WO1991008449A1

WO1991008449A1 - Liquid-level detector for cryogenic liquids

Info

Publication number: WO1991008449A1
Application number: PCT/CH1990/000166
Authority: WO
Inventors: Stefan Siegmann; Hans-Joachim GÜNTHERODT
Original assignee: Institut Für Physik Der Universität Basel Experimentelle Physik Der Kondensierten Materie
Priority date: 1989-12-04
Filing date: 1990-07-09
Publication date: 1991-06-13
Also published as: EP0479836A1

Abstract

The liquid-level detector of the invention makes use of the superconducting effect obtained with a metal oxide ceramic superconductor. A superconducting film (3) is applied over an intermediate substrate layer (2), made of e.g. yttrium-stabilized ZrO2, on a support (1). Flame-spraying is suitable for this and is the preferred process. Both continuously and discontinuously measuring probes can be made. The variation in electrical resistance of the superconducting material is monitored, using a low-frequency-sampling method, and processed in a detection circuit. The superconducting layer (3) carries out pulse-clocked control of a measurement current, calibration pulses for the measurement circuit being inserted between the measurement pulses.

Description

Level detector for cryogenic liquids

The invention relates to a level detector for cryogenic liquids, the liquid level being detected with the aid of superconductivity. The invention also relates to a method for producing such a detector.

It is known that the level in a cryostat with liquid helium can be measured with the aid of a superconducting wire which is immersed in the liquid. When a certain current flows, the part of the wire that is above the helium surface remains in the normal conducting state, while the immersed part becomes superconducting. By measuring the voltage drop across the wire, it is possible to determine how large the proportion of the normal-conducting resistance is in relation to the total length of the wire, from which the level of the liquid helium is inferred.

The fill level for liquid nitrogen has so far been determined primarily using capacitive methods or using carbon or semiconductor resistors. The capacitive method takes advantage of the fact that the capacitance of a capacitor changes when there is liquid nitrogen instead of air between the capacitor plates. These level meters were but very prone to failure. Ice formation on the capacitor plates is particularly troublesome. In the case of measuring devices based on carbon or semiconductor resistances, the fact that these materials have an increase in resistance with decreasing temperature, which is small with carbon resistors and runs linearly, while the resistance in semiconductors increases exponentially, was exploited. Since neither carbon nor semiconductor resistors show a marked change in the resistance curve at the temperature of 77 K which is important for nitrogen, one is forced to record the absolute value of the sensor resistance very precisely. This requires complex calibration procedures. In addition, the resistance curve with these methods is subject to fluctuations in time, which must be compensated for by frequently calibrating the device.

For many applications, a maximum or at most a multi-point measurement is sufficient, e.g. Maximum / minimum. On the other hand, in practice a continuous level measurement is desired in many cases, in extreme cases with probe lengths in the order of 1 meter. The realization of such probes, which have sufficiently high long-term stability and are resistant to various environmental influences, has hitherto been associated with considerable technical difficulties.

The present invention is based on the task of creating a probe that can be produced with reasonable effort, is stable in the long term and exhibits resistance behavior that is as linear as possible, so that simple device arrangements are sufficient for processing measured values. In a preferred embodiment, the probe should also be suitable for use as a continuously measuring probe. According to the invention, this object is achieved by the measures defined in the patent claims.

The main advantage of the new fill level detector is seen in the high measurement accuracy and in the stability and excellent reproducibility of the measured values. This even applies to continuous probes with a detection range of approx. 1 meter. The measurement can take place in open systems. By means of a suitable combination of substrate and superconducting material and by appropriate selection of the manufacturing parameters, the resistance behavior of the probe exhibits a sharp kink in the area of the transition temperature. This results in excellent discriminator properties that contribute to the high measurement accuracy.

By suitable selection of the parameters within the manufacturing process for the probe, certain layer thicknesses can be precisely maintained as desired. This ensures uniform behavior of the probe over its entire active length. With a suitable choice of the superconducting material, continuous probes for measuring the fill level, e.g. of nitrogen, oxygen, hydrogen or argon.

Details of the invention are described below with reference to exemplary embodiments and with the aid of the drawings. The figures show:

1 shows the layer structure of the probe in a schematic representation,

2 shows the comparison of the temperature-dependent resistance behavior of the probe with a carbon resistance,

3 shows a typical evaluated measurement signal as a function of the actual fill level, to demonstrate the linear behavior of the probe, 4 shows a two-point transmitter in a schematic representation, and

5 shows the simplified circuit diagram of an evaluation circuit for the acquisition of resistance measurement values and their preparation for reliable measurement results.

