- Email: [email protected]
An automatic liquid nitrogen filling system
Spectroscopic measurements at low temperature require liquid nitrogen in some type o f research dewar. Often the research dewar must be refilled from a reservoir dewar several times during the course o f an experiment. A system is described which automatically refills a research dewar from a reservoir dewar. Two sensors are used for detecting the liquid nitrogen level in the research dewar, and an external nitrogen gas supply is used to provide the pressure to effect the transfer of liquid nitrogen.
An automatic liquid nitrogen filling system C.P.
Monaghan,
E.J.
O'Brien,
Jr, and
M.L.
Good
Many spectroscopic experiments and other physical measurements must be performed at liquid nitrogen temperatures for extended periods of time. Cryogenic systems ranging from simple static dewars to elaborate automatic coolant fluid transfer devices have been used to provide the necessary hold times. The static dewars suffer from short hold times (usually no more than a few hours) and the reported automatic cooling systems 1'2 or ref'flling systems 3-1° have certain features which make them less than ideal for routine low temperature spectroscopic measurements. In those cases where the liquid nitrogen level in the dewar is maintained with a single sensor, a specified level of liquid nitrogen must be maintained, and the frequent refilling of the dewar results in a considerable waste of liquid nitrogen. Some reported systems transfer liquid nitrogen from a reservoir dewar through solenoid controlled valves to a research dewar. These valves are susceptible to sticking open due to ice formation (especially in humid climates) and are of questionable reliability. Other systems use a heating element in the reservoir dewar to provide the nitrogen gas pressure necessary for fluid transfer to the research dewar. These heating elements are awkward in that they will dissipate heat after being turned off. If pressure in the reservoir dewar is not released immediately, liquid nitrogen will still be transferred. If the pressure is reduced by opening a valve, then boiling liquid nitrogen is wastefully vented as the heater cools. These heater systems present potential safety problems when used in metal dewars where electrical shorts may develop if the heater is accidentally positioned against a wall. These heater systems also suffer from their susceptibility to damage from repeated insertion and removal of the heater from the reservoir dewar and accidental activation outside of the dewar. To provide our Mossbauer spectroscopic experiments with a simple, continuous, and reliable liquid nitrogen supply, we have designed an automatic liquid nitrogen Filling system which overcomes the above described deficiencies. Basically the system described here is a two-sensor device which uses an external gas pressure system for initiating cryogenic fluid flow. The transfer controller is simple to construct and is also very inexpensive. Since the arrangement of gas lines, valves, etc., is similar to transfer systems described in the literature, existing systems can be easily modified to incorporate this transfer controller. The authors are from the Division of Engineering Research, Louisiana State University, Baton Rouge, Louisiana 70803, USA. CPM's permanent address is Department of Chemistry and Physics, Northwestern State University of Louisiana, Natchitoches, Louisiana 71457, USA. Paper received 21 November 1980.
When the transfer controller detects that the lower resistor in the research dewar (see Fig. 1) is not covered with liquid nitrogen, a solenoid opens a valve on the nitrogen gas cylinder and a solenoid closes the pressure release valve on the cap of the reservoir dewar. Pressure in the reservoir dewar builds to 3-5 psig (2109-3515 kg m -2, above ambient) and forces liquid nitrogen through the transfer tube into the research dewar. When the transfer controller detects that the top resistor is covered, one solenoid closes the valve to the cylinder and the other solenoid opens the pressure relief valve on the cap of the reservoir dewar. Pressure in the reservoir dewar is released and the transfer is terminated. When the lower resistor is uncovered again, the cycle repeats itself. A pressure gauge is incorporated in the reservoir dewar cap, and a spring set pressure relief valve, which is adjusted to open at 7 psig (4921.5 kg m'a), is mounted in the cap as a precaution against excess pressure in the reservoir dewar. The sensor probe (see Fig. 2) is constructed from a 1.2 cm diameter wooden dowel rod on which two resistors are mounted with their leads passing through the rod. The two sensors are 470 -+ 5%, 0.25 W carbon resistors which are seated in 2 mm thick styrofoam to prevent splashing liquid nitrogen from prematurely terminating the transfer. Each sensor resistor is in a bridge circuit u which employs a 1 k~2 potentiometer (see Fig. 3) to set the reference voltage at the negative input of a LM339N comparator. When a sen-
L]
2
:3
Fig. 1 A u t o m a t i c liquid nitrogen filling system. 1 -- N 2 gas cylinder; 2 -- liquid N 2 reservoir dewar; 3 -- research dewar; 4 -- N 2 cylinder solenoid controlled valve; 5 -- pressure gauge; 6 -- pressure relief solenoid controlled valve; 7 -- transfer tube; 8 -- sensor probe; 9 -- transfer controller; 10 -- sensor resistors; 11 -- pressure relief valve
0011-2275/81/009540-03 $02.00 © 1981 I PC Business Press Ltd 540
CRYOGENICS. SEPTEMBER 1981
the duration of a pulse enabling the latch is short, the output of the comparator is sampled only briefly and then retained on the latch output. The pulses occur often enough to detect a significant temperature change, but the low sampling rate will allow only a momentary instability in the circuit resulting from any comparator output oscillations.
