivity than 500 microamperes per micron cannot
property of the molecule, heat transfer. Gages in this category have their range of usefulness mainly determined by the physical dimensions of the chamber in which their sensitive elements are enclosed. The range is generally
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S TR Y
May 1948
ionization can occur a t lower pressures than would ordinarily be possible with the same electrode dimensions without the magnetic field. The calibration for the gage used is shown in Figure 7. The current values shown are from a power supply of approximately 2000 volts through a 1-megohm limiting resistor. Thus a large voltage signal is available if a large load resistor is used. This voltage is applied directly to the ignition grid of an OA4G gas triode to actuate a control relay. Adjustment of the operating point
117N7GT
Figure 6.
789
Schematic Diagram of Pirani Pressure Switch
30V.-40V (CONTAINED
SECTION A-A
Figure 7.
Calibration of Philips Gage
minations, because their calibration changes with age and necessitates frequent recalibration. As gross measuring devices, however, they are unsurpassed because of their own simplicity and that of associated controls. This gage was constructed to control the process in an automatic vacuum evaporation machine at a point when roughing pump pressures were low enough to allow the next step in the process to proceed. It was constructed as shown in Figure 3. The resistive elements were connected in series parallel. The whole was enclosed in a vacuum envelope of the same size as a standard 6L6 tube and fitted with a suitable tubulation. A comparison of parallel, series parallel, and series connections is shown in Figure 4. The series parallel hookup offers a slight advantage for an operating point of 500 microns and hence i t was chosen for actual construction. The basic means for controlling from the intelligence available from the gage is shown in Figure 5. Here the low impedance generator (the Pirani gage) is fairly well matched into a light bulb load. The light bulb-photocell combination sets as an impedance transformer-amplifier to give high impedance, moderate voltage signal to the vacuum tube amplifier. With this construction a variable sensitivity is also available by operating the bulb on the knee of its emission curve. The actual control is shown in Figure 6, where R-1 controls the operating point of the amplifier and R-2 the position of the knee of the emission curve with respect to pressure in the gage. This arrangement has proved satisfactory in actual service in spite of its simplicity. The problem of controlling and monitoring the same automatic machine at pressures of 1 and 0.5 micron, respectively, was solved in a slightly different manner. A hot filament ionization gage could have been used for this purpose, but the complication arising from the necessity of emission control made another gage more desirable-namely, a Philips gage (6). A Philips gage is a modified form of a gas discharge gage in which a magnetic field causes long ion paths between the electrodes, so that regenerative
.
F
A
m
SOURCE (a) SCALE OF INCHES O
Figure 8.
Radium Source Ionization Gage
PRESSURE mm HE
.
Figure 9.
Calibration of Gage
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INDUSTRIAL AND ENGINEERING CHEMISTRY
790
Vol. 40, No. 5
is accomplished by means of an ordinary potrritiometer in the grid circuit of the OA4G.
Radium Source Ionization Gage In the field of vacuum dehydration, oil manometers were once used to measure above 1 mm. and below 10 mm. Because these required a reference pressure assumed t o be zero as produced by a small vacuum pump, they did not constitute a convenient method of pressure measurement. Other packaged pumping systems were evolved during the war which also required pressure measurement in this range and as a result of the need the radium source ionization gage was developed (8). In this gage the advantage of calibration linearity for a hot filament gage was retained up to a t least 25 mm. The necessity of emission and acceleration control wa9 also eliminated by the nature of the source. In this gage (Figure 8) there are four major elements: a, the source of ionizing agents; b, the collection elecFigure 10. Elementary Schematic Diagram of Alphatron Amplifier trode; c, the shell that produces the collection field; and d, the vacuum shell. The vacuum shell and the field-producing shell could be combined were it not desirable to 5 maintain a minimum of potential difference across the insulator supporting the collector electrode. It is desirable that this poten4 tial be as small as possible to minimize electrical leakage effects. The calibration of one design of this gage is shown in Figure 9. 3 The ion current must be understood to be a saturation current and to be relatively independent of collection potentials. Departure from linearity at high pressure is attributed to a low of 2 ions by recombination. Increasing the collection field strength offsets the tendency toward recombination losses but also causes 1 a loss of linearity a t low pressures due to regenerative ionization effects a t the field strength required. The magnitude of the ion Ohl ' m'""':I 1. ' 10 100 turrent can be seen to be small: 10-15 ampere per micron prr TIME IN YEARS microgram of radium. With moderate quantities of radium in Figure 11. Calibration of Radium Source Ionization the source, it is still necessary to have an amplifier of high current Gage sensitivity to be able to read pressures in the interval from 1 to 100 microns. To avoid nonlinearity, it is also desirable to have an amplifier whose amplification factor is constant over the range of operating pressures. This is satisfied b r the Rohrr ts amplifier (6) (the basic circuit of which is shown in Figure 70). In this the circuit amplification is equal to the input to output BLOCH ING resistor ratio to within a small error if the voltage gain of the ferdOSCILLIITOR back loop is high. Three factors determine the low pressure limit GRID of this gage as it has been described: (I) physical size and safe radium concentration; (2) a dark current of 2 X l O - l 6 amprre per microgram of radium caused by secondary emission of thr collector electrode under alpha-particle bombardment; (3) a grid current of the amplifier input tube which may be apprcciFigure 12. Radiosonde Adapter able compared to the magnitude of the ion current being measured. With the commercial size gage, the lower limit of preisure sensitivity is 100 microns full scale. Using concentrations of radium up to 1 mg. and large volume ion chambers, gages of 10 microns full scale sensitivity have been made. The upprr limit of pressure usefulness can be extended to almost any valw within the limitations of physical construction. For long term use, the calibration must be modified because of the buildup of radium D in the source ( I ) . I n Figure 11 it can be seen that the error a t the end of 10 years without recalibration is approximately '
'
s
j
'
'
,
m
" ' l ' l l g
'
'
'
t
l
u
7%.
Figure 13.
Calibration Curve of Radiosonde Alphatron
Modifications of the initial design have been made to suit particular needs. One of these has great possibilities in telemetering pressure information t o a control center from a number of sources. I n this design the radioactive element controls the conductance of an electrometer tube that is in the grid to ground connection of a blocking oscillator as in Figure 12. The pressure thus controls the blocking frequency and may be read from a calibration of frequency versus pressure as in Figure 13.
May 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
791
when positive ions were read but added t o 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 a t 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. Cambridge Phil. 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.
