Temperature profile of an experimental helium dewar

meter-relay circuits for driving a solenoid operated valve which is situated in the liquid transfer tube. Commercial copper wire at present available is pure enough to obtain an almost linear change in resistance at temperatures down to 60 K with sufficient reproducibility. The resistance at 77 K was 1/5-1/8 of that at 300 K for 0.10.2 mm dia copper wires with Formvar insulation. For these reasons, copper wires are suitable for monitoring of liquid nitrogen (or oxygen, air etc) level. Fig. 1 shows the resistance of a copper coil versus the distance between the coil and liquid nitrogen level as a typical example. It can be seen in the figure, as the liquid level is lowered, the temperature or the resistance of the coil increases quite rapidly. Using a single coil, both the upper and lower limits of liquid level can be easily determined to within 1 cm by rough monitoring of the resistance. The change in room temperature did not affect the resistance versus distance relation when the coil is placed 5 cm below the top of the dewar. A choke coil of about 50/all inductance works satisfactorily as a liquid level monitor. Fig. 2 shows the schematic diagram of the automatic filling device. The upper and lower limits of the liquid level is selected by adjusting Low and High settings of the meter-relay. A sheet of Mylar tape wound about the coil adjusts the thermal response time and protects against liquid splash. The solenoid operated valve used is the Tomco Model S90R580R with 115 V ac coil, 7 whose coil housing is wrapped with aluminium foil to protect the coil from moisture condensation. The pressure inside the container is maintained at about 0.4 kg cm -2 throughout the operating period. Pressurization immediately after filling the container can be effected by sending warm nitrogen gas into the liquid through the liquid transfer tube. The present apparatus with a 150 1 container has been operating for over a week or more without attention in our laboratory.

-~A Pressure guage ['o)

~ B ~] rUq Solenoid operated [__~valve

regulator~ II [ JI II Ik 111r

Common @ J. /"~l(ohm)

_~=ll'-~ll~ " ~ ,~'lO fl II ,-Jl|l "qr~" II II Italic . . . . f

[ t\1

Meter-relay 4- ± I MA

~ ,

i: !t

i

i

. Meter-relay Eoatacts

I L-T--J[ C°mm°r anual

I~ ''~-~ I .............. t_lqula nnrogenconramer

~-J'J v Research dewar or trap

I ~ ~

~ ~

'

3 ? ~ a Relay ' 6V &5 MA

Fig. 2 Schematic diagram of the automatic filling device. The resistance of the copper coil, made of 0.2 mm dia wire, is 3.5 ohm at 3 0 0 K and 0.5 ohm at 77 K. The sensitivity for finding the liquid level can be raised up to 4 times b y increasing the current through the bridge

References 1. 2. 3.

Moss,SJ., Johnson, W.T.K. Rev Scilnstr 35 (1964) 909 Seguin,H., Leonard, R.W., Imae, J. Rev Sci Instr 37 (1966) Alon, Y. R ev Sci lnstr 40 (196 9) 20

4.

Bose, A., Sthanapati, J., Ghoshal, A.K., Pal, D., Pal, A.K.

5.

Cryogenics 14 (1974) 577 Gteaves,G . J S c i l n s t r 4 0 (1963) 425

6.

Grasyuk, A.Z.,Snfitnov, V.G. Cryogenics 16(1976) 181

7.

Tomco Coupler Division, Commercial Screw Products Co, 712 East 163rd St, Cleveland, Ohio 44110, USA

Temperature profile of an experimental helium dewar J.V. Prodan and J.R. Long We have measured the temperature profile inside a liquid helium dewar under various conditions. The dewar contained a superconducting magnet and a lead system. We believe the results of our measurements are relevant to many cryogenic situations. Also, we tested the performance of the disk type current lead system we constructed as compared to a tubular system. Cryogenic literature contains numerous methods for minimization of liquid loss, thermal groundings etc. Seldom is any information given about the temperature distribution to be expected in the dewar, although some approximation to the distribution is essential in most design calculations. We built a counterflow gas cooled current lead system for our superconducting solenoid optimized for continuous operation at a current of 40 A. Counterflow systems were recently reviewed by Buyanov et al 1 and Rauh. 2 Our system uses a sequence of exchange disks similar to those of Eckert The authors are w i t h the Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA 24061. JVP is at present w i t h GTE Sylvania, Needham, Mass 02194, USA. Received 23 July 1976.

CRYOGENICS.

