Cryogenics38 (1998) 289–291 1998 Elsevier Science Ltd. All rights reserved

Printed in Great BritainPII: S0011-2275(97)00162-8 0011-2275/98/$19.00

Simple continuous level meter forcryogenic liquidsM. Medeova*, V. Pavlık and P. Skyba

Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47,04353, Kosice, Slovakia

Received 20 July 1997

Very simple continuous level meter with capacitive transducer for cryogenic liquids(LO2, LN2 and LHe) is presented. The principle of the level meter operation is basedon the phase-locked loop (PLL) technique of capacity measurements which results ina high linearity and good stability of the level measurement. 1998 Elsevier ScienceLtd. All rights reserved

Keywords: continuous level meter; cryogenic liquids

So far, the large variety of the cryogenic liquids level met-ers with discrete or continuous level indication has beendescribed in1,2 and requirements for these instruments havealready been pointed out in detailed form in3. Therefore, inthis article we describe only the principle of the PLL leveldetector operation and present its characteristics.

Introduction

The principle of the level measurement using a capacitivetransducer is very well known. It is based on the differencein the value of the dielectric constanter for the gas andliquid, the level of which is measured. The capacitive trans-ducer is usually designed and constructed in the form oftwo coaxial tubes insulated to each other by suitable insu-lator, for example by a fishing line. The capacity of suchtransducer is given by expression

C =2pe0er

ln(D/d)·H (1)

where e0 is the dielectric constant of vacuum,er is thedielectric constant of the medium between the electrodes,D andd are diameters of outer and inner electrode, respect-ively, andH is the height of the transducer.

If the transducer is filled up to heighth by the cryogenicliquid, the corresponding change of the capacity is

DC =2pe0

ln(D/d)·(erl − erg)·h (2)

*To whom correspondence should be addressed.

Cryogenics 1998 Volume 38, Number 3 289

hereerl anderg denote the dielectric constant of the liquidand gas, respectively.

Several methods and techniques are possible to use forthe capacity measurement, including AC bridges, pulsemethods, etc. In this paper the use of a modified phase-locked loop (PLL) method for capacity measurement ispresented.

Principle of operation and characteristics

The standard scheme of the PLL-detector consists of thephase comparator, the lowpass filter and the voltage-con-trolled oscillator (VCO) (seeFigure 1). In the ‘locked’condition the frequency of VCO follows the frequencychanges of the input signal because the feedback loop holdszero phase difference between the frequency of VCO andthe input signal frequency. In such a way, the VCO con-trolled voltage VVCO is proportional to the frequencychange of the input signal. This is a standard application ofthe PLL technique in the detection of frequency modulatedsignals. Using Laplace notation (V(s) = L [v(t)]) one caneasily derive equation for VCO controlled voltage4:

VVCO(s) =s·ur(s)

K0·H(s) (3)

whereH(s) is the closed-loop transfer function:

H(s) =K0KdF(s)

s + K0KdF(s)(4)

HereK0 is the VCO gain factor,Kd is the phase detectorgain factor,F(s) is the filter transfer function andur(s) isthe phase of the incoming signal.

The presented level meter is based on a modification of

Simple continuous level meter for cryogenic liquids: M. Medeova et al.

Figure 1 The principle of phase-locked loop detection technique

the PLL scheme which is analogous to the principle ofregulation in the voltage controlled voltage or currentsources5. This modification consists in the use of a refer-ence generator oscillating with a constant (reference) fre-quencyfr as the input signal and in the connection of thecapacity transducerCx to the VCO circuit as the detectionelement. Thus, the VCO frequency depends on both thecontrolled voltageVVCO and the capacity of the transducerCx. Owing to the changes in the cryogenic liquid level, theVCO frequency is set by the PLL to hold zero phase differ-ence between the reference frequencyfr and VCO fre-quency fx. As a result, the VCO controlled voltageVVCO

follows the level changes in the cryogenic liquid. This maybe seen from Equation (3) by replacing VCO gain factorK0 with expressionK0 = k0 /Cx and assuming the highamplification of the loop, i.e.H(s) | 1. Then Equation (3)may be written in the form:

VVCO(s) =s·ur(s)

k0·Cx (5)

As seen from Equation (5) theVVCO controlled voltage

Figure 2 The scheme of the PLL continuous level meter

290 Cryogenics 1998 Volume 38, Number 3

is linear with the changes of transducer capacityCx,because the frequency of input (reference) signal is con-stant (L−1[s·ur(s)] vr = const.).

