Vacuum ValveI. Estermann and S. N. Foner Citation: Review of Scientific Instruments 18, 64 (1947); doi: 10.1063/1.1740822 View online: http://dx.doi.org/10.1063/1.1740822 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/18/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Large Ultrahigh Vacuum Valve Rev. Sci. Instrum. 30, 602 (1959); 10.1063/1.1716703 A Vacuum Valve for Glass Systems Rev. Sci. Instrum. 22, 703 (1951); 10.1063/1.1746046 High Vacuum Valve and Seal Rev. Sci. Instrum. 19, 83 (1948); 10.1063/1.1741223 Tire Valve in Vacuum Application Rev. Sci. Instrum. 13, 534 (1942); 10.1063/1.1769964 Greaseless Vacuum Valves Rev. Sci. Instrum. 9, 428 (1938); 10.1063/1.1752384

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THE REVIEW OF SCIENTIFIC INSTRUMENTS

Laboratoryand Shop Notes

Vacuum Valve I. ESTERMANN AND S. N. FONER*

Research Laboratory of Molecular Physics. Carnegie Institute of Technology. Pittsburgh. Pennsylvania

September 24. 1946

I N many vacuum systems consisting of several inter­connecting sections, it is desirable to control the flow

between sections from a few cc/sec. to the order of hundreds of liters/sec. It is obviously impractical to do this by means of stopcocks because of the large size required to allow a flow of the order of a hundred liters/sec. Moreover, the presence of stopcock grease may be objectionable in the high vacuum system.

The valve to be described in this note is generally ap­plicable to those cases where a flow of several cc/sec. in the closed position is allowable and a flow of a hundred liters/sec. or more in the open position is required. The principal features of this valve are: (a) The simplicity of construction, (b) the ease and reliability of operation, and (c) the absence of gaskets, greases, etc., that may con­tam ina te the system.

As shown in Fig. I, the valve is essentially a conically ground circular brass piece which is moved relative to the valve seat by a lever brought through a sylphon bellows. The seat is machined from a brass plate. If the valve is subject to chemical attack, monel metal or stainless steel should be used instead of brass. Stresses produced in the bellows by valve motion are minimized by locating the pivot of the moving arm well inside the bellows. In an earlier model the pivot was located somewhat inside the large cylinder housing the valve, with the result that the bellows cracked after many operations. The position of the valve is fixed by placing a pin in one of several holes in an attached bracket. Positive action in closing is assured by looping a piece of wire around the lever arm and through a hole in the valve stem.

For our particular application, the valve was arranged between the main section of a molecular beam apparatus (volume approximately 100 liters) and a charcoal trap. This trap consisted of a Pyrex tube 6 cm in diameter and 50 em long, containing a coconut charcoal cartridge. It was kept cooled with liquid nitrogen continuously ... for

FIG. 1. Vacuum valve. Scale full size.

64

VOLUME 18. NUMBER 1 JANUARY. 1947

several weeks in order to maintain a high vacuum in the system. The cooled charcoal acts as a pump with a maxi­mum capacity of 40 liters of air per second. The flow resistance of the valve in the open position can be neglected compared to the flow resistance of the Pyrex tube.

The valve was used for the following purposes: During experiments on the scattering of molecular beams by permanent gases, it was closed in order to prevent the adsorption of the scattering gas by the charcoal. \Vithout the valve mechanism, it would have been necessary to pump for many hours after closing the gas entrance valve in order to reduce the pressure in the system to the operat­ing value of about 10-7 mm Hg. It was also closed for the degassing of the charcoal through a bypass stopcock (not shown) by means of a separate mercury pump, without spoiling the vacuum in the main apparatus.

The leakage through the closed valve was determined by measuring the pressures on both sides of the valve during the degassing of the charcoal, taking the pumping speed of the oil diffusion pumps on the high vacuum side into account. The leakage was about 20 cc/sec. and was not objectionable in our apparatus. No attempt to lap the valve and seat was, therefore, made, but it is felt that by proper grinding the leakage could be reduced to less than 1 cc/sec.

* Now at Applied Physics Laboratory. Johns Hopkins University Silver Spring, Maryland. •

Infra-Red Absorption Cell for Quantitative Analysis

N. B. COLTHUP

Stamford Research Laboratories, American Cyanamid Company. Stamford. Connecticut November 12. 1946

T HIS. cell was .de~ig?ed specifically for rapid quanti­tative analYSIS m mfra-red spectroscopy, but it has

also proved ideal for ordinary sampling of all liquids except the very viscous or corrosive ones. I ts characteristics are: (a) it is easy to fill and may be rapidly and thoroughly cleaned; (b) it is tight enough to hold very volatile liquids; (c) it is simple to construct, the machined parts requiring no close tolerances; (d) it uses no cement and hence can be used for any liquid which does not attack metal or the cell window material; and (e) the sample can be recovered if desired.

