Technique of High Vaccum, Part Five

Transcriped to Web format by Bruce Conover
SAS Technical Illustrations Department.

Vacuum gauges. A vacuum gauge determines the pressure in an evacuated apparatus by a measurement of some physical property of the residual gases, such as viscosity, heat conductivity, and so forth. The measurement of the response of a gauge to the residual gas naturally becomes more delicate as the gas becomes more and more tenuous Finally, below a certain pressure limit (which is characteristic of a given gauge) the gauge does not behave measurably different from what it would if the vacuum were perfect. For example, a discharge tube will give qualitative indications of pressure down to about 10^-3 mm of mercury. Below this pressure the tube becomes nonluminous and non-conducting. The characteristic limits for some of the other gauges are as follows:

The operation of the McLeod gauge depends on a definite volume of residual gases being compressed, so that as the volume decreases, the pressure is increased to a value at which the hydrostatic head of mercury can be measured with an ordinary scale.

The ionization gauge measures with a galvanometer the positive ions that are formed in an electric field when the residual gas is bombarded with electrons. The Langmuir gauge depends on the measurement of viscosity, and the Pirani gauge on the measurement of heat conduction of the residual gas. The Knudsen absolute manometer measures the momentum transferred from a hot to a cold surface by the gas molecules.

Of the above gauges, only the McLeod and Knudsen are absolute manometers in the sense that their geometry and other measurable characteristics of construction and operation determine their response at a given pressure. The McLeod gauge is the simplest and most reliable for permanent gases, but it has the disadvantage of giving erratic response or no response at all to water vapor, carbon dioxide, ammonia, and pump oil vapors which adsorb on the walls of the gauge or condense to a liquid. This disadvantage is serious, inasmuch as water vapor, carbon dioxide, and so forth are often of importance in the last stages of obtaming a high vacuum. The Knudsen gauge responds to gases and vapors alike.

The response of an ionization gauge is difficult to predict from its construction details, and it must be calibrated with a MeLeod gauge using permanent gases. Furthermore, before the pressure can be inferred, it is necessary to make corrections for the molecular weight of the gas and also for the possibility that the gas may be dissociated by the electron bombardment. Quantitative application of the gauge is unreliable to the degree to which these corrections are uncertain. Likewise, the response of the Pirani gauge depends on the molecular weight of the residual gas, and it must be calibrated with a MeLeod gauge that uses permanent gases. The same is true for the viscosity gauge.

The McLeod gauge. [45] Although many improvements have been made in the McLeod gauge, they have seldom been applied. The gauge as ordinarily used today is essentially the same as it was originally.


We will discuss here the simple form of the gauge illustrated in Fig. 34. It is madeof glass as shown and is mounted on a vertical board. The difference in the heights of the mercury levels in the gauge and in the reservoir is approximately equal to the barometric pressure B. As the reservoir is raised, the mercury level in the gauge comes above the Y-branch, thus isolating a definite volume V1 of the residual gas. This is isolated at the unknown pressure P1, the pressure of the residual gas in the apparatus to which the gauge is connected. As the mercury reservoir is further raised, the isolated residual gas is compressed, and when its volume has been reduced to a volume V2, the pressure is great enough to produce a sensible difference in the height of the mercury meniscus in the two capillaries, A and B. At the left, in Fig. 34, the mercury levels are shown at the beginning of a measurement, and at the right they are shown in two different positions corresponding to two methods of making readings. In one, if the meniscus in B is adjusted to the same height as the top of capillary A, the final volume, V2, is equal to Dh s when s is the cross-section area of the capillary. The decrease in volume from V1 to V2 is ordinarily of the order of one-hundred-thousandfold, with a corresponding increase of pressure in the capillary over that which obtained originally. The construction of the gauge with the comparison capillary B of identical bore with A eliminates the necessity of making corrections for surface tension. Referring to Eq. 1, we see that the product P1V1 is, in this case, a constant. The original product, P1 V1, is equal to the final product, P2 V2. From this we get the expression connecting the unknown pressure with the observed manometer difference, Dh



V1 and s are constants of the gauge determined when it is constructed. s is obtained by measuring the length of a known volume or weight of mercury in the capillary. V1 isdetermined by filling the gauge with mercury. These original data may be recorded on the board to which the gauge is attached. Here they will not be lost. Values of P1 determined by Eq. 14 are usually laid off on a nonlinear scale, which is mounted behind capillary A in order that pressures may be read directly.

