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Thermal conductivity leak detectors suitable for testing equipment by overpressure or vacuum
Thermal Conductivity Leak Detectors suitable for testing Equipment by Overpressure or Vacuum* W. STECKELMACHER
and D. M. TINSLEY
Instrument Research Division, Edwards High Vacuum Ltd., Crawley, Sussex (Received 23 January 1962: accepted II ApriI 1962) A new principle of leak detection was investigated depending on the differences in thermal conductivities between a search gas and air. A hot wire element exposed to air containing traces of search gas changes its temperature and hence its resistance and this change was detected by comparison with a reference element exposed to air. For leak testing by the overpressure method, the equipment was pressurized with a suitable search gas. The sensitive elements of the detector were mounted in a hand-held probe unit with which gas samples could be collected. The sampled gas was made to flow over the sensing element while ambient air The elements heated to about 200°C were was made to flow over the reference element. arranged in a bridge circuit and out of balance voltages amplified and indicated as leakage A number of search gases were used but hydrogen or on a meter or as an audible response. Some results are also reported of the use of the detector helium showed the best sensitivities. with elements mounted in vacuum systems (Pirani type vacuum gauges) for vacuum leak detection.
Un nouveau principe de detection de fuite base sur la difference des conductibilites thermiques de Pair et d’un gaz traceur. Un filament chauffe, expose a de I’air contenant des traces de gaz traceur, change de temperature et par consequent de resistance ; ce changement est detect& par comparaison a l’aide d’element de reference expose’ h Pair. Pour la detection de fuite par la methode de surpression, l’equipement a tester est pressurise’ a l’aide d’un gaz traceur approprie’. Les filaments sont fixes a l’interieur d’une tete de detection montee sur une Le gaz traceur passe sur le filament detecteur tandis que Pair ambiant passe sur le poignee. filament de reference. Les filaments port&s a une temperature de 200°C environ sont branches dans un circuit en pont ; la tension de desequilibre, amplifiee, est enregistree par un appareil Un certain nombre de gaz traceurs fut de mesure sous forme de signal audible et visible. employ& mais les meilleures sensibilitb sont obtenues avec de l’hydrogene ou de I’helium. On donne quelques r&bats obtenus avec ce de’tecteur pour la detection de fuite sous vide, I’PIPment sensible &ant une jauge de Pirani.
Introduction
Spectrometer leak detectors, and it is advisable to consider a method of test best adapted to each particular application. The methods described in this paper have the advantages of using search gas and indicating leakage by electrical means. In addition, the apparatus is simple to use and maintain though naturally not having the sensitivity of some other types of more complex equipment. For leak detection of pressurized equipment the detector unit can be incorporated into a hand held search probe. It will be shown that the electrical supply and indicator unit developed for this purpose may be used with advantage also for vacuum testing by mounting the detector in the vacuum system.
The methods used for leak detection are widely diverse, for, not only are they required in the testing of vacuum equipment, but also in the examination of hermetically sealed containers for gases or liquidsi-4. These containers may be of any size and shape and have widely differing permissible leakage rates. For example, leak detection has been carried out on small transistor devices at one extreme, to large liquid gas, vacuum insulated, storage vessels at the other. Clearly, the different methods used will have varying degrees of complexity, ranging from simple soap bubble methods to relatively complex Mass *This paper
was presented
in part at the Congr&s International
des Techniques 153
et Applications
du Vide, in Paris on 24 June 1961.
W.
154
The thermal conductivity over pressure testing
STECKELMACHER
leak detector
used for
Theory Any overpressure method of testing applied to equipment intended to be evacuated may be criticized because of the inverse pressure differentials during the test compared with the final use of the equipment. The method does find wide application however, even for vacuum applications, particularly as relatively simple equipment may be used and also in cases where the long flow paths in the component would make vacuum testing difficult. The method depends on detecting the differences in thermal conductivity between a search gas and air. The equipment under test is pressurized with search gas so that search gas appears in the atmosphere in the immediate vicinity of a leak. Gas is continuously sampled by the detector probe and made to flow past the sensing elements which respond to small changes in thermal conductivity. It is well known that the thermal conductivity of a gas may be measured by a hot wire bridge method. In this method a resistance element, usually in the form of a thin wire or filament, is heated electrically and exposed to the gas. With a given power input into the wire, the temperature, and therefore resistance of the wire, depends on the thermal conductivity of the surrounding gas. Alternatively, the temperature (and resistance) of the wire may be maintained at a constant value and the required power input may either be measured directly or indirectly in terms of the applied voltage or current. In another modification of this method, a thermocouple is used which may be heated directly and the temperature measured in terms of the e.m.f. developed across the junction. Alternatively, the thermocouple may be connected to the hot wire and used to measure the temperature of the junction of the hot wire and thermocouple. Thermal conductivity measurements have been the basis of several types of gas analysis apparatuss-6. In principle the effect due to one gas is compared with that due to a reference gas. The gas is placed in contact with a heated wire, or filament, and the electrical conductivity of this filament is compared directly with the electrical conductivity of a similar filament in contact with the reference gas. In
and D. M. TINSLEY
some devices this has been repeated using up to four filaments there being three varying effects observed against a reference effect. The filaments form part of the Wheatstone bridge network, the out of balance voltage being a measure of the differences in gas concentration appearing at these filaments. This leak detector is based on a simple hot wire bridge in which two Katherometer resistance elements form two arms of the bridge network. One element is exposed to air containing traces of search gas while the other is exposed only to air and serves as a reference to compensate for changes in ambient conditions. The arrangement is shown schematically in Fig. 1. If the bridge voltage is E, the resistance of the Katharometer elements R and the resistance change in the presence of search gas is AR, then the out-ofbalance voltage v of the bridge is given by the approximate expression : E AR v
=zz
4
R
(the assumption is made that the impedance of the bridge balance indicator, i.e. the input resistance of the amplifier, is large compared with the bridge arm resistances). With the Kathatrometer elements at t”C above ambient and a temperature coefficient of resistance of a, we have R
= R. (1 f
ut)
where Ro is the resistance at room temperature (say 20°C). Then the resistance change due to the change in element temperature 1I is given by d R = Ro (1 At and hence E 4
(1 At
1+ut
To see the connexion between the temperature change At and the presence of search gas, consider the heat transfer Q by thermal conductivity of a heated wire in a gas. For a hot wire in air of temperature tr, above its surroundings, we have Q = Gql tl where
ql G
= thermal conductivity of air, = constant depending on area, diameter and geometrical configuration of hot wire. Now consider that some of the air is replaced by search gas of thermal conductivity 92. The heat transfer is held constant so that the wire temperature is changed from tl to t. With a concentration of search gas C (where 0 < C < 1) the concentration of air becomes I -C, then Q = C (6’ Hence (q2C + q1 (1 -C))
+ 41 (1-C)
) t
t = qltl
and therefore (42
-41)
ct
=
41
(t1
-0
At = change in wire temperature as a result of the presence of search gas with concentration C. Put qz/ql = K, i.e. the thermal conductivity of the search gas is referred to air (K = 1 for air). Hence, At (K-1)C = t tl
FIG. 1. Schematic
diagram
of leak detector
for overpressure
testing.