sor resistor is covered with liquid nitrogen, the resistance of the sensor is high, and the output of the comparator goes to logic one. When the resistor is warm, the output of the comparator goes to logic zero. Since the temperature of a sensor varies slowly and since a slowly varying input to the comparator can cause the output to oscillate,12,13 a sample and hold circuit after the comparator is necessary for stable operation.
The cycling operation of the system is controlled by the RS4027 J-K flip-flop.14 When the RUN-STANDBY switch is at STANDBY, the logic one input at R makes the Q output of the flip-flop go to zero, and the transistor is turned off. When the transistor is off, the solenoid controlled valve on the nitrogen gas cylinder is closed, and the pressure relief solenoid valve on the reservoir dewar cap is opened. To start the cycle, the RUN-STANDBY switch is set to RUN and then RESET is momentarily opened. With the RUN-STANDBY switch in the RUN position, logic zero is applied to the R input of the flip-flop. Since the top resistor is warm (it is not yet covered with liquid nitrogen), the output of the top sensor comparator is at zero, the Q output of the top sensor latch is at logic zero, and Q is at logic one. when RESET is pushed, the clock input of the flip-flop (input C) goes to one. The Q output of the flip-flop goes to one which turns on the transistor. When the transistor conducts, the solenoid controlled valve on the nitrogen gas cylinder opens, the pressure relief solenoid valve on the reservoir dewar cap closes, and liquid nitrogen transfers to the research dewar. When the bottom resistor becomes covered with liquid
The sample and hold circuit consists of a pulse generator, 13 which also uses the LM339N comparator, in conjunction with a DM7475N latch. The pulse generator produces several logic one pulses per second which enable the latch. Since
Carbresi ons~h~ tor ~-Styr0foom pad ®
Fig.2 Mountingdetailsof asensor resistor
N Cylinder 5V NC
, oiu
l Pressure relief
O.luf 12V "
2.2k
470
5V I 8k
9"82uf I "~'~'M
~ "',i~
4.7k
:v vv
l [ LA'
~
Bottom
125~>
I 5V
470~
5VDPDT
~ I,,~ -Z
~! Standby ~
LA2
Reset
_
RI
N 5704
:,PI; 3=;LM~,9N FF = RS4027 LAI - 2 = DM7475N
Fig.3 Schematidi cagramofthetransfercontroller CRYOGENICS.SEPTEMBER1981
541
nitrogen, the output of the bottom sensor comparator goes to one, and the Q output of the bottom sensor latch goes to zero causing the C input of the flip-flop to be at zero. When the top resistor becomes covered with liquid nitrogen, the output of the top sensor comparator goes to one, and the Q output of the top sensor latch goes to one (Q'goes to zero). With the S input at zero (it is wired to ground) and with the R input now at one, the Q output of the flip-flop goes to zero. The transistor is turned off and the transfer is terminated. As liquid nitrogen gradually boils off during an experiment, the top resistor becomes uncovered and logic zero is applied to the R input of the flip-flop. When the bottom resistor eventually becomes uncovered, the clock input, C, of the flip-flop goes to one, and the transistor is turned on again. The cycle will repeat itself as long as liquid nitrogen is in the reservoir dewar or until RUN-STANDBY is set to STANDBY. The comparators, latch, and the flip-flop are connected to a 5 V regulated power supply, and the transistor-relay circuit is connected to a 12 V regulated power supply. Voltage regulation is achieved by using a RCA SK3591/960 regulator in the 5 V power supply and by using a RCA SK3592/966 regulator in the 12 V power supply. Probably any IC regulated 5 V or 12 V power supply which can deliver about 250 mA would be satisfactory. The 1 k potentiometers are set by dipping the probe into liquid nitrogen and by observing the outputs of the comparators with a voltmeter. A single
turn potentiometer is barely adequate; a ten turn potentiometer would make the adjustment much more sensitive. We would like to acknowledge the helpful suggestions made by John C. Carter. This work is a result of research sponsored by the Office of Naval Research and the NOAA Office of Sea Grant, Department of Commerce, under Grant No R/MTR-1.
