A ConstantLevel Control for Liquid NitrogenMackenzie Lounsbury Citation: Review of Scientific Instruments 22, 533 (1951); doi: 10.1063/1.1745986 View online: http://dx.doi.org/10.1063/1.1745986 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/22/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Nonelectrical Liquid Nitrogen Level Controller Rev. Sci. Instrum. 33, 121 (1962); 10.1063/1.1717636 Liquid Nitrogen Level Control Rev. Sci. Instrum. 30, 598 (1959); 10.1063/1.1716698 Liquid Nitrogen Liquid Level Controller Rev. Sci. Instrum. 30, 371 (1959); 10.1063/1.1716629 Automatic Level Controller for Liquid Nitrogen Rev. Sci. Instrum. 21, 400 (1950); 10.1063/1.1745599 A Simple ConstantLevel Suspension Rev. Sci. Instrum. 5, 191 (1934); 10.1063/1.1751824

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LABORATORY AND SHOP NOTES 533

now used essentially in connection with cosmic rays, to nuclear­physics experiment. It has been already used successfully.'

A more detailed description and theoretical duscussion will appear in Journal de Physique et Ie Radium.

1 G. Albouy, Compt. rend. 230, 1351 (1950); G. Albouy ap.d H. Faraggi, Conference on Fundamentals Mechanisms of Photographic Sensitivity, Bristol, England (March, 1950), to be published.

2 Fremlin and Walter, Proc. Phys. Soc. (London) 63, 1178 (1950). 3 H. Yagoda. Radioactive Measurements with Nuclear Emulsions (New

York, 1949). • Lacassagne et al. Compt. rend. 231, 89 (1950); Teillac and Albouy,

Compt. rend. 232, 326 (1951).

A Constant-Level Control for Liquid Nitrogen MACKENZIE LoUNSBURY

Atomic Energy Project, National Research Council of Canada, Chalk River, Ontario, Canada

(Received March 5, 1951)

T RAPS cooled by a refrigerant such as liquid nitrogen are frequently used in the laboratory. When the trap must be

refrigerated continuously for a long period of time, as in the high vacuum system of a mass spectrometer, it is convenient to have an automatic device which will maintain the refrigerant at a constant predetermined level in the Dewar flask surrounding the trap. Numerous devices designed for this purpose and with various features of interest have been described in the Iiterature.1- 7 The device to be described in this note is very simple in construction, and positive and reliable in operation.

A block diagram of the apparatus is shown in Fig. 1. A source of dry compressed air is regulated to 5 lb/sq in. by a reducing valve and regulator A, and connected to a magnetic valve B (Type V437 A1K2, 115 volts, 10 watts, 60 cycle; Minneapolis­Honeywell Regulator Company, Minneapolis, Minnesota). When valve B opens, the compressed air flows into the 50-liter Dewar res­ervoir C, which is capped with a 2-hole rubber stopper. The air pressure on the surface of the liquid nitrogen forces the refrigerant through the S-mm Pyrex tube D and into the Dewar flask E surrounding the trap F. A small needle valve G is provided to allow the venting of the pressure caused by the normal evapora­tion of the liquid nitrogen in reservoir C. A negligible volume of compressed air escapes through G during the transfer of refrigerant.

The temperature-sensitive element H, is a Cenco-DeKhotinsky Bimetallic Thermoregulator (Catalog No. 99015), from which the stationary contacts have been removed. A short extension of the moving contact, attached at right angles to the movable arm, extends through the case of the thermoregulator and contacts the lever of a microswitch J (Type BZ-RLS, First Industrial Corporation, Freeport, Illinois), which is mounted at the side of the regulator. The "normally open circuit" contacts of the micro­switch are connected to the solenoid of valve B and to a source of US-volt, 6O-cycIe power.