-t
=
Thermal Conductivity
Leak Detectors
The bridge out-of-balance may therefore terms of search gas concentration and conductivity as follows :
suitable for testing Equipment
be expressed in relative thermal
by Overpressure
155
or Vacuum
TABLE I
Relative
thermal
conductivity and viscosity compared with air*
of some gases
E V
=
4
1 I
at
w
-
1) c
Relative
This is essentially the voltage available at the input to the leak detector amplifier. A number of conclusions may be drawn from this result : For high sensitivity, the filament should be of a material having a large temperature coefficient of resistance. The filament temperature is controlled by the voltage applied to the bridge. The bridge out-of-balance, for a given resistance change, is proportional to the bridge voltage. To obtain a large out-of-balance current and high sensitivity it is therefore desirable to increase the bridge voltage. However, the temperature of the filament must be limited to less than 200°C to prevent oxidation and other gas reactions at its surface. In principle any search gas having a thermal conductivity different from that of air could be used. The sensitivity depends on the relative differences of thermal conductivities and some figures for common gases are compared in Fig. 2 and Table I. It is apparent that both hydrogen and helium show outstandingly large relative differences and are therefore the most sensitive search gases with this method. It is sometimes desirable however, for special applications to
Gas
Acetylene Air Ammonia Argon Benzene Carbon dioxide Carbon monoxide Carbon disulphide Chlorine Chloroform Ethane Ethylene Ethyl alcohol Ethyl chloride Ethyl ether
thermal conductivity at 0°C
Relative viscosity at 0°C
0.77 1.00 1.15 0.68 0.37
0.55 1 0.56 1.23 0.42
0.59
0.80
0.96
0.97
0.31 0.37 0.26 0.88 0.70
0.52 0.76 0.55 0.50 0.56
0.71
0.49
0.39
0.52
0.55
0.42
*Based on Chemical Edition, McGraw-Hill,
Relative thermal conductivity at 0°C
Gas
Freon “12” Helium Hydrogen Hydrogen sulphide Methane Methyl chloride Oxygen Nitrogen Nitric oxide Nitrous oxide n-hexane n-pentane isopentane Sulphur dioxide
Engineers’ Handbook, New York, 1950.
Relative viscosity atO”C
0.39 6.08 7.35
0.69 1.1 0.49
0.65 1.13
0.68 0.70
0.38 1.01 1.01
0.58 1.1 0.98
0.99
1.05
0.62 0.51 0.53
0.79 0.29 0.29
0.51
0.29
0.41
0.68
Edited J. H. Perry,
3rd
- -
-
mmonq
methone
Ir, oxygen.
orbon
ydroqen
hexone,
n1troqen, nitric
monoxide
oxide
sulphide
so-pentane
t I
L
m
3.2
FIG. 2. Diagram
showing
thermal
conductivities
employ one of the other possible search gases and the diagram It is clear gives some indication of results to be expected. that either gases with a thermal conductivity greater than air (such as hydrogen, helium, neon, methane, coal gas etc.) or those with thermal conductivities less than air (Freon, argon, carbon dioxide etc.) would be suitable.
referred
to air for different
gases.
If the detector responds to D divisions on the meter for a search gas with thermal conductivity Kl relative to air when X parts per million of search gas are present at the sensing elements, then D1 = AX(KI -1) (1) The response where A is a constant for the apparatus used.
156
W. STECKELMACHER
would be D2 when a search gas with thermal conductivity Kz is used at the same gas concentration where Dz is given by D2 = AX(K2-1)
(2)
Hence 02
=
D
!!k ' WI
‘) -
XU x 10-6 cm3/min at N.T.P. XU 1.27 x 10-s torr l./sec
D. M. TINSLEY
For example, assume constant temperature in a vessel filled with gas at a pressure of pi atmospheres. Let C be a constant dependent only on the size of the hole and VI the viscosity of the gas ; then Ll, the quantity of gas appearing outside the leak per minute is given by :
1)
(P,2
showing how the response is affected by the thermal properties of the search gas used. The search gas evolved from the leak is induced into the sampling probe by the action of a small pump. To obtain a good response the sensing elements should be small enough to be sited in chambers of small volume. As it is intended to detect changes in gas concentration rather than rates of flow, the gas should be made to flow past the entrance of the element chambers rather than through them. The minimum detectable leak, in terms of quantity of search gas per unit time, depends on the rate of flow of the gas through the leak detector and the minimum concentration to which the detector will respond. If u cm3/min is the rate of flow of search gas through the probe at N.T.P. and the detector will respond to X parts per million of search gas then this gas is detected at the probe with a sensitivity of or
and
(4)
(1 cm3/min = 12.7 x 10-3 torr l./sec) (5) On the most sensitive range for hydrogen as search gas approximate values are : X = 60 parts per million u = 8 cm3/min By reducing the rate of flow u smaller leaks can be detected. However, there is a practical limit since it is important in leak location that the detector should respond quickly when the probe is made to traverse the position of the leak. Reducing the rate of flow lengthens the response time and hence beyond a certain point the indications from the leak detector’ become meaningless. The minimum size of leak that can be detected, assuming that all the gas from the leak enters the probe*, may be expressed in terms of the quantity of search gas appearing at the entrance to the sampling probe. The actual size of hole that is detected depends on : (1) The sensitivity of the detector to the search gas ; (2) The concentration of search gas in the vessel or component under test ; (3) The excess test pressure ; and (4) The viscosity of the gas (and possibly the temperature if this should not be constant). Generally for the size of leak to be detected one can assume that the flow conditions are viscous and non-turbulent, so that the quantity of gas leaking is proportional to the viscosity of the gas and the difference in the squares of the absolute pressures. With these assumptions it is possible to estimate the expected leakage rates of a certain search gas at given pressure and temperature conditions from test carried out with a different search gas at other pressure and temperature conditions.
