Construction, testing, and refining of vacuum gauges - ionization gauge cross section ratio

Coad, D. E.

Culton, J. W.

Peacock, R. N.

Propst, F. M.

Schuemann, W. C.

English

NASA Technical Reports Server

REPORT 
Required under the terms of NASA Research Grant NsG 376 
"Theoretical and Experimental Studies 
of the Underlying Processes and Techniques 
of Low Pressure Measurement" 
June 1, 1966 to November 30, 1966 
December 1, 1966 to May 31, 1967 
Personnel: 
R. N. Peacock W. C. Schuemann 
F. M. Propst D. E. Coad 
J. W. Culton G. G. Tibbetts 
r' 
B (ACCESSION NUMBE? 
(THRU) ;=.&&---+- (CODE) c L
(NASA CR OR TMX OR AD NUMBER) 
(CATEGORY) 
https://ntrs.nasa.gov/search.jsp?R=19670022380 2020-03-12T10:57:32+00:00Z
1 
I. Schuemann Gauge Development 
The program of Schuemann gauge refinement started during the last 
period has been continued. It is the aim of this work to make the gauge a 
more practical instrument for measuring very low pressures to perhaps 10 
Torr. 
- 13 
A total of twelve gauges have been built and tested or, in some 
cases, are still under test (See Fig. 1). The gauges have been designed to 
overcome certain objections to the former models, namely continued outgassing 
of the gauge materials and difficulty of heating by electron bombardment. 
Recent models of this gauge have: 
(1) A large spherical bulb to help keep the walls cool during 
outgassing, and also during normal operation at low pressure. 
(2) Separate filaments, one of pure tungsten, one of thoriated 
tungsten. Use of the latter, low temperature cathode is 
necessary if extremely low pressures are to be observed. 
(3) A collector shield closed at the top to eliminate the influence 
of stray wall pocentials, and also to reduce photo emission 
from the upper surface resulting from scattered x-rays. 
( 4 )  A conducting tin oxide film on the inner surfaces of the 
bulb. This may not be necessary for future gauges, but has 
been very helpful in understanding the importance of wall 
potentials and their relation to operating conditions. 
Four of these gauges from a recent batch will be used on the research described 
in Section I11 of this report. 
2 
11. Low Pressure Absolute Manometer 
A feasibility study has been made for a new type of absolute 
pressure gauge capable of detecting pressures as low as Torr. All 
components of the gauge are inert and compatible with UHV techniques. The 
gauge uses technology which already exists for molecular beam measurements 
in addition to some new ideas. The two main applications for this gauge 
would be the absolute calibration of ionization or other gauges below loe4 Torr, 
or use as the primary pressure gauge for ionization cross section measurements. 
In addition, it is possible that the inert nature of the gauge would make 
accurate sticking probability experiments possible. 
At the present time there is no satisfactory absolute gauge for 
use at pressures below Torr. The McLeod gauge is the most commonly 
-5 used instrument in the pressure range of Torr to 10 Torr. Available 
publications state that an accuracy of 5% at Torr is possible with the 
McLeod. The variation of published values for ionization cross sections, 
where the McLeod was used for the pressure calibration, suggests that the 
-5 actual accuracy is 20% to 30% at 10 Torr. 
The basic device, which is illustrated in Fig. 2, consists of a 
metallic vane mounted on the end of an arm suspended at the center of a 
torsion fiber. The unknown pressure is on on8 side and a high vacuum on the 
other side of the vane. Molecules rebounding from the vane give rise to a 
torque about the axis of the torsion fiber. A cmducting grid is placed in 
front of the vane and a torque, opposing that caused by the molecules, can be 
produced by applying a voltage between the grid and the vane,, 
placed on the arm at the torsion fiber and optical techniques are used to 
A mirror is 
3 
determine a null position. 
made negligible by placing the vane in a wall between two vacuum systems. 