References 1
2 3 4 5 6
7 8 9 10 11 12 13 14
Wiedemann, W., Mundt, W.A., Kullman, D. Cryogenics 5 (1965) 94 Nozik, AJ., Kaplan, M.Anal Chem 39 (1967) 854 Chhatwal,H.L., Chaudhuri, N. Cryogenics 13 (1973) 619 Ben 'yaminovich, SJ~l., Fisher, L.M. Cryogenics 12 (1972) 240 Battakov, B.P., Ktavchenko,V.A., Etmakov, E.B. Cryogenics 16 (1976) 501 Yoshihito, K., Motita, S., Mitsugi, K., Kinoshita, J. Cryogenics 16 (1976) 684 Vandetkooy, J., Kang, C.S.Rev Scilnstrum 49 (1978) 1746 Dey, T.K., Bhattacharyya, SJ(. Cryogenics 20 (1980) 191 Bose, A., Sthanapati, J., Ghoshai, A.K., Pal, D., Pal, A.K. Cryogenics 14 (1974) 577 Pal, A.K., Pal, D., Bhattacharyya, S.N. Cryogenics 20 (1980) 108 Schwab, M.,Spencer, L.Rev Sci Instrum 42 (1971) 884 Pepp.er,C.S. Popular Electronics (January 1980) 67 Smathers, R.T., Frederiksen, T.M., Howard, W.M. Linear Applications Handbook, National Semiconductorvl (1973) AN-74 An equivalent component is a RCA SK4027
a n e w imprint for s c i e n c e and t e c h n o l o g y b o o k s
IPC Science and Technology Press have pleasure in announcing the establishment of the WESTBURY HOUSE imprint for all new books published after 1 October 1980 complete catalogue available from Martin Key Books Manager
WE STBURY HOUSE PO Box 63, Bury Street, Guildford, Surrey GU2 5BH, England Telephone: 0483 31261 Telex: 859556 Scitec G W e s t b u r y H o u s e is t h e b o o k s i m p r i n t o f IPC Science and Technology Press Ltd
542
CRYOGENICS. SEPTEMBER 1981
An automatic liquid nitrogen filling system C.P.
Monaghan,
E.J.
O'Brien,
Jr, and
M.L.
Good
Many spectroscopic experiments and other physical measurements must be performed at liquid nitrogen temperatures for extended periods of time. Cryogenic systems ranging from simple static dewars to elaborate automatic coolant fluid transfer devices have been used to provide the necessary hold times. The static dewars suffer from short hold times (usually no more than a few hours) and the reported automatic cooling systems 1'2 or ref'flling systems 3-1° have certain features which make them less than ideal for routine low temperature spectroscopic measurements. In those cases where the liquid nitrogen level in the dewar is maintained with a single sensor, a specified level of liquid nitrogen must be maintained, and the frequent refilling of the dewar results in a considerable waste of liquid nitrogen. Some reported systems transfer liquid nitrogen from a reservoir dewar through solenoid controlled valves to a research dewar. These valves are susceptible to sticking open due to ice formation (especially in humid climates) and are of questionable reliability. Other systems use a heating element in the reservoir dewar to provide the nitrogen gas pressure necessary for fluid transfer to the research dewar. These heating elements are awkward in that they will dissipate heat after being turned off. If pressure in the reservoir dewar is not released immediately, liquid nitrogen will still be transferred. If the pressure is reduced by opening a valve, then boiling liquid nitrogen is wastefully vented as the heater cools. These heater systems present potential safety problems when used in metal dewars where electrical shorts may develop if the heater is accidentally positioned against a wall. These heater systems also suffer from their susceptibility to damage from repeated insertion and removal of the heater from the reservoir dewar and accidental activation outside of the dewar. To provide our Mossbauer spectroscopic experiments with a simple, continuous, and reliable liquid nitrogen supply, we have designed an automatic liquid nitrogen Filling system which overcomes the above described deficiencies. Basically the system described here is a two-sensor device which uses an external gas pressure system for initiating cryogenic fluid flow. The transfer controller is simple to construct and is also very inexpensive. Since the arrangement of gas lines, valves, etc., is similar to transfer systems described in the literature, existing systems can be easily modified to incorporate this transfer controller. The authors are from the Division of Engineering Research, Louisiana State University, Baton Rouge, Louisiana 70803, USA. CPM's permanent address is Department of Chemistry and Physics, Northwestern State University of Louisiana, Natchitoches, Louisiana 71457, USA. Paper received 21 November 1980.