NOVEMBER

1976

et al. 3 The solenoid is contained in a 150 mm pyrex dewar. Approximately four litres of liquid helium fills the annular space between the glass and the 88 mm outer diameter of a stainless steel insert. The top of the solenoid is 46 cm below the top of the double wall sections of the dewar and hangs by three stainless steel tubes. Spaced along the tubes are the disks of OFHC copper that block all but the 2 mm annular spaces adjacent to the dewar walls, see Fig. 1. These ten disks are spaced at intervals of 38 mm near the top of the dewar double walls (Fig. 1) denoted level zero in Fig. 2. Thermal anchoring and counterflow exchange is accomplished by 40 strand cables of No 28 copper wire glued to each disk, Fig. 1. Heat conducted down the input to one of these cables is removed from the dewar by the cold gas passing over the disk. The disks are connected by cables of No 34 copper wire with strand count as shown, Fig. 2. Leads from the final disk into the liquid helium are a copper-superconductor composite s employing copper strips 0.38 mm thick by 14 mm wide in parallel with copper and NbTi wire.

685

The temperature profile of a tubular counterflow device was obtained by Rauh 4 and was qualitatively similar to the profile of our device. The temperature profile of our system was measured by thermocouples of chromel versus Au + 0.07 at % Fe attached to the midpoint of one of the pair of cables cemented to each disk. A representative set of results is shown in Fig. 2. The two solid curves are zero current quasi-equilibrium profiles at extremes of the liquid helium level. The dashed curve was taken at an intermediate helium level after a current of 40 A had been maintained for five minutes. The 40 A curve was found to be relatively unchanged by longer times or by variations in liquid height. The solid curves should be representative of typical experimental situations involving a relatively high thermal conductance into liquid helium while the dashed curve is representative of a high boil-off situation. The temperature of the lowest disk is always low, never rising above 11 K, and approaching the liquid temperature when the flux of effluent gas is large or the liquid interface high. There are two changes of slope of the curve which appear to be associated with features of the glass dewar system. The double wall section of the helium dewar terminated 1 1 cm below the liquid nitrogen level. The effluent helium gas was thus in thermal contact with the liquid nitrogen over this distance. The features of the slope changes which occur in this same region could be changed

23o 2io 1=0 Helium level ot -51 cm

19o

i7o Helium level of -17 cm

150

"~ ~ =30 E ~..o Top of

E

9o 70

-I=40 A f f Helium level~ ot - 2 8 e m / /

50

//" S

L - Approximote nitrogen level

S' ~ / ,"

Number wire (34B~ S ) segments

3o ~,~

Io I - 15

Tr( tu

1

mim~= I

--tO

~/

Disk positions

~ I "5

I 0

I 5

I

I

I

10

15

20

Position in dewor, cm Fig. 2 Temperature distribution among the copper heat exchange disks as a function of position in the magnet dewar relative to the position of the top of the double wall section of the dewar. The cross-sectional area of copper lead connecting any pair of disks is given in terms of the number of 34 gauge wire segments in each of the pair of lead segments making the connexion. The 40 A dashed curve was taken prior to modifications of the composite leads from the bottom disk into the helium and corresponds to a high boil-off of 85 ml A -~ h -~. At the present rate of 5 ml A -~ h -~ the dashed curve approaches the continuous curve appropriate to the liquid level

Hang~ tube

by a variation in the cross-sections (strand count) of the No 34 current lead segments. For sufficiently undersized wire a hump could be produced. The irregularity always occurred in the region where the helium vapour was in nearly direct contact through a single glass wall with the liquid nitrogen. Apparently, the enthalpy of the vaporized helium cannot be fully used unless the transition from double to single wall occurs above the point at which the elements of a counterflow exchange device have reached liquid nitrogen temperature. After substantial time and effort spent in adjusting the current lead count and configuration the liquid helium loss rate of our disk system at 40 A was reduced to 5 ml A -1 h -1 . Although this is a reasonable rate and could be improved with some effort, tubular lead sets are available commercially4 with loss rates of 3 ml A-1 h-i or less and would be a better choice at this time. 19

15 cm

PI

References Fig. 1 Simplified drawing with top and side views of the counterf l o w lead system in the region of the glass helium dewar where the double wall terminates into a single wall in contact with liquid nitrogen. Copper disks hang from three stainless steel tubes not shown in the side view. O n l y one of the t w o leads is shown in the side view. Insulators for the passage of leads through a disk are also omitted

686

1 Buyanov, Yu.L., Fradkov, A.B., Shebalin, l.Yu. Cryogenics 15

193 (1975) 2 Rauh, M. ETH-Zurich, Dissertation No 4656, summarized by Grassman, P., yon Hoffmann, T. Cryogenics 14 (1974) 349 3 Eckext,D., Endig, M., Lange, F. Cryogenics 10 (1970) 138 4 American Magnetics, Inc, Oak Ridge, TN 37830, USA

C R Y O G E N I C S . NOVEMBER 1976