-
1) ';
L1
=
If the vessel is filled with another gas at a pressure Pz, its viscosity being Vz, then (P22 and
1) F2
=
Lz
(7)
(42
-
1)
v2
Ll
032
-
1)
VJ =
Lz
In practice Lz, V2, Pz, may be related to the specification to which the vessel is to be tested. Then L1 gives the required sensitivity of the detector, i.e.
where L1 and VI, PI now correspond to the required conditions when testing at the pressure PI. If a vessel is pressure tested to a pressure PI and also subjected to high vacuum (pressure PO assumed to be 0), we have PI2 -
1
Ll
vo
. v,
1
=
(10)
Lo
where LO is now the leakage rate (usually expressed in Torr l./sec) in the vacuum test with the viscosity of the gas in question VO. The negative sign is taken into account by the fact that in a vessel under vacuum the leakage is into the system whereas in a pressurized vessel the leakage is out from the system. The viscosity of some gases relative to air are listed in Table I. For a practical evaluation of a given leak test problem the short conversion table (Table II), listing different ways in which leakage rates are commonly expressed, will be found useful. TABLE II
Conversion 1 1 1 cm3 1 cm3 1 ft3 1 ft3
of leakage =l = = = = =
lusec clusec (STP)/min (STP)/year (STP)/year (STP)/hr
1 1.27 2.4 6.82 5.97 18.3 = EM
1 g of gas/set (M = Molecular
rates
x IO-3 torr x 10-s torr x 10-Z torr x lo-8 torr X 10-4 torr torr I./set
l./sec l./sec l./sec l./sec L/SK.
x 103 torr l./sec
weight of gas at 25 “C)
Description of the leak detector The leak detector sensing probe
Referring to Fig. 1 and Fig. 3, the sensing elements were mounted in a metal block inside a hand held probe unit. Gas samples were drawn up by a narrow bore tube which allowed the precise location of a leak to be determined.
*With the very small size leaks being considered
this is quite a valid assumption.
Thermal Conductivity Leak Detectors suitable for testing Equipment by Overpressure or Vacuum
effect of a change in hot wire temperature as a result of any change in bridge voltage. In an instrument, such as this in which small out-of-balance effects are to be measured, it is necessary to ensure very constant bridge voltage conditions. Essentially two different leak detectors were investigated, an a.c. and a d.c. operated instrument.
The sampling pump consisted of a small centrifugal blower which was at first separately mounted but in a later development incorporated in the sensing probe. It was found convenient to run the pump at two speeds : a fast speed for maximum speed of response, and a slower speed to give an increased detection sensitivity at some sacrifice in response time. The sensing elements consist of coils of thin tungsten wire mounted on glass-metal seals in a compact assembly into which the pump connection is made. The sensing probe was also fitted with a small meter to repeat the leak indication of the amplifier unit. This facility was found to be convenient to operators particularly when testing awkwardly shaped equipment.
(1) In the a.c. design, a saturated core type of constant voltage transformer was used to supply a stable bridge voltage. In this case the bridge was supplied with an a.c. voltage and the bridge out-of-balance was fed into a special a.c. amplifier and indicator unit. Clearly an a.c. method is useful in mains operated instruments. (2) In the other design the bridge is supplied with a stabilized d.c. voltage or a dry cell of suitably slow discharge characteristics. This calls for the slightly more awkward The method lends itself for a problem of d.c. amplification. truly portable battery operated equipment. The design of a mains operated a.c. amplifier and leak indicator is shown in Fig. 4. The a.c. amplifier provides straight amplification of the input signal in the first two stages. The third stage is a phase sensitive rectifier the output d.c. being fed to a moving coil centre reading milliameter*. This d.c. signal is also used to activate a relaxation oscillator which provides a note increasing in frequency as the leakage indication rises. This is convenient for remote operation where the operator cannot see the meter indication. The amplifier sensitivity is such that on its most sensitive range $ mV out-of-balance gives full scale deflection on the output meter. As this amplifier was designed to operate at 50 c/s (or 60 c/s) and is connected to the a.c. mains, the usual precautions in design and layout had to be taken to ensure minimum a.c. pick-up effects within the amplifier. The use of a phase sensitive rectifier actually allows some latitude. Thus the amplifier is not very sensitive to say 100 c/s components,
The leak detector power supply and amplifier
The sensing elements form two arms of the bridge network and are incorporated in the probe unit. The other two arms are conveniently made of high stability wire wound fixed and variable resistors in the amplifier unit. The variable resistors allow the bridge to be balanced when both elements operate under identical conditions. The bridge out-of-balance is, among other things, proportional to the bridge voltage applied ; apart from the indirect
FIG. 3. Cross-sectional
view of gas detector
hand
unit.
Amplifier
I
FIG. 4. Leak test amplifier
157
Ph
ose sens~i~ve detector 1.c. REFERENCE
1
circuit diagram
for ax. operation.
*It may be noted that if the output from the a.c. amplifier had been converted to d.c. by straight rectification, then the sense of any bridge outof-balance would have been lost and an ordinary end zero reading meter would have been used. Such an arrangement is generally very inconvenient when trying to find the bridge balance condition.
158
W. STECKLEMACHER AND D. M. TINSLEY
e.g. from the full wave rectified HT supply. In practice production problems associated with the manufacture of sensitive equipment of this type have been solved by the use of printed circuits in which the precise component layout is determined once and for all. When testing equipment suffering from relatively large leakage, the precise location of a leak can be made difficult if the sensitivity of the detector is so high that the indicator is swamped when in close proximity to the leak. One way would be to reduce the overpressure of search gas in the equipment. A more convenient method is to attenuate the bridge output fed to the amplifier. A simple potential divider giving four sensitivity ratios 3.3 : 1 each has been found convenient. In the alternative completely portable equipment, d.c. operation was used as mentioned above. The circuit diagram is shown in Fig. 5. The d.c. amplifier is a straight amplifier using three emitter coupled pairs of transistors. The input pair are silicon Amplher OUTPUT I
transistors having a low leakage current which minimizes the temperature drift of the amplifier. The sensitivity of the amplifier is such that a change of 1 mV in the bridge out of balance gives full scale deflection in the output meter. The transistors are mounted in pairs in good thermal contact so that any drift in one will be balanced by a similar drift in the other thus giving a minimum change in the output. The amplifier is on a printed circuit measuring less than 2 in. x 1 in. It is fed by a 6 V battery consuming about 20 mA current. The amplifier, batteries, indicator, motor and fan are all contained in the small hand unit holding the probe assembly. To keep down the size of the instrument no audio indication is provided. Performance b complete instrument (a.c. type) is shown in Fig. 6. Typical results obtained using this leak detector with various search gases are shown in Table III. In use much depends TABLE III
Detectable
ML TER -
leakage for some selected gases (indicating
1/lo F.S.D. on the most sensitive range) Gas
cm3 (STP)/min
Torr I./set -
Hydrogen Freon Town gas Argon
FE.