The basic structure of a fill level sensor selected as a preferred example is first explained with reference to FIG. 1. A mechanical carrier 1 is coated with a substrate 2. Any body, ribbon, wire or the like can be selected as the carrier. In the example, the carrier consists of a steel tube of approx. 3-8 mm in diameter. The superconducting material 3 is located on the substrate 2. The outside of the sensor is protected by a protective layer 4. In the interior of the support tube there is an electrical resistance heating element 5, which is separated by an electrical insulator 6, e.g. Made of magnesium or aluminum oxide, is supported on the inner wall of the support tube. In the area of the two pipe ends, electrical connections 7 to 10 are led to the superconducting layer 3.

The heating element 5 serves to heat the sensor area directly above the liquid level to be detected. The heating element ensures that the superconducting layer 3 does not become superconducting in the area of the cold liquid vapors directly above the liquid level and maintains its normal, finite resistance properties in this area. This ensures the good discriminator properties of the probe and thus the measurement accuracy and reliability of the measurement. The arrangement of the heating element, which extends over the extends the entire length of the active layer on the probe. This results in a short reaction time of the probe as soon as the liquid level changes. Appropriate resistance values for the heating element 5 are between 100 and 1000 ohms. The heating power can be between 10 mW and 100 mW.

By measuring the voltage drop, e.g. that part of the sensor which is above the liquid level and whose electrical resistance has not dropped to zero can be determined by a four-point measurement, as is the case in the immersed part of the sensor. The liquid level in the container is directly derived from this measurement.

The superconducting material of layer 3 was selected, for example, from the group of the metallic oxides Y-Ba-Cu-0, with the ratio Y: Ba: Cu = 1: 2: 3, for example with the structure YιBa ₂ Cu3θ ₇ _ _χ . A part of barium can be replaced by strontium, which makes the material completely superconducting at 81 K (± 2 K). The point of use of superconductivity is approximately 84 K. The superconducting material is applied as a film of, for example, 20-80 μm to a suitable substrate 2. The desired superconductor film is finally formed only after a subsequent heat treatment. Suitable material for a substrate 2 applied as a layer is, for example, yttrium-stabilized ZrO 2. It has been shown that the transition temperature T _{c of} the above-mentioned superconducting material depends strongly on the substrate used and the tempering conditions. In the example shown here, depending on the setting of the process parameters described in more detail below, layers with defined offset T _c values in the range between 80 K and 85 K can be produced reproducibly. The jump temperature in this example is thus approx. 4 K above the temperature of liquid nitrogen (77 K), so that probes with the jump temperature mentioned here as examples are excellent for both continuous and discrete detection or monitoring of the fill level in containers or tanks with liquid nitrogen.

A similarly suitable superconductor for use as a nitrogen probe is BiCaCuSrO _χ , whose transition temperature T _c is 80 K.

In FIG. 2, for comparison, the temperature dependence of the resistance profile of the new sensor using _χ is high-temperature superconductor as compared to known carbon resistors be¬ C shown a 7_ Y-lBa2Cu3θ. The advantage of the new sensor lies in the sharply defined crunch in the resistance behavior, which results in excellent discriminator properties without the need for complex electronics for evaluating the measured values. Due to the sharp kink in the resistance curve, a particularly high measuring accuracy can be achieved.

For monitoring other cryogenic liquids, similar suitable high-temperature superconductors and combinations with substrate material can be selected from the group of substances given above, the superconducting transition temperature of which is close to the temperature of the cryogenic liquid to be monitored. However, it is expedient to choose the transition temperature at most 5 K above the boiling point of the liquid, since otherwise the sensor material would remain superconducting if the sensor is located directly above the liquid level in the area of the supercooled vapor which forms. As already mentioned, the reaction between the substrate layer 2 and the superconductor layer 1 also plays a decisive role in achieving the desired properties of the probe, in addition to other process parameters. The desired superconductivity properties only form in the course of the heat treatment. According to a preferred example, a yttrium-stabilized ZrO 2 coating 2 is applied to a carrier 1 made of steel, for example made of stainless steel, and then the actual superconductor layer 3 is sprayed on. These steps are followed by the heat treatment mentioned and described in more detail later.

Various methods, such as laser vapor deposition, electron beam vapor deposition, shock spraying, plasma spraying or flame spraying, are fundamentally possible for thin or thick film preparation. Flame spraying has proven to be particularly advantageous for the production of longer-acting, longer probes. The spray particles are heated and accelerated to the speed of sound. The particles then impact the substrate, and their rough surface mechanically adheres to them when they cool.