The second procedure of making the observations on V2 and P2 is illustrated at the right in Fig. 34. The gas is cornpressed to a definite mark on capillary A at a distance Dho from the top, so that the final volume, V2, is the same for every measurement. The final pressure necessary to com-press volume V1 to V2 is Dh, and the pressure P1 in the system is determined by these quantities, according to the following equation:



A linear pressure scale computed from this formula is ordinarily mounted behind capillary B. The McLeod gauge is thoroughly reliable for the permanent gases from 10^-1 mm to 10^-4 mm of mercury. It is less reliable to 10^-5 mm. Below this the indications are only qualitative, and at 10-6 the mercury often sticks in the top of capillary A.

The gauge is most reliable after it has been outgassed by gently warming it with a soft flame. Three gauges with different values of V1 are necessary to cover adequately the complete pressure range from 10^-1 to 10^-3 mm. Many of the designs of McLeod gauges are more elaborate than the one shown in Fig. 34. For example, three bulbs may be mounted together with one reservoir, one for low pressures, one for intermediate pressures, and one for high pressures.

The McLeod gauge is fragile. If it breaks, not only is the gauge lost but what is often more serious, mercury may get into the vacuum system. In glass vacuum systems using mercury pumps this is not as serious as it may be in kinetic vacuum systems. These systems, fabricated of brass with soft-soldered joints, are attacked by mercury and the joints are destroyed. Accidents with this gauge are usually caused by bringing the reservoir up too quickly. Then mercury in V1 acquires enough momentum to shatter the bulb when the metal surface arrives at the opening of the capillary tube with no cushion of air to soften the shock.

Admitting air into the vacuum system is to be avoided when the mercury is not completely out of V1. The admission of air will have the same result as carelessness in raising the reservoir. Sometimes a mercury pellet will remain in capillary A when the reservoir is lowered. It can usually be brought down by tapping the capillary (after the mercury is all out of V1). If this treatment fails, the capillary should be heated with a soft gas flame. In the latter case, a sheet of asbestos is placed behind the capillary to protect the calibration scale from the flame.

The capillary tubes used for the construction of McLeod gauges are seldom larger than 2 or 3 mm or smaller than 1/2 mm bore. The volume of the bulb, V1, ordinarily varies from 50 to 500 cc. Only pure distilled mercury should be used. Mercury is attacked by the sulphur present in rubber hose, so that dross is produced which adheres to the inside of the gauge and may become very annoying. A gauge contaminated with this sulphide may be cleaned out by the combined action of zinc dust and nitric acid. Rubber hose for use on a gauge should be cleaned before it is used by passing hot caustic potash solution back and forth through it for a quarter of an hour or so. The tubing should be thoroughly washed free of caustic and dried before use.

In cases where it is necessary to avoid contamination of the vacuum system with mercury vapor, a liquid air trap should be connected between the vacuum system and the gauge. For kinetic vacuum systems this precaution is often omitted. A stopcock between the gauge and the system which is kept closed when the gauge is not in use minimizes contamination.

The ionization gauge. [46] Ionization gauges are triodes mounted in a glass bulb connected to the apparatus in which the pressure is to be measured. They are electrically connected as shown in Fig. 35.


Electrons emitted from the filament are accelerated to the grid, and their momentum would carry them to the plate if an inverse field more than sufficient to prevent this were not impressed between the grid and the plate. They therefore return to the grid and are finally collected on it. However, while they arc between the grid and the plate, they bombard and ionize some of the molecules of the residual gas present there. These ions are collected on the plate and measured with a sensitive galvanometer. The ratio of this ion current to the current of bombarding electrons or grid current is proportional to the pressure at pressures below about 10^-4 mm.

An ionization gauge may be made from an ordinary three-element radio tube equipped with a glass connection to the vacuum system. Such gauges are useful for the pressure range from 10-3 to 10-6 mm of mercury.