The first system contains the gas to be measured and the grid to produce the 
electrostatic force. The second system contains the suspension fiber 
mounting and a pump with pumping speed very much larger than the leakage 
conductance from system 1 to 2 through the gap around the vane. 
The gas pressure on the back of the vane is 
The vane is made co-planar with the wall at the null position, 
and the grid is parallel to the wall giving a uniform electric field. The 
electrostatic force then is proportional to the square of the voltage. The 
minimum usable voltage is probably about 0.1 volts, since contact potential 
differences could be of this magnitude, 
1000 volts in order to avoid field emission or breakdowns. 
pressure range of 10 There is no reason why 
the absolute accuracy could not be better than 1% over this entire raoge 
because all of the variables are known, can be measured, or calculated exactly. 
The maximum usable voltage is about 
This gives a 
-a , or from lom4 to 1<12Torr. 
These are: 
(1) 
(2) 
(3) System temperature. 
(4) 
(5) 
Voltage between vane and grid, 
Electrostatic field from voltage and geometry. 
Vane null positio9 with no pressure differential. 
Approximate pressure on the back of the vane. 
The above assumes steady state operation. In nonequilibrium 
situations, the accuracy of the gauge would suffer because of a net gas 
flow onto or off of the surface of the vane. 
e .  . ' .  , 
3 
determine a null position. 
made negligible by placing the vane in a wall between two vacuum systems. 
The first system contains the gas to be measured and the grid to produce the 
electrostatic force. The second system contains the suspension fiber 
mounting and a pump with pumping speed very much larger than the leakage 
conductance from system 1 to 2 through the gap around the vane. 
The gas pressure on the back of the vane is 
The vane is made co-planar with the wall at the null position.. 
and the grid is parallel t o  the wall giving a uniform electric field. The 
electrostatic force then is proportional to the square of the voltage. The 
minimum usable voltage is probably about 0.1 volts, since contact potential 
differences could be of this magnitude. The maximum usable voltage is about 
1000 volts in order to avoid field emission or breakdowns. 
pressure range of 10 , or from to 10 Torr. There is no reason why 
the absolute accuracy could not be better than 1% over this entire range 
because all of the variables are known, can be measured, or calculated exactly. 
This gives a 
-8 -1 2 
These are: 
(1) 
(2) 
(3) System temperature. 
(4)  
(5) 
Voltage between vane and grid. 
Electrostatic field from voltage and geometry. 
Vane null positio2 with no pressure differential. 
Approximate pressure on the back of the vane, 
The above assumes steady state operation. In nonequilibrium 
situations, the accuracy of the gauge would suffer because of a net gas 
flow onto or off of the surface of the vane. 
4 
From basic mechanics the torsion of the suspension fiber due to 
a torque is 
where T is the torque 
A is the effective length of the fiber 
r is the radius 
M is the torsional rigidity. 
To relate 
lo6 
or 1.3 
pressure and force per unit area 
dynes/cm2 is approximately equal to 760 Torr 
A 
3 dynes/cmL 
lo Torr 
assuming a .0005" tungsten wire with an effective length of 2 cm, a vane 
with a 4 cm diameter, and a 5 cm moment arm (as shown in Fig. 2)  gives 
2 (  e x  1.3 x 10 3 x 5 ) " )  
, - r  
0 =  -4 4 1 2  n(6 .25  x 10 ) (1.5 x 10 
5 
8 = 4.5 x 10 P, where P is in Torr. 
Experimenters using optical levers in normal environments are 
able to detect radians. Simple auto collimating telescopes ($100) 
are capable of resolving 10 -5  radians, while better ones ($800) can resolve 
radians. More complicated, but not difficult, techniques 1 can resolve 
lo-' radians. 
R. V. Jones, J. Sci. Instr. 38,  37 (1961). 1 
a 
5 
-8 For a pressure of 10 Torr, the angle through which the arm would 
move if the electric field were off is 