The spray particles are introduced into the spray device as powder and there with an inert carrier gas, for example with _? , whirled up and injected concentrically into a spray nozzle. The hot combustion gases emerging from a combustion chamber take the powder mixture with it and melt it. The particles reach a speed of 200 to 400 m / s. After a flight distance of approx. 20 to 40 cm, they hit the material to be coated. The dwell time in the hot gas jet is therefore approximately 1 millisecond. After this short flight time, the melted powder particles hit the substrate surface. Upon impact, the particles cool instantly through heat transfer transfer to the cooler material. They are pushed wide by their kinetic energy; they shrink and freeze immediately.

The layer 3 applied with a constant thickness of between approximately 20 to 80 μm thus adheres predominantly by mechanical clamping and only locally by chemical binding forces with the carrier layer.

The preparation of the coating material described in more detail below also plays a decisive role in achieving the uniform layer thickness and the good adhesive properties. After the powdery basic substances have been mixed together, the powder is sintered for about 5 times 100 hours at about 940 degrees C. with the addition of oxygen. After cooling, the material was ground to fine powder in an agate mill. After a further sintering process, the powder was ground and sieved with two sieve fabrics to grain sizes between 80 and 30 / m. When the coating method described above is used, the substance thus prepared results in a particularly uniform melting of the powder grains.

The substrate 2 used was selected according to the criteria described below and, in the example, applied in a coating process. To avoid cracking, the thermal expansion coefficient of the substrate should be close to that of the superconductor used. Furthermore, the substrate should be as non-conductive as possible and should not react with the superconductor during annealing. To meet these complex requirements, 7% yttrium-stabilized ZrO 2 has proven to be particularly advantageous in the present example. The subsequent heat treatment can in principle be carried out at different temperatures and residence times. Since Zr0 ₂ would react with the superconductor at too high temperatures, longer treatment times at lower temperatures have proven to be advantageous. Good results were obtained with a thermal aftertreatment which took place at 900 degrees C for 1/2 hour. With decreasing temperature, the decrease in resistance is linear up to the onset at approx. 93 K. Thereafter, the resistance drops rapidly to zero, and it reaches this value at approx. 82 K. At room temperature, the probes had resistance values in the order of 10 to 100 Ohm up. These values depend on the length of the film and its layer thickness.

The probe structure thus obtained was covered with a protective layer for stabilization. For this purpose, polytetra-fluorethylene-containing lacquer was sprayed onto the surface of the probe. A heat treatment was then carried out at approx. 150 degrees C for 3/4 hours. AL is suitable as a paint the commercial product "GREBLON" No. 05-1215-700.9440.

Overall, the adhesive properties and crack resistance of the applied layer were excellent. The resistance behavior of the probe has the desired linearity, as can be seen in Fig. 3.

The modified example described below is suitable for minimum-maximum probes or for limit value displays or simple safety switches. As shown in FIG. 4, discrete probe zones 13, 15 can be defined on certain areas on a common carrier 1, each of which is provided with its own electrical connections for recording measured values. In this example, each probe zone comprises a superconducting Layer 3 and an electrical heating resistor 5, which is separated from the superconducting layer 3 by an insulating layer 6.

E.g. the upper sensor zone 13 in the area of the highest permissible liquid level and the second sensor zone 15 in the area of the lowest level in a container 11 results in a combined minimum / maximum display for the liquid level to be monitored.

In the example, four external electrical connections are provided for each sensor zone, of which connections I and I_ are used for applying a measuring current I and connections U ₊ and U_ for voltage tapping over the superconducting layer. The voltage drop across the connections U ₊ and U_ is evaluated in a subsequent detector circuit.

A detector circuit according to FIG. 5 is particularly advantageous for detecting and evaluating the changes in resistance on the superconducting material of the sensor. It is able to detect the smallest resistance values in the range of milli-ohms as well as resistance changes in this order of magnitude safely and without being influenced by external disturbances and drift. For example, the resistance values of the above-mentioned strontium-doped superconducting material are approx. 50 milli-ohms for room temperature and approx. 15 milli-ohms directly above the jump. With a measuring current of approx. 10 mA used in the example, the voltage drop to be recorded in the middle of the transition range was approx. 75 μV.

5 consists of a circuit I to which the sensor, consisting of the superconducting layer 3 and the resistor 5, is connected is. The circuit contains a switch A controlled by a clock generator T. The measuring circuit consists of an amplifier 20, a decoupling capacitor 21, a second amplifier 22 and an RC element with a downstream output amplifier 23. Between the second amplifier 22 and the RC element is a second switch B, controlled in clock T and in synchronism with switch A, between the decoupling capacitor 21 and the second amplifier 22 there is a normalizing resistor 24, which is controlled via a third switch N controlled in clock T. leads to mass. However, this third switch is switched in push-pull to the first two switches A and B, that is, switch N remains open when switches A and B are closed, and vice versa. A value between 400 and 500 per second has proven to be an advantageous clock frequency in the example.