The helix of the thermoregulator is half immersed in the liquid nitrogen of Dewar E, which is filled to the desired level. When the liquid level falls slightly, the temperature of the helix, in the region between the original level and the new level, rises. The helix expands, actuating the moving contact of the thermoregulator to depress the lever of the microswitch. This closes the electrical

o

c FIG, 1. Block diagrllm of constant-level control for liquid nitrogen.

circuit, energizing the solenoid of valve B and thus opening the magnetic valve in order to allow compressed air to pass. As already described, the compressed air forces liquid nitrogen to transfer from reservoir C to Dewar E. When sufficient liquid nitrogen has been transferred to regain the original level in E, the bimetallic helix contracts, opens the microswitch, and thus closes the mag­netic valve. When liquid nitrogen is not being transferred, there is no consumption of compressed air or electrical power.

The reason for using the microswitch is to decrease the sensi­tivity of the thermoregulator. An earlier model of the device employed only the thermoregulator to open and close the electrical circuit. This was found to be sensitive to very small temperature changes, and caused the transfer of liquid nitrogen every several minutes, resulting in an unnecessary waste of refrigerant. The present combination of thermoregulator and microswitch causes the transfer of refrigerant once every half-hour, and the duration of transfer is about 30 seconds. The level of liquid nitrogen in Dewar E remains constant to ±O.S cm. To raise or lower the predetermined level, one has only to set the adjusting knob of the thermoregulator. The reservoir Dewar usually needs to be re­plenished only once a week.

Perhaps the most important feature of this simple controlling device is that its components are capable of long-term, dependable operation. In this laboratory, it has given completely reliable, failure-free service continuously for the past year.

1 A. C. Egerton and A. B. Ubbelohde, Trans. Faraday Soc. 26, 236 (1930). 'R. B. Scott and F. G. Brickwedde, Rev. Sci. lnstr. 2,171 (1931). • G. O. Jones, J. Sci. lnstr. 25, 239 (1948). • M. S. Fred and E. G. Rauh, Rev. Sci. lnstr. 21, 258 (1950). • E. H. Quinnel and A. H. Futch, Rev. Sci. lnstr. 21, 400 (1950). • A. Wexler and W. S. Corak, Rev. Sci. lnstr. 21, 583 (1950). 1 G. F. H. Box and M. S. Walker, J. Sci. lnstr. 27, 314 (1950).

The Thyratron Grid Spike M. PARKER GIVENS*

Applied Physics Laboratory, Johns Hopkins University, Silver Spring, Maryland

(Received November 22, 1950)

I T has been known for some time that when a thyratron fires, the control grid assumes a high positive potential for a short

time.! The duration of this grid spike is usually less than a micro­second and depends upon the construction of the thyratron and the circuit in which it is used. Although a qualitative explanation of this phenomenon is fairly simple, it does not seem to be widely known. The purpose of this note is to propose and give some evidence for what the author considers a reasonable explanation of the qualitative features of this phenomenon.

When the thyratron is nonconducting, the grid has control in the sense that the tube may be held nonconducting by a suitable negative bias on the grid in spite of a positive potential on the plate. The tube goes into conduction, or "fires," when (1) either the plate potential is increased beyond a critical value or (2) the negative bias on the grid is decreased below a critical value. When the tube fires, the grid loses control and the tube remains con-

-3$0 V DC

FIG, 1. Circuit for demonstratin~ the ~rid spike,

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534 LABORATORY AND SHOP NOTES

ducting until the plate potential drops essentially to zero. In some applications the thyratron is used to discharge a condenser through a fiashlamp, or to discharge a line, and in these cases the plate potential drops to zero very abruptly.

At discharge the grid is observed to take on a positive potential essentially equal to the plate potential and then drop quickly to zero. This effect may be easily demonstrated with any of the common thyratrons and a circuit similar to the following (see Fig. 1). The values of the circuit constants have been selected to give good operation with the type 884 thyratron, but the values are not critical. The transformer T is used to trigger the thyratron and synchronize it with the oscilloscope sweep. Any transformer giving a secondary voltage of 5 or 6 volts has been found satis­factory. Triggering at 60 cycles per sec is satisfactory, but 1000 cycles per sec is preferred if a signal near this frequency is readily available. A high spike will be observed on the oscilloscope.