It is obvious 
is adequate. 
The 
also is not a 
The 
= 4.5 x radians 
= . 2 6  degrees 
then that the theoretical sensitivity of the proposed device 
natural noise limit due to Brownian motion is Torr and 
serious limitation. 
attachment of the tungsten fiber can be accomplished by 
2 
3 electroplating copper over the wire. 
A .0005 inch tungsten wire will support approximately 10 gms 
which is sufficient for the vane shown. 
On November 10, 1966, a device was tested which had all of the 
-4  essential features outlined above. The sensitivity was S = 4 x 10 or 
approximately 2 degrees at 10 Torr. The device was stable, easily damped, 
and unaffected by the rather bad vibration present in the test system. 
It was surprisingly rugged and no particular construction problems were 
encountered. 
-6  
G o  Ising, Phil. Mag. 1, 827 (1926) .  2 
3D. W Bassett and A .  J. B. Robertson, J. Sci. Instr. - 36 ,  321 (1959) .  
- 
6 
111. A Laboratory Study of Ionization Gauges Related to Spurious Effects Found 
in Upper Atmospheric Measurements 
During a visit to Goddard Space Flight Center on July 26, 1966, 
George Newton presented several pressure measurement problems of interest to 
his group. These are relevant to the accurate determination of conditions 
in the upper atmosphere measured by satellite or rocket probes. 
decided that the vacuum group at the Coordinated Science Laboratory would 
collaborate on certain experiments to aid in making more accurate low pressure 
measurements under space conditions. 
It was 
The final selection of experiments, ranked roughly in order of their 
importance, was: 
(1) Ionization gauge pressure indications in gas mixtures, 
particularly for the NASA flight gauge types. 
Evaluation of surface ionization effects occurring on the 
grid of the flight B-A gauge. This would include behavior 
at different grid currents and duty cycles. 
Relative sensitivities of various ionization gauges for 
different gases commonly usable in the laboratory. This 
question arose because published sensitivity ratios for 
noble gases agreed well with cross section ratios, but 
active gases do not. 
(2) 
(3) 
Basic to both problems (1) and (3) is data on ionization cross 
Therefore, sections for gases at the electron energies found in gauges. 
before starting experimental work, a brief literature search on ionization 
gauge calibration and measurements of absolute ionization cross sections 
7 
was made, and an attempt was made to evaluate the worth of the various 
published data. This collected information has considerable value and, in 
fact, has given tentative answers to some of the problems listed above. A 
table of data and a bibliography follow. 
is continuing, and the final report will be as complete as possible in listing 
pert inenti literature. 
The collection of similar information 
< 
. . ! .  . . 
Conventional 
Gauges 
1) FP-62 
FP-62 
VG- 1 
2 )  Commercial 
(unknown) 
Metson 
(guppre s sor 
(suppressor 
Metson 
3) 507 
4) FP-62 
3 to 7 gauge 
average 
Inverted Triode 
5) 5966 
Ionization Cros 
Section Ratios 
6) 100 V 
electrons 
7) 120 v 
e 1 ec t rons 
8) 120 V 
electrons 
9) &negatron , 
sensitivity 
ratios 
LO) 120 v 
electrons 
11) 120 v 
el ec t rons 
TABLE I 
IONIZATION GAUGE AND IONIZATION CROSS-SECTION RATIOS 
(NITROGEN SET EQUAL TO 1.00) 
- 
A 
- 
1.12 
1.20 
1.24 
1.07 
1.23 
1673 
1.5 
1.11 
1.24 
1.18 
1.10 
1.11 - 
- 
He 
- 
.157 
.160 
.158 
.190 
.135 
.136 
.21 
- 
.16 
e 123 
.163 
.134 
.147 -
Ne 
.221 
.246 
.255 
.236 
.209 
.33 
- 
.23 
.282 
,269 
e 287 
- 
H2 
- 
.437 
.478 
.475 
.532 
.522 
.523 
.382 
.42 
- 
38 
.314 
,363 
309 
.348 -
- 
N2 
- 
1.00 
1.00 
1.00 
1.00 
1.00 
1.00 
1.00 
1.00 
1.00 
1.0 
- 
1.00 
1.00 
1.00 
1.00 
1.00 - 
- 
Kr 
- 
1.82 
1.91 
1.81 
- 
1.70 
1.61 - 
Xe 
- 
2.45 
2.96 
2.79 
- 
253 
2.16 - 
Hg 
- 
3.07 
3.30 
3.95 
- 
2.12 
- 
- 
co 
1.06 
1.03 
1.11 
1.06 
1.07 
e .  
1.05 - 
.852 
.769 
1.14 
1.02 
1.1 
1.07 
1.08 - 
c02 
1.36 
1.39 
1.27 
1.41 - 
8 
h Y  
Air 
- 
