Device for emptying liquid nitrogen dewarsW. M. Miller, B. D. Terris, and D. M. Ginsberg Citation: Review of Scientific Instruments 52, 611 (1981); doi: 10.1063/1.1136647 View online: http://dx.doi.org/10.1063/1.1136647 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/52/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A Dewar for the heat of vaporization of liquid nitrogen Phys. Teach. 14, 571 (1976); 10.1119/1.2339500 Single Walled, Space Saving, Liquid Nitrogen Dewars Rev. Sci. Instrum. 40, 1362 (1969); 10.1063/1.1683799 Simple Instrument for TwoLevel Control of Liquid Nitrogen in Dewars Rev. Sci. Instrum. 40, 20 (1969); 10.1063/1.1683738 Simple Liquid Nitrogen Dewar for Optical Studies Rev. Sci. Instrum. 36, 1652 (1965); 10.1063/1.1719420 Pyrex Dewars for Liquid Helium Rev. Sci. Instrum. 18, 522 (1947); 10.1063/1.1740991

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Device for emptying liquid nitrogen dewars W. M. Miller, B. D. Terris, and D. M. Ginsberg

Department of Physics and Materials Research Laboratory. University of Illinois at Urbana-Champaign. Urbana. Illinois 61801

(Received 13 October 1980; accepted for publication II December 1980)

We have invented a device for quickly emptying a dewar of liquid nitrogen without having to invert the dewar. The liquid is forced out by compressed nitrogen gas, using a type of pump which does not require the dewar to be sealed.

PACS numbers: 07.20.Mc

It is frequently necessary to empty a dewar of liquid nitrogen in order to warm up equipment without waiting for the nitrogen to boil away. A tolerable boiling rate is difficult to obtain by means of an electric heater be­cause of the sizable latent heat of liquid nitrogen (1.6 x 105 1/1). The nitrogen cannot be siphoned out of the dewar because the liquid is at its boiling point. If the dewar is large, it may be difficult to empty it by remov­ing it from its support and inverting it. We have in­vented a device which is easily inserted into a dewar, and which enables us to empty it easily and rather quickly.

We force the liquid nitrogen out of the dewar by means of compressed nitrogen gas. Our method does not re­quire the dewar to be sealed at the top; we have found it difficult to make such a seal, especially with a liquid helium dewar situated in the nitrogen dewar. Our device is basically a tube which goes from the top of the dewar to the bottom and then back to the top again, with holes in the bottom side of the tube, at the bottom of the dewar. After some nitrogen flows into the tube through these holes, gas is forced into one end of the tube to blow the liquid nitrogen out of the other end. This is repeated periodically by an automatically oper­ated solenoid valve. It is useful, but not absolutely necessary, to provide the holes in the side of the tube with a simple valve to prevent any of the liquid nitrogen from flowing back out of the holes when the gas pressure is applied.

The device is built from two pieces of rubber tubing, connected to a metal tube of square cross section, as shown in Fig. 1. The metal tube has six holes drilled through its bottom side. A piece of Mylar, 2 mils thick, is positioned inside the metal tube, and attached to it at one end with glue,l as shown. Between pulses of gas pressure, the hydrostatic pressure of the liquid nitrogen deflects the valve and forces some liquid into the metal tube.

The required pulses of gaseous nitrogen are supplied by a solenoid valve,2 connected to a gas supply which is at the pressure of 5.0 psi. The solenoid is operated by 110 V electrical power, regulated by a triac3 and a square-wave signal generator. The circuit is shown in Fig. 2. The generator can be adjusted to control the

FIG. 1. Lower end of the device. The middle of it is a metal tube with square cross section, % in. on a side, with six holes drilled in it, 0.086 in. in diameter and lis in. apart. The Mylar valve is attached at one end with glue.) When gas pressure is applied to the left side of the device, it forces the liquid to the right, closing the valve.

frequency! and asymmetry parameter A of the square wave signal. We define A as the ratio of the length of time during which the flow of gas is off to the length of time when it is on.