In Figure 2, the scheme of the PLL continuous levelmeter is presented. The crystal controlled reference gener-ator oscillates with the constant frequency 4 MHz, whichis set to the value of 250 kHz by the divider (7493 typeintegration circuit (IC)). This reference signal is theincoming signal for the phase comparator of the monolithicphase-locked loop IC LM565. The output error voltagefrom the phase comparator is filtered by a second orderlowpass filter. This filtered signal controls the frequency ofVCO. The low pass filter and its parameters (Cf, Rf) weredesigned using recommendations of an application notepresented in6. To reduce the influence of ambient air tem-perature on the VCO controlled voltage the LM565 iswarmed above room temperature using a small heater.Increase of the cryogenic liquid level will increase thecapacity of the sensor. The VCO must remain in ‘locked’condition for the minimum as well as for the maximumcapacity of the sensor. To fulfill this condition the VCOfrequency is set using resistorRa so that the phase differ-

Simple continuous level meter for cryogenic liquids: M. Medeova et al.

Figure 3 The dependence of the level-meter output voltage onthe capacity change. The solid line corresponds to the fit ofexperimental data by linear function f(x) = a + b·x with para-meters a = 14.56 ± 4 and b = 170.7 ± 1.1

ence between the reference and VCO signals is less than±90° for the whole range of the sensor capacity changes.

The differential amplifier (MAC 524 type IC) is used toprovide a level shift, and, if necessary, also to provide anadditional gain of the voltage signal according to the typeof cryogenic liquid, the level of which is being measured.The amplifier measures the difference between the VCOcontrolled voltage and the reference voltage. As a referencevoltage the source of stable voltage (MAC01 IC) is used.The level shift is adjusted byRl. This serves to set zerovalue of the DC output voltage at the zero level of thecryogenic liquid. The DC output voltage can be measuredby an A/D converter, a panel meter, etc.

The level sensor is made of the stainless steel wire witha diameter of 1 mm situated in the pair of coaxial thin-walled stainless steel tubes with diameter of 2.5 mm and4 mm. The wire and the inside tube, both with length of440 mm, create the sensor, and the outside tube with lengthof 1 m shields the sensor. The wire and tubes are held apartby the insulator. The initial capacity of the transducer is|100 pF. The measuring circuit is mounted directly on thetop of the transducer to reduce parasitic capacitance of theconnection cable and it is connected to the sensor viatwisted-pair wire.

In Figure 3, the dependence of theVVCO on the capacitychange of the transducerCx is presented. The measurementswere performed using additional capacitors connected par-allel to the transducer. The fit of the measured values withlinear function showed good linearity of the measurementand very good sensitivity of the level meter,| 171 mV/pF.This sensitivity allows to measure the level of the liquidhelium. The corresponding change of the voltage for sensorfully filled with LHe is | 800 mV. A long-time stability ofthe electronics measured after power-on is presented inFig-ure 4. The instabilities of the voltage difference betweenVVCO and reference voltage are in the range of± 1 mVwithin 9 hours. The instabilities are caused mainly due tochanges in ambient air temperature. If we are taking into

Cryogenics 1998 Volume 38, Number 3 291

Figure 4 The long-time stability of the voltage differencebetween VVCO and reference voltage measured after power-on.The dashed lines show deviation in range of ± 1 mV

account that the output level ofVVCO is about 4 Volts andwe neglect the reference voltage instabilities, then the devi-ations in range of± 1 mV represents the stability of± 2.5× 10-4. This corresponds to the temperature stability of theLM565 voltage controlled oscillator which, according to6,is in the range of 200 ppm/°C.

Conclusion

The continuous PLL level meter’s linearity and very goodsensitivity allows the measurement of all the cryogenicliquids including liquid 4He. In the case of LO2 or LN2

level-measurement the sensitivity of the instrument can beset by the reference frequency. Time stability of the outputvoltage (better than± 1 mV) is appropriate for standardlevel measurements in cryostats. The PLL principle of thecapacity measurement presented above is also suitable formeasurements of other physical quantities which aredetected by the capacitive sensor.

Acknowledgements

This work was supported by the Slovak Grant AgencyVEGA grant No. 2/4178/1997. We gratefully acknowledgethe material support provided by VSZˇ OCEL a.s. Kosice.

References

1. Richardson, R.C. and Smith, E.N.,Experimental Techniques in Con-densed Matter Physics at Low Temperatures. Adison-Wesley Pub-lishing Company, Inc. New York, 1988.

2. White, K.G.,Experimental Techniques in Low-Temperature Physics.Clarendon Press, Oxford, 1989

3. Velichkov, I.V., Drobin, V.M., Cryogenics, 1990, 30, 538.4. Gardner, M.F.,Phase-lock Techniques. John Wiley & Sons, New

York, 1979.5. Horowitz, P. and Hill, W.,The Art of Electronics. Cambridge Univer-

sity Press, Cambridge, 1989.6. Mills, B.T., Linear Applications Handbook. National Semiconductor

Corporation, 1986, 141.