A device for the measurement and automatic control of the level of liquid gases

A device for the measurement and automatic control of the level of liquid gases A. E. Rovinskii and G. I. A b r a m o v A large number of capacity-type level indicators for cryogenic liquids-N2, H2, and He-are described which differ not only in probe construction but in the transmission system. 1-6 It is most convenient to use for this purpose mass produced apparatus. We describe below a level measurer and controller for liquefied gases based on an ESU-2M level indicator. The ESU-2M indicator is an electronic relay with two high frequency oscillators using 6N8S double triodes. The sensitivity of the oscillators, that is, the change in probe capacity leading to the cut off of the oscillation and the setting offthe electromagnetic relays, is not more than 5 pF. A solenoid valve which shuts off the vapour outlet from a storage vessel is connected to the ESU-2M through an intermediate relay, such as a R E S - 6 (Figure 1). The capacitance is made of rectangular metal plates; it was found convenient to use 0"08 mm thick safety razor blades for this purpose. 0 " 2 - 0 ' 3 mm diameter copper wires are soldered to the edge of each plate and a strip of paper is stuck (88 adhesive) to the opposite edge, 0 " 2 - 0 ' 2 5 mm thick for N2 and 0" 1 - 0 ' 15 mm for H2 and He (Figure 2). The time constant of the probe increases if thinner spacers are used, since liquid remains between the plates some time after the probe is lifted out into the vapour. The plates are stacked so that the wires are turned in different directions. The packet is firmly pressed together and bound on both edges with thread covered with adhesive. The wires are twisted together, soldered into two plaits and connected to a coaxial lead. Non-magnetic probes are made similarly from 8 x 8 mm 2 unannealed copper foil, 0"2-0"3 mm thick. As the differences between the dielectric constants of liquid and vapour (et - ev) for N2, H2, and He are about 0"4, 0"2, and 0"05, it is necessary to have initial capacities of not less than 12"5, 25, and 100 pF, in order to obtain the required capacity change AC = 5 pF, when the probe is immersed in liquid. The corresponding number of plates, n, required for such a condenser can be calculated from the equation AC= 0"8 A n (et - e v ) / 8 pF, where A c m 3 is the plate area in contact with the liquid and 6 mm the distance between the plates. The design described has been used successfully for automatic level control of liquid N2 and for indicating the level of liquid helium. In the latter case the probe consisted of forty-five 8 x 12 mm 2 plates, 0"15 mm apart with a capacity of ~200 pF. The authors thank L. M. Fisher for providing facilities for the helium experiment, and V. V. Lavrova for help in carrying it out. The authors are with the Moscow Energy Institute, Moscow, USSR.

Prib i Tekh EksperNo 2, 247 (1970). Received 18 August 1970. CRYOGENICS . APRIL 1971

2

i =4

3---

= 5

1, 2, 3 to the ESU-2M contact panel

4, 5 to electromagnetic value 6 RES-6 relay

Figure I. Arrangement of connexions for the valve of the automatic level controller to the ESU--2M relay

/I

I

aQoet

3

1 2 3

plates paper washers thread

4 5

screw for attaching probe wires, plaited together

Figure 2. Capacity level probe

REFERENCES

1. DASH, J. G., and BOORSE, H. A. Phys Rev 82, 85 (1951) 2. MEIBOOM, S., and O'BRIEN, I. P. Rev SciInstrum 34, 811 (1963) 3. MAXWELL, E. Rev S c i l n s t r u m 25, 111 (1954) 4. ZIMMERMAN, I. E., and ROOT, R. C. Rev Sci Instrum 32,853 (1961) 5. KHOLINOV, YU. M. Prib i Tekh F,ksper No 1, 192 (1963) 6. ROZENBERG, Ya. I., and KASATKIN, V. G. Prib i Tekh Eksper No 3, 203 (1963) 141