The device was tested by determining how quickly it pumped liquid nitrogen from a dewar filled to a height of approximately 70 cm. We alternately varied each of the two variables! and A to increase the pumping speed S. With! = 0.85 Hz and A = 1.6, we obtained a rate S = 42 IIh. We were able to empty completely a dewar containing 4.8 I of liquid nitrogen, starting at a height of 71 cm, in 12.5 minutes, with an average pumping speed. of 23 IIh. With either of the variables! or A fixed, varia-

FIG. 2. Circuit used to supply pulses of nitrogen gas to the device.

I

I I I I I I I I I L ___ ..J

\solenold Valve

110 V.O.c.

611 Rev. Sci. Instrum. 52(4), Apr. 1981 0034·6748/81/040611·02$00.60 © 1981 American Institute of Physics 611

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tions of 10% in the other variable changed S by roughly 10% near its optimum value. In this respect, S is not a very sensitive function off or A.

In cases where clearance problems make it impossible to use the pump described here, a smaller one could be used, presumably providing a lower pumping speed. Also, a coaxial geometry could be used for the inlet and outlet tubes, although the pump would then be more ex­pensive to build.

This work was supported in part by the U.S. Depart­ment of Energy under Contract DE-AC02-76EROl198.

J The glue was Alscobond Y-725. Alloy Supply Company. Rochester, New York.

2 Solenoid air valve, type V-24. Johnson Service Company, Milwaukee, Wisconsin.

"Triac, type T41l5D. RCA Corporation, Somerville, New Jersey.

Variable power microwave discharge and cavity Joseph W. Bozzelli and Robert Barat

Department a/Chemical Engineering and Chemistry, New Jersey Institute a/Technology, Newark, New Jersey 07102

(Received 18 September 1980; accepted for publication 15 December 1980)

The construction of a variable power electrodeless microwave discharge and cavity using a microwave oven magnetron and power supply is demonstrated. The apparatus is convenient to use, durable, and inexpensive.

PACS numbers: 52.80.Pi

Electrodeless microwave discharges of gases are used as transient atom and free radical sources in the initiation of chemical lasers and in studies on the chemical ki­netics of these species. I They have been used as sources of light emission in resonance fluorescence studies for monitoring low concentrations of reactive atoms2 and diatomic radicals. 3 They have been used as atomic emis­sion sources in the identification of gas chromatograph effluent composition4 and as excitation sources in optical emission spectroscopy. i; These microwave-induced plasmas have been utilized in studies on reactions of solids with high-temperature reactive gases. 6 They also serve as a reactive system for the destruction of highly toxic and/or carcinogenic chemicals. 7

McFarlane has reported the use of a commercial micro­wave oven-type magnetron to sustain an electrodeless discharge and make atomic species for use in chemical lasers. We would like to report on several modifications to this type of microwave oven magnetron, that makes an electrodeless discharge which is inexpensive, facile to construct, and convenient to use. The modifications include the use of a household-type microwave oven for the power supply, magnetron mounting, and cooling fan, plus the connection of a variac or triac (light dimmer) for output power variation. The variac or triac is used to vary the voltage or power input to the magnetron tube, through the step up transformer, allowing power output control from essentially zero to the maximum

wattage of the tube (approximately 400 W). The previ­ously described systems using a 2.45 GHz oven-type magnetron required a large amount of cooling, usually an awkward water-cooled heat exchange apparatus.

Chilukuri and LichtenH have also presented a method to construct a variable power electrodeless discharge source. Their design, however, requires considerable fabrication and electronic assembly, including the con­struction or purchase of a power supply duplicating the one provided in the oven. It also requires purchase or fabrication of an output antenna, attenuation electronics, and a power meter.

The microwave-induced plasma is an efficient mech­anism for coupling electromagnetic power into a gas. It is a clean atom source that involves no metal elec­trodes to contaminate the apparatus or light source. This is particularly attractive since metal impurities from any electrodes often cause loss in the desired atom product.

Commercial microwave discharge systems, equipped with variable power are available,9 but they are approxi­mately an order of magnitude more expensive than the design presented here.

A household-type microwave oven was purchased. The original waveguide, directing the energy into the oven compartment, was removed and the opening into this compartment blocked off with a 3/32 in. aluminum plate. This plate also served as the base for the redesigned waveguide shown in Fig. 1. This cavity (waveguide) was

612 Rev. Sci. Instrum. 52(4), Apr. 1981 0034-6748/81/040612-03$00.60 © 1981 American Institute of Physics 612

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