5. Leak test amplifier
circuit diagram
x IO-4 x 10-Z x IO-4 x 10-3
lo-6 1.3 x lo-4 1 x 10-S 1 x lo--4
6.5
x
on the atmosphere being free from search gas. If a very leaky component is being tested the atmosphere itself may become concentrated with search gas. Whilst this is inherently balanced out, using the controls, the ultimate sensitivity is bound to suffer. Thus, when referring to this
for d.c. operation.
FIG. 6. Photograph
5 1 8 8
of complete
leak detection
instrument.
Thermal Conductivity Leak Detectors suitable for testing Equipment by Overpressure or Vacuum table it should be borne in mind that the atmosphere is taken to be quite free from search gas. This method has produced many good results and enabled leaks of reasonable size to be located quite readily and with certainty. The mains operated unit has proved its worth on many widely varying applications, saving considerable time and trouble. The portable unit, though still a laboratory development, is intended to provide a quick means of leak detection in places where mains supplies are difficult to find or even non-existent. It incorporates only a single range of sensitivity and is handled almost as easily as a pocket torch. It is expected that the ultimate sensitivity of this instrument will be slightly less than that of the mains unit. The relatively low operating temperature of the filaments makes the detector quite safe to use under most industrial conditions. Also the life and long term stability of the sensing elements should be very good and in practice the only effect which has been noted after long periods of operation in industrial conditions was the accumulation of a dust deposit in the intake line which was easily removed. The thermal conductivity vacuum testing
leak detector
used for
Thermal conductivity methods are well known both for low pressure measurement (Pirani gauges, thermocouple gauges etc. have been discussed’-8) and to a lesser extent for leak detection-12. These gauges are widely used industrially especially in the pressure range from 1 torr to 1 m torr. In their application to leak detection, their sensitivity to the overall pressure change as well as to changes in the partial pressure of the search gas is a decided limitation and often sets a practical limit to the minimum size leak which may be detected. The effect can be minimized by the use of a refrigerated charcoal trapiz. In vacuum testing the detector element takes on a particularly simple form. The normal Pirani gauge head (as used for pressure measurement) may be used and is inserted in a suitable position in the vacuum system. In systems involving a vapour pump the gauge is best inserted into the backing line with a manually operated throttle valve to the backing pump. Reference should be made to earlier work for detailsis 2, ii. The electrical problems of hot wire bridge supplies and indication are exactly analogous to those discussed for the overpressure detector above and in practice the same power supply and amplifier equipment already described may be used. In fact this has considerable advantages over the use of a sensitively suspended and mechanically very delicate galvanometers previously used. The amplifier can be easily arranged to be quite insensitive to large overload conditions which can notoriously arise under low pressure conditions when for leak test purposes the indicator has been made sensitive to very small pressure changes. For vacuum testing, a Pirani gauge head and suitable fixed resistance take the place of the sampling probes and compensator of the Katherometer head shown in Fig. 1.
159
The only additional requirement is that the bridge balance controls must now have sufficient range to be able to obtain a bridge zero at any prevailing system pressure from atmoThe balance spheric right down to very low pressures. controls have therefore been split up into a coarse balance to cover the whole range and a fine balance to achieve easy control on the more sensitive ranges of the amplifier. An alternative arrangement, to overcome sensitivity to pressure fluctuations, is to use two Pirani gauge heads in place of the Katherometer elements to obtain a differential set up (see also Ref. 7). Both are connected into the same vacuum system and therefore respond equally to pressure fluctuations. By using a chemical trap in the pumping line of one head and an equivalent constriction in the other the instrument gives a preferential indication of search gas, but this was not so far fully investigated. In practice using the single head arrangement down to 10-s torr l./sec may be detected with hydrogen as search gas. The actual sensitivity achievable depends very much on the design of the particular vacuum system and on any precautions that have been taken to ensure minimum pressure fluctuations. The instrument has been found very useful in testing all kinds of vacuum equipment and plant and has also been applied to the production testing of components on a leak test pumping station specially designed for this method of test. Acknowledgements
Acknowledgment is made to Mr. F. D. Edwards, Managing Director of Edwards High Vacuum Ltd. for encouragement and permission to publish this paper and to the various members of the Instrument Research Division who have contributed to the development of the instruments here Particular mention is made of J. M. Parisot described. working on the earlier design, T. Chapman, P. Gittins and J. English who have all made significant contributions. References
1 A. Guthrie and R. K. Wakerling; Vacuum Equipment and Techniques, McGraw-Hill, New York, 1949, 190-242. 2 S. H. Cross and W. Steckelmacher; Leak Detection by Vacuum Techniques, Research, 1956, p. 124-l 3 1. 3 W. Steckelmacher; Testing of Vacuum Equipment and Hermetically Sealed Components for Leak Tightness, Schweizer Archiv 27, Feb. 1961, 179-188. 4 H. Kronberger; Vacuum Techniques in the Atomic Energy Industry, Proc. I. Mech. E. 172, 1958. 5 H. A. Daynes; Gas Analysis by Measurement of Thermal Conductivity, Cambridge University Press, 1933. 6 A. Farkas and H. W. Melville; Experimental Methods in Gas Reacfions, Macmillan, London, 1939. 7 J. H. Leek; Pressure Measurement in Vacuum Systems, Institute of Physics, London, 1957. * W. Steckelmacher; The Measurement of Pressure in Industrial High Vacuum Systems, Instrument Practice, May 1960. 9 T. R. Cuykendall; Use of Pirani Gauges in Finding Vacuum Leaks, Rev. Sri. Instrum. 6, 1935, 371-372. 10 J. Blears and J. H. Leek; Differential Methods of Leak Detection, Brit. .I. App. Phys. 2, Aug. 1951, 227-232. tt N. Ochert and W. Steckelmacher; Leak Detection Practice, Vacuum 2. 1952. 125-136. t* T. B.‘Kent;’ A Hydrogen Pirani Leak Detector Using a Charcoal Trap, J. Sci. Instrum. 32, Apr. 1955, 132-134.