The novel feature of this cell is the shape of the spacer which has a partition in it extending almost to the bottom of the rectangular opening dividing it into two connected sections. This allows both the entrance and exit to be at the top of the cell without leaving any hard-to-clean traps out of the line of flow as exist in the ordinary u-shaped cell. Since both openings are at the top and above the liquid, they do not have to be stoppered against gravity, thus eliminating the need for a carefully machined valve or plug. In quantitative work where the time required per analysis is short, the cells are usually left unstoppered, a distinct time saver when many samples are to be run.

Details of construction are shown in Fig. 1. The entrance and exit are made from bases of hypodermic needles so that tapered plugs may be used if desired. The cell is

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LABORATORY AND SHOP NOTES 6S

INFRARED ABSORPTION CELL FOR LIQUIDS

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FiG. 1. Construction details showing the partition spacer and its position in the assembled cell.

cleaned with a tapered nozzle used in connection with a vacuum pump. The method for recovering a sample with a syringe and a tapered "recovery" plug is shown in the figure. The sample is blown out with the syringe and collected through the plug. If desired, the cell can be rinsed with a syringe full of solvent which can, in turn, be collected.

Cells have been made with rocksalt and potassium bromide in thicknesses from 0.5 mm to 0.03 mm with amalgamated tinfoil for spacers. Below that down to 0.01 mm, amalgamated silver is used. Thicker cells have been made but they require cementing.

These cells have been used satisfactorily for some time and have proved to be so easy to clean and to construct that they have replaced all other types in this laboratory.

A Level and Differential Attenuator R. H. LEFKOVICH

Ordnance Research Laboratory. School of Engineering. The Pennsylvania State College. State College. Pennsylvania

October 9, 1946

I N the design and test of certain multi-channel amplifier systems it is frequently desirable to vary the differ­

ential input signal level to two of the channels, in addition to varying the absolute level by means of an ordinary attenuator system. The device described below provides four output signals, to ground, from a two-terminal signal generator connected to the input of the device. The differ-

ential in level of either pair of the four output signals may be adjusted in 0.1 db steps to a maximum of 11 db. The absolute level of either of the pair may be made greater than the other by means of a switch. The mean output level may be varied over a range of 110 db in 1-db steps. In the case of a four-channel system, the signal level applied to the channels without differential is equal to the mean of the differential signal levels. The devise is comprised of commercially obtainable components.

The schematic diagram lists the manufacturer and type numbers of the components for the device constructed. The oscillator, or signal source, is connected to the input terminals of the attenuators. R1 and R2 are standard T pads, having a range of 100 db in lO-db steps and 10 db in 1-db steps, respectively. The output of R2 branches into two circuits. One side goes directly to an input terminal of a differential attenuator, R4; the other side being connected through a fixed attenuator, R3, to the other input terminal of R4. R3 is a fixed s-db T pad which compensates for the insertion loss of R4 at one of its output terminals. R8 is a rheostat which is inserted in either lead A or lead B of the schematic to provide a zero differential output adjustment. This compensates for small inaccuracies in the attenuators. Level terminals are pro­vided which permit the measurement of absolute level of the signal at the differential attenuator input.

Attenuator R4 is a differential attenuator having a range of 10 db in 1-db steps. The attenuator is designed so that the mean of the differential output is constant. For example, for a lO-db output differential, the level on one side is 5 db below the mean level while the output on the other side is 5 db above the mean level. Differential attenuator Rs is inserted in one of the outputs of R4 and is a T pad having a range of 1 db in b.1-db steps. Adjust­ment of the latter attenuator obviously upsets the mean of the differential output by a maximum of 1 db. For most applications this factor is of negligible importance. Re­sistors R6 and R7 provide proper termination for the device.

Switch S1 permits connection of the differential output to either of the output connection pairs 1-3 or 2-4. When the differential output is impressed on one .pair, the mean output level is impressed on the other pair. Switch S2 provides reversal of the differential for the given pair.

FIG. 1.

RI lOO-db attenuator. The Daven Company, No. T2514 R2 IO-db attenuator. Daven. No. T2512 R3 5-db fixed attenuator. Daven, No. T154 R4 IO-db differential attenuator. Daven. No. BH230 Spec. No. 1627 R5 I-db differential attenuator. Daven. No. T250 Spec. No. 1626 R6-R7 looon -I watt. I percent resistors R8 200-n potentiometers Jl-J2 General Radio Company. No. 138VD(2) +No. 274 - Y(2) J3-J4-J5-J6 American Phenolic Corporation. No. 83-IR (CPH-49194) Sl Federal A.C. Switch Corporation, No. 1424 S2 Federal A.C. Switch Corporation. No. 1425

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