Fig. 36 shows the construction details of a gauge designed to have higher insulation of the plate than an ordinary radio tube. Measurements with it are possible to a pressure of 10^9 mm of mercury. The upper end of a glass bulb supports the plate assembly, while the lower end supports the combined grid and filament assembly. The grid is made from a piece of nickel screen rolled to form a cylinder. This is bound mechanically to the central glass tube through the bottom by wrapping it with wire, and it is connected electrically to the grid electrode with one loose end of the wrapping wire. There are two filaments, but only one is used. The other is held in reserve to be used if the first is accidentally burned out. The filaments may be replaced by cutting the central tube at S.

Expensive auxiliary electrical instruments are required for this gauge They should be protected with Littelfuses as shown in the wiring diagram (Fig. 35).

The plate may be outgassed with high-frequency currents or by electron bombardment. In the latter case, an alternating potential of 500 volts is applied between the filaments and the plate. The amount of heat developed depends on the emission from the filament, and this is controlled by the filament current. Outgassing of the plate and glass walls of the gauge is necessary if quantitative measurements are to be made. However, for hunting leaks it is necessary only to outgas the plate once.

Dunnington has made a gauge using 30-mil helices of tungsten wire for both plate and grid. These helices are outgassed simply by passing a current through them for a few seconds. He found that such a gauge did not have a linear relationship between pressure and ratio of plate to grid currents. Once calibrated, however, it was found to be very reliable.

At a given pressure, the ratio of plate to grid current is different for different values of the grid current. For this reason, it is necessary to adjust the grid current to some definite value, usually in the range of 10 to 50 milliamperes.

The Pirani gauge.[47] The Pirani gauge consists of a heated filament of platinum, tungsten, or some other metal with a high temperature coefficient of electrical resistance. The filament is exposed to the residual gases and is cooled by them. The temperature of the filament is determined by the heat conductivity of the residual gas, which, in turn, depends on the pressure. The filament may be operated in several ways.


The most satisfactory method is to connect the filament to one arm of a Wheatstone bridge and heat it by a constant current as shown in Fig. 37. If the bridge is balanced at one temperature of the filament, a change of its temperature caused by a change in the heat conductivity of the residual gases will unbalance it. Thus, the deflection of the bridge galvanometer indicates the pressure of the residual gases.

Ordinarily, the filament is mounted in a bulb fitted with a connecting tube and is balanced with an identical compensating filament mounted in an adjacent arm of the bridge. This auxiliary bulb is evacuated and sealed off at a very low pressure. The use of an auxiliary bulb serves to make the gauge insensitive to variations in room temperature. Changes in the over-all temperature of one bulb are the same as changes in the other, so that the galvanometer does not respond to these changes but only to the changes produced by the residual gas in the one bulb.


Fig. 38 shows a calibration curve of a Pirani gauge manufactured by E. Leybold Nachfolger. The pressure range over which it is useful extends from 1/10 mm to 10^-4 mm.

The construction of the Pirani gauge, together with the theory of its use, is treated in detail by several authors, who should be consulted by anyone planning to use the gauge for quantitative measurement. A gauge useful for qualitative work, as for hunting leaks, can be improvised, from twoordinary 20- to 40-watt vacuum tungsten lamps, one of which is fitted with a connecting tube.


Fig. 39 shows the construction details for this gauge. The bridge galvanometer, should have a sensitivity of about 10^-8 ampere division. Sometimes uncertain contact to the supporting wires may cause variable heat loss from the filament, and this should be suspected if the gauge is erratic. Tapping will often define the contact.