and D. M. TINSLEY
Instrument Research Division, Edwards High Vacuum Ltd., Crawley, Sussex (Received 23 January 1962: accepted II ApriI 1962) A new principle of leak detection was investigated depending on the differences in thermal conductivities between a search gas and air. A hot wire element exposed to air containing traces of search gas changes its temperature and hence its resistance and this change was detected by comparison with a reference element exposed to air. For leak testing by the overpressure method, the equipment was pressurized with a suitable search gas. The sensitive elements of the detector were mounted in a hand-held probe unit with which gas samples could be collected. The sampled gas was made to flow over the sensing element while ambient air The elements heated to about 200°C were was made to flow over the reference element. arranged in a bridge circuit and out of balance voltages amplified and indicated as leakage A number of search gases were used but hydrogen or on a meter or as an audible response. Some results are also reported of the use of the detector helium showed the best sensitivities. with elements mounted in vacuum systems (Pirani type vacuum gauges) for vacuum leak detection.
Un nouveau principe de detection de fuite base sur la difference des conductibilites thermiques de Pair et d’un gaz traceur. Un filament chauffe, expose a de I’air contenant des traces de gaz traceur, change de temperature et par consequent de resistance ; ce changement est detect& par comparaison a l’aide d’element de reference expose’ h Pair. Pour la detection de fuite par la methode de surpression, l’equipement a tester est pressurise’ a l’aide d’un gaz traceur approprie’. Les filaments sont fixes a l’interieur d’une tete de detection montee sur une Le gaz traceur passe sur le filament detecteur tandis que Pair ambiant passe sur le poignee. filament de reference. Les filaments port&s a une temperature de 200°C environ sont branches dans un circuit en pont ; la tension de desequilibre, amplifiee, est enregistree par un appareil Un certain nombre de gaz traceurs fut de mesure sous forme de signal audible et visible. employ& mais les meilleures sensibilitb sont obtenues avec de l’hydrogene ou de I’helium. On donne quelques r&bats obtenus avec ce de’tecteur pour la detection de fuite sous vide, I’PIPment sensible &ant une jauge de Pirani.
Introduction
Spectrometer leak detectors, and it is advisable to consider a method of test best adapted to each particular application. The methods described in this paper have the advantages of using search gas and indicating leakage by electrical means. In addition, the apparatus is simple to use and maintain though naturally not having the sensitivity of some other types of more complex equipment. For leak detection of pressurized equipment the detector unit can be incorporated into a hand held search probe. It will be shown that the electrical supply and indicator unit developed for this purpose may be used with advantage also for vacuum testing by mounting the detector in the vacuum system.
The methods used for leak detection are widely diverse, for, not only are they required in the testing of vacuum equipment, but also in the examination of hermetically sealed containers for gases or liquidsi-4. These containers may be of any size and shape and have widely differing permissible leakage rates. For example, leak detection has been carried out on small transistor devices at one extreme, to large liquid gas, vacuum insulated, storage vessels at the other. Clearly, the different methods used will have varying degrees of complexity, ranging from simple soap bubble methods to relatively complex Mass *This paper
was presented
in part at the Congr&s International
des Techniques 153
et Applications
du Vide, in Paris on 24 June 1961.
W.
154
The thermal conductivity over pressure testing
STECKELMACHER
leak detector
used for
Theory Any overpressure method of testing applied to equipment intended to be evacuated may be criticized because of the inverse pressure differentials during the test compared with the final use of the equipment. The method does find wide application however, even for vacuum applications, particularly as relatively simple equipment may be used and also in cases where the long flow paths in the component would make vacuum testing difficult. The method depends on detecting the differences in thermal conductivity between a search gas and air. The equipment under test is pressurized with search gas so that search gas appears in the atmosphere in the immediate vicinity of a leak. Gas is continuously sampled by the detector probe and made to flow past the sensing elements which respond to small changes in thermal conductivity. It is well known that the thermal conductivity of a gas may be measured by a hot wire bridge method. In this method a resistance element, usually in the form of a thin wire or filament, is heated electrically and exposed to the gas. With a given power input into the wire, the temperature, and therefore resistance of the wire, depends on the thermal conductivity of the surrounding gas. Alternatively, the temperature (and resistance) of the wire may be maintained at a constant value and the required power input may either be measured directly or indirectly in terms of the applied voltage or current. In another modification of this method, a thermocouple is used which may be heated directly and the temperature measured in terms of the e.m.f. developed across the junction. Alternatively, the thermocouple may be connected to the hot wire and used to measure the temperature of the junction of the hot wire and thermocouple. Thermal conductivity measurements have been the basis of several types of gas analysis apparatuss-6. In principle the effect due to one gas is compared with that due to a reference gas. The gas is placed in contact with a heated wire, or filament, and the electrical conductivity of this filament is compared directly with the electrical conductivity of a similar filament in contact with the reference gas. In
and D. M. TINSLEY
some devices this has been repeated using up to four filaments there being three varying effects observed against a reference effect. The filaments form part of the Wheatstone bridge network, the out of balance voltage being a measure of the differences in gas concentration appearing at these filaments. This leak detector is based on a simple hot wire bridge in which two Katherometer resistance elements form two arms of the bridge network. One element is exposed to air containing traces of search gas while the other is exposed only to air and serves as a reference to compensate for changes in ambient conditions. The arrangement is shown schematically in Fig. 1. If the bridge voltage is E, the resistance of the Katharometer elements R and the resistance change in the presence of search gas is AR, then the out-ofbalance voltage v of the bridge is given by the approximate expression : E AR v
=zz
4
R
(the assumption is made that the impedance of the bridge balance indicator, i.e. the input resistance of the amplifier, is large compared with the bridge arm resistances). With the Kathatrometer elements at t”C above ambient and a temperature coefficient of resistance of a, we have R
= R. (1 f
ut)
where Ro is the resistance at room temperature (say 20°C). Then the resistance change due to the change in element temperature 1I is given by d R = Ro (1 At and hence E 4
(1 At
1+ut
To see the connexion between the temperature change At and the presence of search gas, consider the heat transfer Q by thermal conductivity of a heated wire in a gas. For a hot wire in air of temperature tr, above its surroundings, we have Q = Gql tl where
ql G
= thermal conductivity of air, = constant depending on area, diameter and geometrical configuration of hot wire. Now consider that some of the air is replaced by search gas of thermal conductivity 92. The heat transfer is held constant so that the wire temperature is changed from tl to t. With a concentration of search gas C (where 0 < C < 1) the concentration of air becomes I -C, then Q = C (6’ Hence (q2C + q1 (1 -C))
+ 41 (1-C)
) t
t = qltl
and therefore (42
-41)
ct
=
41
(t1
-0
At = change in wire temperature as a result of the presence of search gas with concentration C. Put qz/ql = K, i.e. the thermal conductivity of the search gas is referred to air (K = 1 for air). Hence, At (K-1)C = t tl
FIG. 1. Schematic
diagram
of leak detector
for overpressure
testing.