When the transfer controller detects that the lower resistor in the research dewar (see Fig. 1) is not covered with liquid nitrogen, a solenoid opens a valve on the nitrogen gas cylinder and a solenoid closes the pressure release valve on the cap of the reservoir dewar. Pressure in the reservoir dewar builds to 3-5 psig (2109-3515 kg m -2, above ambient) and forces liquid nitrogen through the transfer tube into the research dewar. When the transfer controller detects that the top resistor is covered, one solenoid closes the valve to the cylinder and the other solenoid opens the pressure relief valve on the cap of the reservoir dewar. Pressure in the reservoir dewar is released and the transfer is terminated. When the lower resistor is uncovered again, the cycle repeats itself. A pressure gauge is incorporated in the reservoir dewar cap, and a spring set pressure relief valve, which is adjusted to open at 7 psig (4921.5 kg m'a), is mounted in the cap as a precaution against excess pressure in the reservoir dewar. The sensor probe (see Fig. 2) is constructed from a 1.2 cm diameter wooden dowel rod on which two resistors are mounted with their leads passing through the rod. The two sensors are 470 -+ 5%, 0.25 W carbon resistors which are seated in 2 mm thick styrofoam to prevent splashing liquid nitrogen from prematurely terminating the transfer. Each sensor resistor is in a bridge circuit u which employs a 1 k~2 potentiometer (see Fig. 3) to set the reference voltage at the negative input of a LM339N comparator. When a sen-
L]
2
:3
Fig. 1 A u t o m a t i c liquid nitrogen filling system. 1 -- N 2 gas cylinder; 2 -- liquid N 2 reservoir dewar; 3 -- research dewar; 4 -- N 2 cylinder solenoid controlled valve; 5 -- pressure gauge; 6 -- pressure relief solenoid controlled valve; 7 -- transfer tube; 8 -- sensor probe; 9 -- transfer controller; 10 -- sensor resistors; 11 -- pressure relief valve
0011-2275/81/009540-03 $02.00 © 1981 I PC Business Press Ltd 540
CRYOGENICS. SEPTEMBER 1981
the duration of a pulse enabling the latch is short, the output of the comparator is sampled only briefly and then retained on the latch output. The pulses occur often enough to detect a significant temperature change, but the low sampling rate will allow only a momentary instability in the circuit resulting from any comparator output oscillations.
sor resistor is covered with liquid nitrogen, the resistance of the sensor is high, and the output of the comparator goes to logic one. When the resistor is warm, the output of the comparator goes to logic zero. Since the temperature of a sensor varies slowly and since a slowly varying input to the comparator can cause the output to oscillate,12,13 a sample and hold circuit after the comparator is necessary for stable operation.
The cycling operation of the system is controlled by the RS4027 J-K flip-flop.14 When the RUN-STANDBY switch is at STANDBY, the logic one input at R makes the Q output of the flip-flop go to zero, and the transistor is turned off. When the transistor is off, the solenoid controlled valve on the nitrogen gas cylinder is closed, and the pressure relief solenoid valve on the reservoir dewar cap is opened. To start the cycle, the RUN-STANDBY switch is set to RUN and then RESET is momentarily opened. With the RUN-STANDBY switch in the RUN position, logic zero is applied to the R input of the flip-flop. Since the top resistor is warm (it is not yet covered with liquid nitrogen), the output of the top sensor comparator is at zero, the Q output of the top sensor latch is at logic zero, and Q is at logic one. when RESET is pushed, the clock input of the flip-flop (input C) goes to one. The Q output of the flip-flop goes to one which turns on the transistor. When the transistor conducts, the solenoid controlled valve on the nitrogen gas cylinder opens, the pressure relief solenoid valve on the reservoir dewar cap closes, and liquid nitrogen transfers to the research dewar. When the bottom resistor becomes covered with liquid