H29 
.894 
1.22 
6.55 
9 
REFERENCES FOR TABLE I 
1. 
2. Wagener and Johnson, J. Sci. Instr. 28, 278 (1951). 
3. Riddiford, J. Sci. Instr. 28, 375 (1951). 
4. Reynolds, Physics 1, 182 (1931) (now J. Appl. Phys.). 
Dushman and Young, Phys. Rev. - 68, 278 (1945). 
5. Schulz, J. Appl. Phys. 28, 1149 (1957). 
6. .Langmuir and Jones, Phys. Rev. - 41, 357 (1928). 
7. Smith, Phys. Rev. 36, 1293 (1930). 
8. Tate and Smith, Phys. Rev. 3!3, 270 (1932). 
9. Klopfer and Schmidt, Vacuum - 10, 363 (1960). 
10. Schram, Moustafa, Schatten, DeHeer, Physica 32, 734 (1966). 
11. Rapp and Englander-Golden, J. Chem. Phys. 43, 1464 (1965). 
See Also: 
Kieffer and Dunn, Rev. Mod. Phys. 38, 1 (1966). 
Brombacher, "A Survey of Ionization Vacuum Gauges and Their Performance 
Characteristics;" N.B.S. Technical Note 298 (1967). 
Brombacher, "Bibliography and Index on Vacuum and Low Pressure Measurement," 
N.B.S. Monograph 35 (1961). 
Kieffer, "Bibliography of Low Energy Electron Collision Cross Section Data," 
N.B.S. Miscellaneous Publication 289 (1967). 
Barnett, et al., "Bibliography of Atomic and Molecular Processes for 1963,'' 
Oak Ridge National Laboratory-Atomic and Molecular Processes Information 
Center (1965). 
Barnett, et al., "Bibliography of Atomic and Molecular Processes for 1964," 
Oak Ridge National Laboratory - Atomic and Molecular Processes 
Information Center (1966). 
10 
IV. Procedure 
Since the facilities of the Laboratory do not permit making accurate 
comparisons of gauges in an absolute sense, it was decided to limit the work 
to making relative comparisons. Good relative data should be sufficient to 
answer the questions posed by Newton. The first problem considered before 
starting the design of the apparatus was the choice of the instrument to be 
used as the relative standard. 
An examination of the data on the table shows that the sensitivities 
of the omegatron seem to be in good agreement with cross section data. It is 
also well known that the omegatron has little effect on the system under 
measurement. These results should not be surprising since the omegatron has 
a monoenergetic electron beam, and the beam current is small. Some omegatrons 
are also made of platinum alloy to reduce the effects of changing surface 
potentials, as well as chemical effects. The properties of the omegatron 
have been well studied, particularly by K10pfer.~ 
the omegatron would be applicable to the measurement of gas mixtures (Problem 3). 
However, the omegatron is somewhat unique as a Spectrometer in that: it can also 
be used as a total pressure gauge, meanwhile preserving its basic properties 
of freedom from spurious effects. 
Used as a spectrometer, 
5 
Thus, the system for studying these gauge effects could consist 
of a high conductance manifold carrying the omegatron and the desired 
4A. Klopfer and W. Schmidt, Vacuum 10, 363 (1960). 
5A. Klopfer, Transactions of the Am. Vac. SOC. 1961, p. 439, Part I. 
11 
ionization gauges. The system shoulc be capable of attaining pressures in 
the low 10'' Torr range, and should have a gas admission system. Besides 
the NASA flight gauges, the manifold should have other gauges for comparison, 
such as the normal Bayard-Alpert Westinghouse type 5966, and Scheumann ganges. 
The latter is useful because it is dependable at low pressures and is also 
less subject to surface ionization effects. Figure 3 is a photograph of the 
system. 
In order to select the most suitable type of omegatron for comparison, 
a small system was set up and several omegatrons tested. One tube from those 
on hand was selected, and a range of operating parameters determined. 
Since it is desired that the comparisons be as accurate as possible, 
errors in ion current measurement should be held to about 1%. A digital 
voltmeter will be used to monitor the output voltages from the electrometers. 
This is more accurate than the panel meter. 
source belonging to the laboratory was returned to Keithley for standardization. 