-t
=
Thermal Conductivity
Leak Detectors
The bridge out-of-balance may therefore terms of search gas concentration and conductivity as follows :
suitable for testing Equipment
be expressed in relative thermal
by Overpressure
155
or Vacuum
TABLE I
Relative
thermal
conductivity and viscosity compared with air*
of some gases
E V
=
4
1 I
at
w
-
1) c
Relative
This is essentially the voltage available at the input to the leak detector amplifier. A number of conclusions may be drawn from this result : For high sensitivity, the filament should be of a material having a large temperature coefficient of resistance. The filament temperature is controlled by the voltage applied to the bridge. The bridge out-of-balance, for a given resistance change, is proportional to the bridge voltage. To obtain a large out-of-balance current and high sensitivity it is therefore desirable to increase the bridge voltage. However, the temperature of the filament must be limited to less than 200°C to prevent oxidation and other gas reactions at its surface. In principle any search gas having a thermal conductivity different from that of air could be used. The sensitivity depends on the relative differences of thermal conductivities and some figures for common gases are compared in Fig. 2 and Table I. It is apparent that both hydrogen and helium show outstandingly large relative differences and are therefore the most sensitive search gases with this method. It is sometimes desirable however, for special applications to
Gas
Acetylene Air Ammonia Argon Benzene Carbon dioxide Carbon monoxide Carbon disulphide Chlorine Chloroform Ethane Ethylene Ethyl alcohol Ethyl chloride Ethyl ether
thermal conductivity at 0°C
Relative viscosity at 0°C
0.77 1.00 1.15 0.68 0.37
0.55 1 0.56 1.23 0.42
0.59
0.80
0.96
0.97
0.31 0.37 0.26 0.88 0.70
0.52 0.76 0.55 0.50 0.56
0.71
0.49
0.39
0.52
0.55
0.42
*Based on Chemical Edition, McGraw-Hill,
Relative thermal conductivity at 0°C
Gas
Freon “12” Helium Hydrogen Hydrogen sulphide Methane Methyl chloride Oxygen Nitrogen Nitric oxide Nitrous oxide n-hexane n-pentane isopentane Sulphur dioxide
Engineers’ Handbook, New York, 1950.
Relative viscosity atO”C
0.39 6.08 7.35
0.69 1.1 0.49
0.65 1.13
0.68 0.70
0.38 1.01 1.01
0.58 1.1 0.98
0.99
1.05
0.62 0.51 0.53
0.79 0.29 0.29
0.51
0.29
0.41
0.68
Edited J. H. Perry,
3rd
- -
-
mmonq
methone
Ir, oxygen.
orbon
ydroqen
hexone,
n1troqen, nitric
monoxide
oxide
sulphide
so-pentane
t I
L
m
3.2
FIG. 2. Diagram
showing
thermal
conductivities
employ one of the other possible search gases and the diagram It is clear gives some indication of results to be expected. that either gases with a thermal conductivity greater than air (such as hydrogen, helium, neon, methane, coal gas etc.) or those with thermal conductivities less than air (Freon, argon, carbon dioxide etc.) would be suitable.
referred
to air for different
gases.
If the detector responds to D divisions on the meter for a search gas with thermal conductivity Kl relative to air when X parts per million of search gas are present at the sensing elements, then D1 = AX(KI -1) (1) The response where A is a constant for the apparatus used.
156
W. STECKELMACHER
would be D2 when a search gas with thermal conductivity Kz is used at the same gas concentration where Dz is given by D2 = AX(K2-1)
(2)
Hence 02
=
D
!!k ' WI
‘) -
XU x 10-6 cm3/min at N.T.P. XU 1.27 x 10-s torr l./sec
D. M. TINSLEY
For example, assume constant temperature in a vessel filled with gas at a pressure of pi atmospheres. Let C be a constant dependent only on the size of the hole and VI the viscosity of the gas ; then Ll, the quantity of gas appearing outside the leak per minute is given by :
1)
(P,2
showing how the response is affected by the thermal properties of the search gas used. The search gas evolved from the leak is induced into the sampling probe by the action of a small pump. To obtain a good response the sensing elements should be small enough to be sited in chambers of small volume. As it is intended to detect changes in gas concentration rather than rates of flow, the gas should be made to flow past the entrance of the element chambers rather than through them. The minimum detectable leak, in terms of quantity of search gas per unit time, depends on the rate of flow of the gas through the leak detector and the minimum concentration to which the detector will respond. If u cm3/min is the rate of flow of search gas through the probe at N.T.P. and the detector will respond to X parts per million of search gas then this gas is detected at the probe with a sensitivity of or
and
(4)
(1 cm3/min = 12.7 x 10-3 torr l./sec) (5) On the most sensitive range for hydrogen as search gas approximate values are : X = 60 parts per million u = 8 cm3/min By reducing the rate of flow u smaller leaks can be detected. However, there is a practical limit since it is important in leak location that the detector should respond quickly when the probe is made to traverse the position of the leak. Reducing the rate of flow lengthens the response time and hence beyond a certain point the indications from the leak detector’ become meaningless. The minimum size of leak that can be detected, assuming that all the gas from the leak enters the probe*, may be expressed in terms of the quantity of search gas appearing at the entrance to the sampling probe. The actual size of hole that is detected depends on : (1) The sensitivity of the detector to the search gas ; (2) The concentration of search gas in the vessel or component under test ; (3) The excess test pressure ; and (4) The viscosity of the gas (and possibly the temperature if this should not be constant). Generally for the size of leak to be detected one can assume that the flow conditions are viscous and non-turbulent, so that the quantity of gas leaking is proportional to the viscosity of the gas and the difference in the squares of the absolute pressures. With these assumptions it is possible to estimate the expected leakage rates of a certain search gas at given pressure and temperature conditions from test carried out with a different search gas at other pressure and temperature conditions.