The sample and hold circuit consists of a pulse generator, 13 which also uses the LM339N comparator, in conjunction with a DM7475N latch. The pulse generator produces several logic one pulses per second which enable the latch. Since
Carbresi ons~h~ tor ~-Styr0foom pad ®
Fig.2 Mountingdetailsof asensor resistor
N Cylinder 5V NC
, oiu
l Pressure relief
O.luf 12V "
2.2k
470
5V I 8k
9"82uf I "~'~'M
~ "',i~
4.7k
:v vv
l [ LA'
~
Bottom
125~>
I 5V
470~
5VDPDT
~ I,,~ -Z
~! Standby ~
LA2
Reset
_
RI
N 5704
:,PI; 3=;LM~,9N FF = RS4027 LAI - 2 = DM7475N
Fig.3 Schematidi cagramofthetransfercontroller CRYOGENICS.SEPTEMBER1981
541
nitrogen, the output of the bottom sensor comparator goes to one, and the Q output of the bottom sensor latch goes to zero causing the C input of the flip-flop to be at zero. When the top resistor becomes covered with liquid nitrogen, the output of the top sensor comparator goes to one, and the Q output of the top sensor latch goes to one (Q'goes to zero). With the S input at zero (it is wired to ground) and with the R input now at one, the Q output of the flip-flop goes to zero. The transistor is turned off and the transfer is terminated. As liquid nitrogen gradually boils off during an experiment, the top resistor becomes uncovered and logic zero is applied to the R input of the flip-flop. When the bottom resistor eventually becomes uncovered, the clock input, C, of the flip-flop goes to one, and the transistor is turned on again. The cycle will repeat itself as long as liquid nitrogen is in the reservoir dewar or until RUN-STANDBY is set to STANDBY. The comparators, latch, and the flip-flop are connected to a 5 V regulated power supply, and the transistor-relay circuit is connected to a 12 V regulated power supply. Voltage regulation is achieved by using a RCA SK3591/960 regulator in the 5 V power supply and by using a RCA SK3592/966 regulator in the 12 V power supply. Probably any IC regulated 5 V or 12 V power supply which can deliver about 250 mA would be satisfactory. The 1 k potentiometers are set by dipping the probe into liquid nitrogen and by observing the outputs of the comparators with a voltmeter. A single
turn potentiometer is barely adequate; a ten turn potentiometer would make the adjustment much more sensitive. We would like to acknowledge the helpful suggestions made by John C. Carter. This work is a result of research sponsored by the Office of Naval Research and the NOAA Office of Sea Grant, Department of Commerce, under Grant No R/MTR-1.
References 1
2 3 4 5 6
7 8 9 10 11 12 13 14
Wiedemann, W., Mundt, W.A., Kullman, D. Cryogenics 5 (1965) 94 Nozik, AJ., Kaplan, M.Anal Chem 39 (1967) 854 Chhatwal,H.L., Chaudhuri, N. Cryogenics 13 (1973) 619 Ben 'yaminovich, SJ~l., Fisher, L.M. Cryogenics 12 (1972) 240 Battakov, B.P., Ktavchenko,V.A., Etmakov, E.B. Cryogenics 16 (1976) 501 Yoshihito, K., Motita, S., Mitsugi, K., Kinoshita, J. Cryogenics 16 (1976) 684 Vandetkooy, J., Kang, C.S.Rev Scilnstrum 49 (1978) 1746 Dey, T.K., Bhattacharyya, SJ(. Cryogenics 20 (1980) 191 Bose, A., Sthanapati, J., Ghoshai, A.K., Pal, D., Pal, A.K. Cryogenics 14 (1974) 577 Pal, A.K., Pal, D., Bhattacharyya, S.N. Cryogenics 20 (1980) 108 Schwab, M.,Spencer, L.Rev Sci Instrum 42 (1971) 884 Pepp.er,C.S. Popular Electronics (January 1980) 67 Smathers, R.T., Frederiksen, T.M., Howard, W.M. Linear Applications Handbook, National Semiconductorvl (1973) AN-74 An equivalent component is a RCA SK4027
a n e w imprint for s c i e n c e and t e c h n o l o g y b o o k s
IPC Science and Technology Press have pleasure in announcing the establishment of the WESTBURY HOUSE imprint for all new books published after 1 October 1980 complete catalogue available from Martin Key Books Manager
WE STBURY HOUSE PO Box 63, Bury Street, Guildford, Surrey GU2 5BH, England Telephone: 0483 31261 Telex: 859556 Scitec G W e s t b u r y H o u s e is t h e b o o k s i m p r i n t o f IPC Science and Technology Press Ltd
542
CRYOGENICS. SEPTEMBER 1981