The Langmuir gauge.[48] Langmuir's viscosity gauge is made with a flattened quartz fiber about 50m thick and from five to ten times as wide. This quartz ribbon is about 5 cm long and is mounted in one end of a glass tube about 25 mm in diameter. as shown in Fig. 40. When this ribbon is set vibrating in a high vacuum, the amplitude changes very slowly because the damping by the residual gas is almost negliglble, and, owing to the low internal viscosity of fused quartz, the loss of vibrational energy from this source is also low. From atmospheric pressure down to a few millimeters of mercury, the damping produced by the molecules of the residual gas is nearly independent of pressure. Over the transition range of pressure, where the damping varies from this constant value to zero, the time required for the amplitude of vibration to decrease to half value is an index of the pressure. Within this range the relation between the time, t, the pressure, P, and the molecular weight of the residual gas is given by the following formula:



Here a and b are constants of the gauge. The value of the ratio b/a may be obtained by observing the damping time, to, for an essentially perfect vacuum, that is, a pressure of 10^-6 mm or less. For this pressure the left side of Eq. 16 can be set equal to zero. The values of a and b are deter-mined from a second measurement of the time t1 at a definite pressure P1. This pressure is determined with a McLeod gauge. M is approximately 29 for air. The gauge may also be calibrated by subjecting it to saturated mercury vapor at a definite temperature at which the vapor pressure of mercury is known. The range over which the gauge is most useful lies between the pressures 2 X 10^-2 and 5 X 10^-5

A feature of this gange is its small volume. Because there are no metal parts exposed, the gauge is suitable for measuring the pressure of corrosive gases like the halogens. This gauge, in conjunction with a McLeod gauge, mav be used for measuring the molecular weight of an unknown gas at low pressures.

The flat quartz fibers may be obtained by drawing them out of the aide rather than the end of a quartz tube or by following the technique given in Chapter V.


Figs. 40 and 41 show construction details and the method of mounting the fiber together with a pivoted glass tube, which contains an iron armature operated by an external electromagnet, to start the fiber vibrating.



An optical arrangement for observing the amplitude of vibration is also shown. An image of the quartz fiber is projected on a scale with a simple lens.



The Knudsen gauge.[49] Fig. 42 shows the Knudsen gauge as designed by DuMond.

When this gauge is constructed according to the specifications outlined by him, it is claimed to have a definite sensitivity, so that no preliminary McLeod calibration for it is needed. The gauge shown here differs slightly from DuMond's design in that it is equipped with a permanent (Alnico) magnet for damping.

Also, it has a special liquid air trap for determining what fraction of the pressure indication is produced by condensable vapors.

The Knudsen gauge is to be preferred to the McLeod gauge where it is important to avoid contaminating a vacuum system with mercury. No expensive auxiliary instruments are required with the Knudsen gauge, as with the ionization gauge. Furthermore, the filaments will not burn out and the suspension is not delicate.

It is advisable to modify DuMond's design so that all connections and supports fasten to one end plate. This facilitates making repairs. The metal case thus becomes, in effect, a water-cooled covering "bell jar" fitted with a window.

FOOTNOTES

[45] Gaede, W., Ann. d. Physik, 41, 289 (1913); Hickman, K. C. D., J.O.S.A., 18, 78 (1921); Pfund, A.H., Phys. Rev., 18, 78 (1921).

[46] Buckley, O.E., Nat. Acad. Sci., Proc., 2, 683 (1916); Dushman, S., and Found, C.G., Phys. Rev., 17, 7 (1921); Jaycox, E.K., and Weinhart, H.W., Rev. Sci. Instruments, 2, 401 (1931); Simon, H., Zeits. f. techn, Physik, 5, 221 (1924).

[47] DuMond, J. W. M., and Pickels, W. M., Jr., Rev. Sci. Instruments, 6, 362 (1936); Hale, C. F., Am. Electrochem. Soc., Trans., 20, 243 (1911); vonPirani, M., Deutch. Phys. Gesell., Verh., 8, 24 (1906); Skellett, A. m., J. O. S. A., 15, 56 (1927); Stanley, L. F., Phys. Soc., Proc., 33, 287 (1931).

[48] Beckman, Arnold O., J. O. S. A., 16, 276 (1928); Haber, F., and Kerschbaum, F., Zeits. f. Elektrochem., 20, 296 (1914); Langmuir, I., Am. Chem Soc., J., 35, 107 (1913).

[49] DuMond, J. W. M., and Pickels, W. M., Jr., Rev. Sci. Instruments, 6, 362 (1936); Knudsen, M., Ann. d. Physik, 28, 75 (1909).