-
1) ';
L1
=
If the vessel is filled with another gas at a pressure Pz, its viscosity being Vz, then (P22 and
1) F2
=
Lz
(7)
(42
-
1)
v2
Ll
032
-
1)
VJ =
Lz
In practice Lz, V2, Pz, may be related to the specification to which the vessel is to be tested. Then L1 gives the required sensitivity of the detector, i.e.
where L1 and VI, PI now correspond to the required conditions when testing at the pressure PI. If a vessel is pressure tested to a pressure PI and also subjected to high vacuum (pressure PO assumed to be 0), we have PI2 -
1
Ll
vo
. v,
1
=
(10)
Lo
where LO is now the leakage rate (usually expressed in Torr l./sec) in the vacuum test with the viscosity of the gas in question VO. The negative sign is taken into account by the fact that in a vessel under vacuum the leakage is into the system whereas in a pressurized vessel the leakage is out from the system. The viscosity of some gases relative to air are listed in Table I. For a practical evaluation of a given leak test problem the short conversion table (Table II), listing different ways in which leakage rates are commonly expressed, will be found useful. TABLE II
Conversion 1 1 1 cm3 1 cm3 1 ft3 1 ft3
of leakage =l = = = = =
lusec clusec (STP)/min (STP)/year (STP)/year (STP)/hr
1 1.27 2.4 6.82 5.97 18.3 = EM
1 g of gas/set (M = Molecular
rates
x IO-3 torr x 10-s torr x 10-Z torr x lo-8 torr X 10-4 torr torr I./set
l./sec l./sec l./sec l./sec L/SK.
x 103 torr l./sec
weight of gas at 25 “C)
Description of the leak detector The leak detector sensing probe
Referring to Fig. 1 and Fig. 3, the sensing elements were mounted in a metal block inside a hand held probe unit. Gas samples were drawn up by a narrow bore tube which allowed the precise location of a leak to be determined.
*With the very small size leaks being considered
this is quite a valid assumption.
Thermal Conductivity Leak Detectors suitable for testing Equipment by Overpressure or Vacuum
effect of a change in hot wire temperature as a result of any change in bridge voltage. In an instrument, such as this in which small out-of-balance effects are to be measured, it is necessary to ensure very constant bridge voltage conditions. Essentially two different leak detectors were investigated, an a.c. and a d.c. operated instrument.
The sampling pump consisted of a small centrifugal blower which was at first separately mounted but in a later development incorporated in the sensing probe. It was found convenient to run the pump at two speeds : a fast speed for maximum speed of response, and a slower speed to give an increased detection sensitivity at some sacrifice in response time. The sensing elements consist of coils of thin tungsten wire mounted on glass-metal seals in a compact assembly into which the pump connection is made. The sensing probe was also fitted with a small meter to repeat the leak indication of the amplifier unit. This facility was found to be convenient to operators particularly when testing awkwardly shaped equipment.
(1) In the a.c. design, a saturated core type of constant voltage transformer was used to supply a stable bridge voltage. In this case the bridge was supplied with an a.c. voltage and the bridge out-of-balance was fed into a special a.c. amplifier and indicator unit. Clearly an a.c. method is useful in mains operated instruments. (2) In the other design the bridge is supplied with a stabilized d.c. voltage or a dry cell of suitably slow discharge characteristics. This calls for the slightly more awkward The method lends itself for a problem of d.c. amplification. truly portable battery operated equipment. The design of a mains operated a.c. amplifier and leak indicator is shown in Fig. 4. The a.c. amplifier provides straight amplification of the input signal in the first two stages. The third stage is a phase sensitive rectifier the output d.c. being fed to a moving coil centre reading milliameter*. This d.c. signal is also used to activate a relaxation oscillator which provides a note increasing in frequency as the leakage indication rises. This is convenient for remote operation where the operator cannot see the meter indication. The amplifier sensitivity is such that on its most sensitive range $ mV out-of-balance gives full scale deflection on the output meter. As this amplifier was designed to operate at 50 c/s (or 60 c/s) and is connected to the a.c. mains, the usual precautions in design and layout had to be taken to ensure minimum a.c. pick-up effects within the amplifier. The use of a phase sensitive rectifier actually allows some latitude. Thus the amplifier is not very sensitive to say 100 c/s components,
The leak detector power supply and amplifier
The sensing elements form two arms of the bridge network and are incorporated in the probe unit. The other two arms are conveniently made of high stability wire wound fixed and variable resistors in the amplifier unit. The variable resistors allow the bridge to be balanced when both elements operate under identical conditions. The bridge out-of-balance is, among other things, proportional to the bridge voltage applied ; apart from the indirect
FIG. 3. Cross-sectional
view of gas detector
hand
unit.
Amplifier
I
FIG. 4. Leak test amplifier
157
Ph
ose sens~i~ve detector 1.c. REFERENCE
1
circuit diagram
for ax. operation.
*It may be noted that if the output from the a.c. amplifier had been converted to d.c. by straight rectification, then the sense of any bridge outof-balance would have been lost and an ordinary end zero reading meter would have been used. Such an arrangement is generally very inconvenient when trying to find the bridge balance condition.
158
W. STECKLEMACHER AND D. M. TINSLEY
e.g. from the full wave rectified HT supply. In practice production problems associated with the manufacture of sensitive equipment of this type have been solved by the use of printed circuits in which the precise component layout is determined once and for all. When testing equipment suffering from relatively large leakage, the precise location of a leak can be made difficult if the sensitivity of the detector is so high that the indicator is swamped when in close proximity to the leak. One way would be to reduce the overpressure of search gas in the equipment. A more convenient method is to attenuate the bridge output fed to the amplifier. A simple potential divider giving four sensitivity ratios 3.3 : 1 each has been found convenient. In the alternative completely portable equipment, d.c. operation was used as mentioned above. The circuit diagram is shown in Fig. 5. The d.c. amplifier is a straight amplifier using three emitter coupled pairs of transistors. The input pair are silicon Amplher OUTPUT I
transistors having a low leakage current which minimizes the temperature drift of the amplifier. The sensitivity of the amplifier is such that a change of 1 mV in the bridge out of balance gives full scale deflection in the output meter. The transistors are mounted in pairs in good thermal contact so that any drift in one will be balanced by a similar drift in the other thus giving a minimum change in the output. The amplifier is on a printed circuit measuring less than 2 in. x 1 in. It is fed by a 6 V battery consuming about 20 mA current. The amplifier, batteries, indicator, motor and fan are all contained in the small hand unit holding the probe assembly. To keep down the size of the instrument no audio indication is provided. Performance b complete instrument (a.c. type) is shown in Fig. 6. Typical results obtained using this leak detector with various search gases are shown in Table III. In use much depends TABLE III
Detectable
ML TER -
leakage for some selected gases (indicating
1/lo F.S.D. on the most sensitive range) Gas
cm3 (STP)/min
Torr I./set -
Hydrogen Freon Town gas Argon
FE.
