Level probe for cryogenic liquids

RESEARCH AND TECHNICAL NOTES LEVEL

PROBE

FOR

CRYOGENIC

LIQUIDS

M. SAUZADE, C. GEORGES~, J. PONTNAU, and P. LESAS lnstitut d't~lectronique, Orsay, France Received 26 March 1964

TIlE level of a cryogenic liquid may be determined by using the difference in thermal conductivity of the liquid and its vapour or gaseous environment. The sensing element is a semiconductor whose temperature is maintained higher than the surrounding fluid by an electric current. For a suitably doped semiconductor, the resistivity is a sensitive function of temperature, varying inversely; beyond a certain current a 'thermal avalanche' can occur. The voltage--current relationship typically shows a

I u

sudden bend, and under certain conditions, a negative resistance. If the sensor is shifted from liquid to vapour, this 'knee' voltage is also shifted because of the different thermal conductivities of the two phases. Thus, to determine the boundary between the two phases, the sensor is maintained at constant voltage and one looks for the large change in current as the sensor passes from one phase to the other. This principle has already been employed using 'thermistors' in which the thermal avalanche occurs at room temperature. Such devices are unusable at low temperatures because their 'knee' voltage would be very high. We have developed a sensor which avoids this problem and permits precise level measurements in helium or hydrogen, with very little dissipation of energy. The helium probe consists of a bar of boron-doped silicon (room temperature resistivity 0.017 fl cm) which is 60 It thick, 0.6 mm wide, and 1.5 mm long. It is soldered to the terminals of a model TO-18 transistor case. Figure 1 shows the current-voltage curves of this probe in liquid helium and in the vapour just above it. The indicated temperatures were obtained by extrapolating the measured resistance to its value in the limit of zero measuring current. They 300

2

I

I

I

I

I

I

o

3

5

6

I

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E

//1

o

200

I u 100 0

05 VoltageI. 2. 3. 4. 5.

Probe Probe Probe Probe Probe

1.0 •

1.5 (volts)

in liquid hefium 1"5 m m above liquid helium 3 m m above liquid helium 10 m m above liquid helium 20 m m above liquid helium

Figure 1 I" Soci~t6 lndustrielle de Liaisons I~lectriques, 64 bis, Rue de Monceau, Paris, France.

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80

60 40 20 1

2

3

4 Voltage

7

8 9 (volts)

1. Probe in liquid hydrogen 2. Probe ! cm above liquid hydrogen 3. Probe 2 cm above liquid hydrogen

Figure 2 C R Y O G E N I C S " F E B R U A R Y 1965

are only approximate since we have not precisely calibrated the resistance variation law at low temperatures. Still, it is clear from this graph that, at a constant voltage of about 1 V, the current changes from 200 ~tA with the probe in liquid to more than 3 mA when it is in gaseous helium, 1-5 mm above the surface. The resistance change is so large that level variations much less than a millimetre can be easily seen. Owing to the small size of the probe, thermal inertia is very slight and the response time is a fraction of a second. The power dissipated in liquid helium is of the order of 0-2 mW.

NUCLEAR

QUADRUPOLE

RANGE

RESONANCE

130-320 Mc/s

FOR

LOW

The hydrogen probe is a slightly longer bar (2-5 mm). The results are essentially the same (Figure 2) as for helium. The 'knee' current and voltage are somewhat higher, but the ratio of currents in gaseous and liquid hydrogen is still about 10. Probes of this type are useful for liquids whose boiling points fire under 100° K, a range permitting the negative temperature coefficient of resistance. The probe dimensions and amount of doping must be such that the current and voltage of the 'knee' are kept small enough for power dissipation to be at an acceptable level.

SPECTROMETER TEMPERATURE

PICK-UP

FOR

THE

MEASUREMENTS

D. F. BALSA and S. M. RYABCHENKO Institute of Physics, Academy of Sciences, U.S.S.R. Received 2 June 1964 *

IN studying crystals by the n.q.r, method, measurements have to be made over a wide frequency range-from 1 to 1,000 Mc/s. Such a frequency range can be covered by a few pick-ups. Measurements of n.q.r. spectra at low temperatures provide important information. In the present work a pick-up for the frequency range 130-320 Mc/s for a n.q.r, spectrometer is ? Received by PTI~ Editor 21 June 1962: Pribory i Tekhnika t~ksperimenta No. 4, 107 (1963).

described, suitable for low-temperature measurements. It can work in regenerative or super-regenerative conditions. The system described is similar to those described by DehmelO and by Kojima et al. 2 and is a push-pull oscillator-detector with two-conductor lines resonance circuits. The basic circuit of the pick-up with preamplifier is shown in Figure l(a). Line A is a half-wave section, short-circuited on both sidi:s with a fraction 2/8 substituted by a coil of inductance L = p]¢o. The anodes of the oscillator valves are connected at the voltage

from 3 G

V36N2P ~120k

Line A -

30

b

V3 6N2P

~lOOk

t 680k

~o 68k

from GSS

+

°6.3V o 8 nF,..~

4.7kL

(o)

Figure I(a). Circuit of pick-up with preampl(lfer CRYOGENICS

• FEBRUARY

1965

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