Use of Silicones as Diffusion Pump OilsGordon P. Brown Citation: Review of Scientific Instruments 16, 316 (1945); doi: 10.1063/1.1770299 View online: http://dx.doi.org/10.1063/1.1770299 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/16/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optical Properties of Some Silicone DiffusionPump Oils in the Vacuum Ultraviolet—Using an OpenDish Technique J. Appl. Phys. 42, 4258 (1971); 10.1063/1.1659762 Optical Properties of Some Silicone DiffusionPump Oils in the Vacuum Ultraviolet—Using a ClosedCell Technique J. Appl. Phys. 42, 4252 (1971); 10.1063/1.1659761 On the Use of Diffusion Pump Oil in Mechanical Pumps J. Vac. Sci. Technol. 3, 168 (1966); 10.1116/1.1492469 Use of OilDiffusion Pumps in Mass Spectrometers Rev. Sci. Instrum. 18, 926 (1947); 10.1063/1.1740886 Trap for Use with Oil Diffusion Pump Rev. Sci. Instrum. 8, 263 (1937); 10.1063/1.1752306

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'tttt;: ~EViEW OF SCiENTiFIC iNST~tiMENTS VOLUME 16, NUMBER 11 NOVEMBER, 1945

Use of Silicones as Diffusion Pump Oils

GORDON P. BROWN

National Research Corporation, Boston, Massachusetts

(Received July 18, 1945)

Comparative performances of Litton C, Octoil, Narcoil, and two typical silicones in a non­fractionating diffusion pump are presented with a discussion of vapor pressure, stability to heat and oxidation, and the ability to operate against high forepressures. Extrapolated vapor pressure, temperature data, and test results show the high boiling silicone to produce the highest vacuum as indicated by an un trapped ionization gauge. The silicon was further found to be nearly completely resistant to oxidation when exposed to air while hot. A table showing relative merits of various oils, as applied to vacuum pumping conditions ordinarily encountered, is presen ted.

L'OR a long time there has been need of a r diffusion pump oil that would withstand atmospheric pressure while hot without appreci­able chemical breakdown. Among the family of silicones there have recently been developed members that not only fulfill the stability re­quirement, but also have vapor pressures as low or lower than the best commercial oils now available. One of the high boiling silicones has been operated at atmospheric pressures for twenty minutes and then produced a better vacuum within an hour than any standard oil after twenty-four hours and no exposure to air.

Figure 1 shows vapor pressure versus tempera­ture for two typical silicones (supplied by Dow­Corning) along with other standard oils at elevated temperatures. Litton oil was taken as a representative straight hydrocarbon, Octoil as the ester, and Narcoil as the chlorinated aro­matic hydrocarbon. Exact values of vapor pres­sure for these oils secured by extrapolation to room temperature are meaningless. However, relative positions probably remain the same. In the absence of any device known to be satis­factory for measuring vapor pressures below one micron, a series of tests of various oils in a diffusion pump have been made. The ultimate vacuum after twenty-four hours of pumping as indicated by an untrapped ionization gauge at the pump inlet was taken to be a measure of the vapor pressure. A six-inch, non-fractionating, unbaffled, all-metal pump was used throughout. The figures for ultimate vacuum obtained in such a pump as shown in Fig. 2 are higher by a considerable factor than those which can be obtained from all-glass laboratory equipment

because of incomplete outgassing. Nevertheless, the values have relative meaning and can be measured with ease. Had all-glass equipment been used, it was felt the ultimate vacuums secured might have been in considerable error because of uncertainty in meaning of ion gauge readings below 10-6 mm.

Speed of the pump using the various oils is shown in Fig. 2. These were taken using a test dome and both a high sensitivity McLeod gauge and ionization gauge for pressure measurements. Speeds using a McLeod were not taken below 2 X 10-5 mm because of limiting accuracy of the

316

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FIG. 1. Vapor pressure versus temperature.

,. I .• .,. It

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USE OF SILICONES AS DIFFUSION PUMP OILS 317

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FIG. 2. Speed versus pressure for various pump oils in a typical 6" metal pump_ Data for forepressures as indicated in microns_

gauge. However, in all cases the ultimate McLeod reading came to an indicated 10-6 mm or better. The accuracy of this measurement was estimated as ±200 percent_ A curve of probable per­formance with mercury is included based on trends observed in similar pumps.

The maximum pressure in the fore-line against which a pump will maintain its speed is im­portant in most applications_ While this is

TABLE 1.