With its help the required current measuring precision can be maintained. 
A Keithley picoampere current 
V. Status - Vacuum Instrumentation 
To lessen the chances of damaging the calibrated flight type gauges, 
-10 the system has been tested without them. Pressures as low as 5 x 10 Torr 
(probably the helium limit) have been reached, and the other gauges must now 
be attached. 
input resistors changed to make them consistent. Omegatron control circuits 
and other control panels have been built and tested. One function of some 
of the circuits is to prevent damage to calibrated gauges. 
The electrometers have been calibrated and, where necessary, 
. 
VI. Future Work 
With all gauges in place and the system at low pressures, the 
actual measurement program will begin. It should be possible to obtain data 
for all three problems of Section I11 simultaneously. 
of operations is as follows. 
The planned sequence 
With the desired gas bottles on the system, it will be baked and 
the gauges outgassed. 
a monitor gauge, the pressure will be increased gradually from the base 
pressure to a limit of about 10 
for all gauges under dynamic equilibrium conditions. 
one gas of a pair, it will next be done for the other gass and then for 
several mixtures and pressures of the pair of gases. 
gases will require taking the gas manifold to atmosphere and installing the 
gas bottles. 
(N2, A ) ;  (N2, H ); (A, H ); and last, more reactive gases. Data analysis 
will require considerable effort, partly because of the amount of data to 
be recorded. 
Using a Granville-Phillips pressure controller and 
-6 Torr, with readings made at many points 
After doing this for 
Preparation for dtfferent 
Gas combinations that are proposed for test are (N2, Ne); 
2 2 
VII. Surface Physics 
The studies of the adsorption and desorption of gases on metal 
surfaces have included the study of the adsorption of CO, H 2’ N2, and O2 
on polycrystalline and single-crystalline tungsten surfaces. The single- 
crystalline planes studied have been the (111) and (110) orientations. These 
measurements include studies of adsorption from liquid nitrogen temperatures 
to high temperatures. Substantial information concerning the spectrum of 
. 
t -  ' c . 
13 
a d s o r p t i o n  ene rg ie s  f o r  t hese  gases  has  been obtained.  
i n  gene ra l ,  one i s  no t  a b l e  t o  apply simple k i n e t i c s  t o  t h e  r e s u l t s  obtained.  
Th i s  i s  a r e s u l t  of t he  f a c t  t h a t  p a r t i c l e s  are not  bound wi th  a d i s c r e t e  
energy bu t  t h a t  t h e  e f f e c t i v e  binding energy i s  a func t ion  of coverage. 
Ca lcu la t ions  of desorp t ion  rates f o r  such systems can be c o r r e l a t e d  wi th  the  
exper imenta l ly  measured desorp t ion  rates. 
We have found t h a t ,  
This  work i s  i n  p repa ra t ion  f o r  a t h e s i s  and f o r  pub l i ca t ion .  When 
t h i s  i s  completed, t hese  documents w i l l  be submitted as a p a r t  of the  f i n a l  
r e p o r t  f o r  t h i s  g ran t .  
P u b l i c a t i o n s  : 
-110s and S t e i n r i s s e r ,  J. Vac. Sci. Technol. - 4,  6 (1967). 
S t e i n r i s s e r ,  J .  Vac. Sci .  Technol. 4, 44 (1967). 
Fig. 1 Schuemann, Bayard-Alpert, and magnetron type gauges with mass 
spectrometer on UHV system. 
. 
,-Precision etched grid 
Electrical guard rings / 
Formed copper 
suspension arm 
,001 thick 
6" dia. 
Wire electroDlated 
\ 
Mount lcm. dia. 
tungsten suspension wire 
Wire electroplated 
to suspension arm 
Wire electroplated 
to spring 
F i g .  2 Low Pressure Absolute Manometer. 
Fig. 3 Gauge Comparison System and Associated Equipment. 


Theoretical and experimental studies of the underlying processes and techniques of low pressure measurement  Progress report, 1 Jun. 1966 - 31 May 1967

Theoretical and experimental studies of the underlying processes and techniques of low pressure measurement Progress report, 1 Jun. 1966 - 31 May 1967

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