This circuit provides an unusually simple way be produce micro­second positive pulses.

To explain this observation one need simply remind the reader that when the thyratron fires, the grid loses control and becomes instead a probe projecting into the gaseous discharge between the cathode and anode. Such a probe will take on a potential nearly that of the plasma surrounding it. In a gaseous discharge the potential drop is almost entirely near the cathode and the plasma region which fills most of the space is almost at anode potential. It follows then that the grid will assume a potential very close to the potential of the plate and fall back to zero as the plate poten tial falls.

One test of this explanation would be to hold the plate at a high positive potential for a longer period of time and observe whether or not the grid also remains at a high positive potential. This test has been made and the results are reassuring.

The test was made in the following manner. The high side of a 110-volt 6O-cycle power line was hooked directly to the plate of a type 884 thyratron. The grid was biased so that the tube would fire when the plate reached 50 or 60 volts (Eg=-6 volts). The plate potential was applied to the horizontally deflecting plates of an oscilloscope and the grid potential to the vertically deflecting plates (see Fig. 2). The Lissajous figure so obtained was observed and is drawn in Fig. 3. (The 884 tube cannot stand such abuse as this for more than a few seconds.)

In a gaseous discharge tube, such as is considered here, the potential of the plasma can be more positive than the cathode only by an amount approximately equal to the ionization potential of the gas, unless more current is being drawn from the cathode than it can normally emit. For this application the tube is ob­viously being operated in this high current condition which results in shortened tube life and some loss of stability.

The Lissajous figure may be interpreted as follows. As long as the plate is negative, the grid potential is constant. The grid potential remains constant until the tube fires. The grid then rises with the plate potential and then returns to its steady value along a straight line, showing that the grid potential is proportional to the plate potential. Arrows have been drawn to show the proper direction in which to trace the figure.

~----~r------------------------' 110 V 6'O~AC

1/

FIG. 2. Circuit for demonstrating the prolonged duration of the grid spike.

FIG. 3. The Lissajous figure produced by the circuit of Fig. 2.

That the constant of proportionality between grid and plate potential is very near to unity was checked by measuring these two with a peak reading voltmeter. The agreement was within 3 percent.

This interpretation of the Lissajous figure was checked by observing on the oscilloscope the separate traces of plate potential and grid potential against time.

The test has confirmed the assumption that the grid spike is due to the fact that the grid changes from control grid to probe whenever the thyratron is conducting.

* Now at the Institute of Optics. University of Rochester. Rochester. New York.

1 See. for example. H. Heines. Electronics 19. 101 (1946).

A Simple Pressure Recorder J. TADAYON

Research Deparlmenl. The Melal Box Company. Lid .. London. England (Received February 19.1951)

T HE recent note of Price and Zemanyl describing a simple recording Hg manometer, in which the voltage drop across

a Nichrome wire coaxial with the manometer was amplified by a self-balancing potentiometer and recorded by a photoelectric recorder, prompts me to describe the following simple pressure recorder which has been constructed and is being used in this department. The instrument is simple, and the idea might easily have occurred to other workers; nevertheless, no reference to such a device has been found in the literature.

The instrument, Fig. 1, consists of a manometer U one limb of which is in contact with a long slit S attached to the box A. The change in the liquid level is then directly photographed on a recording paper in A moving behind the slit.

At present, the instrument is being used in conjunction with a pneumatic gauge for measuring the thickness of organic coatings and its variation on tinplate, where manometer changes of up to 30 cm in each limb are frequently employed. In this arrangement the recording paper used is a 12-inch-wide low speed document paper, insensitive to red and subdued lighting, of the type usually used in reflex photography. The roll of paper is placed in drum Dl and is taken up on a cylinder in drum D2 by means of motor M. The manometer is a i-inch bore glass tubing containing water to

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