With Pump liquid tore-

air Without cold pres .. trap trap sure Sta- Sta-all Greater Less re- bility bility

pres- than than quire- to to Type oil :mre-s 5 XIO-' 5 XIO-' ments air heat

Silicone (high boiling) 3 2 3 2 2

Typical Ester 3 2 2 3 4 3

Straight Hydrocarbon 3 2 2 3 3 2

Chlorina ted Aromatic Hydrocarbon 2 3 2 2 2

Mercury

primarily a function of pump design it is also dependent on type of oil used, and amount of heat actually delivered to the oil. The trend between various boiler fluids is for the higher vapor pressure oils to work against the higher forepressure_ Each speed curve in Fig. 2 is marked with the maximum forepressure when loaded to 10-4 mm with 750 watts heat input_

Stability of pump oils can be considered as stability against oxidation while hot, and re­sistance to thermal dissociation in the boilers caused by excessive intensity of heating at the heater and boiler fluid interface. The resistance to oxidation as has been previously pointed out is very high in the case of the silicones and at least equal to that for the chlorinated hydro­carbons. This resistance is, in fact, sufficiently high to make possible distillation at atmospheric pressure as a means of reconditioning the oil. Measurement of the second factor, resistance to thermal breakdown, was made by observing the heat intensity above normal 750-watts pump input at which breakdown began, as indicated by pulsations in the ionization gauge. The pulsa­tions were found to occur at about the same value as for the chlorinated hydrocarbon; con­sequently, there does not seem to be any possi-

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318 DANIEL R. STULL

bility of the pumps working against forepressures as high as with the chlorinated hydrocarbon by using additional heat.

Selection of the best oil for a particular appli­cation must be made carefully. Where the highest vacuums obtainable are required silicones give best performance because their boiling point can be adjusted as required and consequently low vapor pressures secured. In addition the stability remains excellent. In the range of intermediate vacuum created by diffusion pumps,

THE REVIEW 'OF SCIENTIFIC INSTRUMENTS

all the oils are reasonably satisfactory. Where good speed and exceptional forepressure charac­teristics are required at a sacrifice in ion gauge readings the higher vapor pressure compounds remain superior.

Table I is an estimate of the relative merits of the various oils as applied to vacuum pumping conditions normally encountered. Listing is in order of apparent merit, 1 being best. No exact values are given because of uncertainty in the absolute meaning.

VOLUME 16. NUMBER 11 NOVEMBER. 1945

An Automatic Recorder for Resistance Thermometry DANIEL R. STULL

The Dow Chemical Company, Midland, Michigan

(Received July 30, 1945)

A completely automatic recording Wheatstone bridge is designed especially for use in resistance thermometry. It is built into a standard commercial recorder, contains decade coils operated' by a G€neva gear mechanism, and increases the original range of the machine 100-fold. With it a continuous record of resistance versus time is made covering a range of 0 to 100 ohms that may be easily read to 0.001 ohm±O.OOl ohm. Employed in connection with a standard 25-ohm platinum resistance thermometer a temperature range of -190° to 550°C can be covered with a precision of 0.01 °C±O.Ol 0.

INTRO DUeTIO N

T HE precision measurement of temperature is a research problem of long standing, and

is associated with a wide variety of investiga­tions. A reliable method suggested in 1871 by Siemens,! made more precise by Callendar,2 and perfected by a number of others3- 9 depends upon the very regular and reproducible relation exist­ing between the temperature and electrical re­sistance of metals (copper, nickel, and, for highest precision, platinum). This property of platinum has been utilized as the basis of the International

1 K. W. Siemens, Proc. Roy. Soc. London 19,351 (1871). 2 H. L. Callendar, Phil. Trans. London 1'18, 160 (1887);

Phil. Mag. 32, 104 (1891). 3 T. S. Sligh, Jr., Sci. Pap. Bur. Stand. 17,49 (1922). 4 C. H. Meyers, Bur. Stand. J. Research 9, 807 (1932). 6 E. F. Mueller, Bull. Bur. Stand. 13, 547 (1916-17). 6 E. F. Mueller and F. Wenner, J. Research Nat. Bur.

Stand. 15,477 (1935). 7 W. H. Keesom, Comm. Kamerlingh Onnes Lab. Univ.

Leyden 18, 11 (1928-30). 8 W. H. Keesom and B. G. Dammers, Physica 2, 1051

(1935); 2,1080 (1935). 9 M. S. Van Dusen, J. Am.Chem. Soc. 47,326 (1925).

Temperature Scale from -190° to 660°C,1O.1l and is the appropriate means of determining such physical constants as freezing and boiling points.

THE PROBLEM

The attainment of high precision in the meas­urement of temperature with a platinum resist­ance thermometer requires that resistances be measured with a precision usually obtainable only with specially designed bridges. For a 25-ohm platinum resistance thermometer, dR/dT is about 0.1 ohm per degree C, so that measure­ment of temperature to O.Ol°C requires that the resistance of the thermometer be measured to 0.001 ohm.

Bridges capable of this accuracy which are available commercially are for the most part hand operated and hence are of limited usefulness.

10 G. K. Burgess, Bur. Stand. J. Research 1, 635 (1928). 11 R. L. Weber, Temperature Measurement and Control

(The Blakiston Company, Philadelphia, 1941), Chap. XV.

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