If switches A and B are switched on, the voltage drop across superconducting layer 3 is measured, amplified, and stored in capacitor C. If switches A and B are open, the third switch N closes instead, as a result of which a zero point correction is carried out to eliminate interferences, such as thermal instabilities. The voltage building up on the capacitor C is thus directly proportional to the resistance value measured on the superconductor 3. The output amplifier delivers, for example, a signal voltage between 0V and 5V DC voltage. This can be used, for example, to control subsequent digital monitoring units which display the liquid level, operate control circuits for opening and closing refill devices, or read out the level of the liquid and the status of the measuring instrument via a serial or parallel interface for further processing, for example in a computer. In order to reduce heat losses of the liquid to be monitored by the action of the heating resistor 5, it can be expedient to specify certain measuring intervals in which one or more measuring processes are triggered. In between, the detector device remains switched off. The intervals between such intervals can be a few minutes or, in other applications, a few hours. This on / off switching can be controlled with the aid of an autonomously controlled or manually operated switch 25 according to FIG. 5.

Claims

PATENT CLAIMS:

1. Level detector for cryogenic liquids, for detecting the liquid level with the help of superconductivity by a probe to be introduced into the cryogenic liquid, characterized in that the probe consists of a carrier (1) with a substrate (2) thereon and one on the Superconducting layer (3) located on the substrate consists of a high-temperature superconductor selected from the group of metal oxide ceramics and that the superconducting layer (3) is provided with electrical connections (7-10) for measuring its electrical resistance in a detection device.

2. Level detector according to claim 1, characterized in that the superconductor selected for the sensor has a transition temperature which is a few K, preferably about 5 K, above the temperature of the cryogenic liquid to be monitored.

3. Level detector according to claim 1, characterized in that the superconducting layer (3) contains material from the group Y-Ba-Cu-0 in the ratio Y: Ba: Cu = 1: 2: 3, with the adaptation of the transition temperature to the desired application-specific values, a part of barium is at least partially replaced by another element that influences the transition temperature.

4. Level detector according to claim 1, characterized in that the superconducting layer (3) contains material of the composition BiCaCuSrO _χ .

5. Level detector according to claim 1, characterized in that the carrier (1) consists of steel.

6. level detector according to claim 1, characterized in that the superconducting layer (3) over the level area to be detected is distributed discontinuously along the support body (1), whereby discrete individual sensor areas (13, 15) are defined.

7. Level detector according to claim 1, characterized in that a heating device (5) is provided at a certain, uniform distance from the superconducting layer (3), through which those areas of the superconducting layer

(3), which are located directly above the liquid level in the area of liquid vapors, are held in the area of the electrical normal line.

8. Level detector according to claim 1, characterized in that the detection device has a low-frequency clock (T) for switching current measuring pulses (I) through the superconducting layer (3) and for calibrating the measuring circuit for a reference, the calibration clocks between the measuring cycles lie.

9. level detector according to claim 8, characterized gekennzeich¬ net that the clock (T) is set to a clock frequency in the range between 400 and 500 per second.

10. A method for producing a level detector according to claim 1, characterized in that an yttrium-stabilized ZrC ^ coating is applied to a carrier (1) as the substrate (2) and then the superconducting layer (3) in the Flame spraying process is sprayed on, and that these steps are followed by a heat treatment.

11. The method according to claim 10, characterized by the use of 7% yttrium-stabilized Zr0 ₂ as the substrate (2).

12. The method according to claim 10, characterized in that the superconducting material is applied as powdery particles during flame spraying at approximately 200 to 400 m / s from a distance of 20 to 40 cm to the surface of the substrate (2) .

13. The method according to claim 12, characterized in that to obtain the powdery particles, the powdery basic substances are mixed together, sintered for about 5 times 10 hours at about 940 degrees C with the addition of oxygen and, after cooling, ground to a fine powder.

14. The method according to claim 10, characterized in that the superconducting material is applied in a constant layer thickness in the range between approximately 20 and approximately 80 / um.

15. The method according to claim 10, characterized in that the subsequent heat treatment is carried out at about 900 degrees C for about 1/2 hour.

16. The method according to claim 15, characterized in that after the heat treatment, a polytetra-fluorethylene-containing lacquer is sprayed onto the surface of the superconducting layer (3) and baked in an additional heat treatment.