5. Leak test amplifier
circuit diagram
x IO-4 x 10-Z x IO-4 x 10-3
lo-6 1.3 x lo-4 1 x 10-S 1 x lo--4
6.5
x
on the atmosphere being free from search gas. If a very leaky component is being tested the atmosphere itself may become concentrated with search gas. Whilst this is inherently balanced out, using the controls, the ultimate sensitivity is bound to suffer. Thus, when referring to this
for d.c. operation.
FIG. 6. Photograph
5 1 8 8
of complete
leak detection
instrument.
Thermal Conductivity Leak Detectors suitable for testing Equipment by Overpressure or Vacuum table it should be borne in mind that the atmosphere is taken to be quite free from search gas. This method has produced many good results and enabled leaks of reasonable size to be located quite readily and with certainty. The mains operated unit has proved its worth on many widely varying applications, saving considerable time and trouble. The portable unit, though still a laboratory development, is intended to provide a quick means of leak detection in places where mains supplies are difficult to find or even non-existent. It incorporates only a single range of sensitivity and is handled almost as easily as a pocket torch. It is expected that the ultimate sensitivity of this instrument will be slightly less than that of the mains unit. The relatively low operating temperature of the filaments makes the detector quite safe to use under most industrial conditions. Also the life and long term stability of the sensing elements should be very good and in practice the only effect which has been noted after long periods of operation in industrial conditions was the accumulation of a dust deposit in the intake line which was easily removed. The thermal conductivity vacuum testing
leak detector
used for
Thermal conductivity methods are well known both for low pressure measurement (Pirani gauges, thermocouple gauges etc. have been discussed’-8) and to a lesser extent for leak detection-12. These gauges are widely used industrially especially in the pressure range from 1 torr to 1 m torr. In their application to leak detection, their sensitivity to the overall pressure change as well as to changes in the partial pressure of the search gas is a decided limitation and often sets a practical limit to the minimum size leak which may be detected. The effect can be minimized by the use of a refrigerated charcoal trapiz. In vacuum testing the detector element takes on a particularly simple form. The normal Pirani gauge head (as used for pressure measurement) may be used and is inserted in a suitable position in the vacuum system. In systems involving a vapour pump the gauge is best inserted into the backing line with a manually operated throttle valve to the backing pump. Reference should be made to earlier work for detailsis 2, ii. The electrical problems of hot wire bridge supplies and indication are exactly analogous to those discussed for the overpressure detector above and in practice the same power supply and amplifier equipment already described may be used. In fact this has considerable advantages over the use of a sensitively suspended and mechanically very delicate galvanometers previously used. The amplifier can be easily arranged to be quite insensitive to large overload conditions which can notoriously arise under low pressure conditions when for leak test purposes the indicator has been made sensitive to very small pressure changes. For vacuum testing, a Pirani gauge head and suitable fixed resistance take the place of the sampling probes and compensator of the Katherometer head shown in Fig. 1.
159
The only additional requirement is that the bridge balance controls must now have sufficient range to be able to obtain a bridge zero at any prevailing system pressure from atmoThe balance spheric right down to very low pressures. controls have therefore been split up into a coarse balance to cover the whole range and a fine balance to achieve easy control on the more sensitive ranges of the amplifier. An alternative arrangement, to overcome sensitivity to pressure fluctuations, is to use two Pirani gauge heads in place of the Katherometer elements to obtain a differential set up (see also Ref. 7). Both are connected into the same vacuum system and therefore respond equally to pressure fluctuations. By using a chemical trap in the pumping line of one head and an equivalent constriction in the other the instrument gives a preferential indication of search gas, but this was not so far fully investigated. In practice using the single head arrangement down to 10-s torr l./sec may be detected with hydrogen as search gas. The actual sensitivity achievable depends very much on the design of the particular vacuum system and on any precautions that have been taken to ensure minimum pressure fluctuations. The instrument has been found very useful in testing all kinds of vacuum equipment and plant and has also been applied to the production testing of components on a leak test pumping station specially designed for this method of test. Acknowledgements
Acknowledgment is made to Mr. F. D. Edwards, Managing Director of Edwards High Vacuum Ltd. for encouragement and permission to publish this paper and to the various members of the Instrument Research Division who have contributed to the development of the instruments here Particular mention is made of J. M. Parisot described. working on the earlier design, T. Chapman, P. Gittins and J. English who have all made significant contributions. References
1 A. Guthrie and R. K. Wakerling; Vacuum Equipment and Techniques, McGraw-Hill, New York, 1949, 190-242. 2 S. H. Cross and W. Steckelmacher; Leak Detection by Vacuum Techniques, Research, 1956, p. 124-l 3 1. 3 W. Steckelmacher; Testing of Vacuum Equipment and Hermetically Sealed Components for Leak Tightness, Schweizer Archiv 27, Feb. 1961, 179-188. 4 H. Kronberger; Vacuum Techniques in the Atomic Energy Industry, Proc. I. Mech. E. 172, 1958. 5 H. A. Daynes; Gas Analysis by Measurement of Thermal Conductivity, Cambridge University Press, 1933. 6 A. Farkas and H. W. Melville; Experimental Methods in Gas Reacfions, Macmillan, London, 1939. 7 J. H. Leek; Pressure Measurement in Vacuum Systems, Institute of Physics, London, 1957. * W. Steckelmacher; The Measurement of Pressure in Industrial High Vacuum Systems, Instrument Practice, May 1960. 9 T. R. Cuykendall; Use of Pirani Gauges in Finding Vacuum Leaks, Rev. Sri. Instrum. 6, 1935, 371-372. 10 J. Blears and J. H. Leek; Differential Methods of Leak Detection, Brit. .I. App. Phys. 2, Aug. 1951, 227-232. tt N. Ochert and W. Steckelmacher; Leak Detection Practice, Vacuum 2. 1952. 125-136. t* T. B.‘Kent;’ A Hydrogen Pirani Leak Detector Using a Charcoal Trap, J. Sci. Instrum. 32, Apr. 1955, 132-134.