CN100470211C - Buoyancy deep cryogenic liquid level gauge - Google Patents

Abstract

This invention discloses an instrument for measuring deep low temperature liquid levels, which isolates a sensor and the atmosphere in airproof to make it in the same pressure with a tested container, so the level guage is allowed to work at the deep low temperature flume with positive or negative pressure (such as liquid N, liquid O, liquid Ar and liquid H) in which, the reading is stable, reliable. This invented level guage is used in the level measurent of the liquid N tank in a large current lead constant temperature device of a large type of EAST and not influenced by the magnetic field generated by several ten thousands of amperes.

Description

Buoyancy deep cryogenic liquid level gauge

technical field

The invention belongs to the liquid level measuring instrument technology, can be used in conjunction with a liquid level automatic control device, and particularly belongs to a liquid level gauge for deep and low temperature liquid level measurement.

Background technique

Currently on the market, there are differential pressure level gauges and radio frequency capacitance level gauges suitable for liquid level measurement at deep low temperatures (such as liquid nitrogen, liquid oxygen, liquid argon, and helium, etc.). The differential pressure level gauge is based on the principle of changing the pressure difference between the bottom pressure and the liquid surface by changing the height of the liquid level. It is quite convenient and popular for normal temperature liquid level measurement and control differential pressure liquid level gauge, and merchants provide submerged liquid differential pressure sensors and supporting amplifiers and display instruments. However, for deep cryogenic liquids, since the temperature is much lower than the normal working temperature range of the differential pressure sensor probe, the bottom of the liquid pool and the gas phase pressure measuring tube are usually led out separately to connect with the positive and negative ports of the differential pressure sensor. If the low-temperature liquid enters the positive pressure measuring tube and has a certain height difference relative to the bottom of the liquid pool, the differential pressure gauge actually measures the liquid level difference between the liquid pool and the positive pressure tube, which leads to a large measurement error. Therefore, the accuracy of differential pressure liquid level measurement is closely related to the production and installation of the positive pressure tube. If all the low-temperature liquid in the positive pressure tube can be vaporized, the accuracy of the measurement is guaranteed. However, design or construction personnel without professional training often ignore this point, resulting in inaccurate liquid level measurement. In addition, the positive pressure tube must be led out of the cryogenic liquid container, and the tube inserted from the top of the container into the bottom of the liquid pool cannot be simply used, because it is more difficult to avoid the liquid column in the tube, so it is not convenient to test the liquid in the temporary test container. bit measurement.

The radio frequency capacitive liquid level gauge uses the principle of the difference in dielectric constant between cryogenic liquid and steam to measure the change of oscillation frequency to determine the change of liquid level. It can be used for liquid level measurement and control of liquid oxygen, liquid nitrogen and liquid argon. However, in the experiment, it was found that the liquid level sensor of the capacitive liquid level gauge is greatly affected by the magnetic field. Only a few tens of millitesla (mT) magnetic field drops the liquid level reading from 70cm to 0, while the actual liquid level only changes a few millimeters.

The principle of the buoyancy liquid level gauge is simple, the calibration is simple, and it is also resistant to electromagnetic interference. However, the sensor is connected to the atmosphere. If the pressure of the measured liquid increases or decreases relative to the atmospheric pressure, this is equivalent to a change in the buoyancy of the probe, resulting in a false liquid level rise and fall, which will cause a large error, so it cannot be used for liquid level measurement of cryogenic liquids.

Contents of the invention

The present invention provides a buoyancy deep cryogenic liquid level gauge for liquid level measurement of deep cryogenic liquids, which uses a high-precision (0.5%) pull-pressure sensor, and its shell is sealed and isolated from the atmosphere, and the sensor is not connected to the buoyancy probe. Diaphragm isolation is used, which allows positive and negative differential pressures of the low-temperature liquid pool under test relative to atmospheric pressure without affecting measurement accuracy.

The buoyancy type deep cryogenic liquid level gauge includes a pull-pressure sensor, which is characterized in that the pull-pressure sensor is installed in a sealed cavity connected to a deep cryogenic liquid pool at room temperature, and there is a buoyancy probe, a pull-pressure sensor and a deep cryogenic liquid pool. The buoyancy probes are fixedly connected by connecting rods, and the signal line of the tension-pressure sensor is drawn out from the sealed plug seat on the sealed cavity. The gravity of the buoyancy probe is less than the maximum tension range of the tension-pressure sensor, and the maximum buoyancy is less than the gravity of the probe.

When the cryogenic liquid is liquid nitrogen, liquid oxygen, liquid argon and liquid hydrogen, the buoyancy probe adopts thin-walled stainless steel pipe fittings sealed at both ends; when the cryogenic liquid is liquid helium, the buoyancy probe adopts epoxy pipe fittings sealed at both ends.

The buoyancy probe can be made of materials such as stainless steel or epoxy tube.

In order to increase the measurement sensitivity, a larger probe diameter d can be selected according to the conditions, because the buoyancy F is proportional to the gravity of the liquid displaced by the probe. In the following formula, ρ is the density of the liquid, H is the height of the liquid level, and the product of it and the denominator on the right side of the equation is the mass of the liquid. If the units of d and ρ are respectively selected as cm and g/cm ³ , then the unit of F is g·f or ×9.81mN.

$\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; f</span>}}{\mathrm{<span\; class="notranslate">\; 0.7854</span>}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="d"\; filenames="C200610085887E00071.png"\; state="\{\{state\}\}">d</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&rho;</span>}}$

Vapors of deep cryogenic liquids such as liquid nitrogen, liquid oxygen, liquid argon, liquid neon, liquid hydrogen and liquid helium are non-toxic to tension-pressure sensors. For liquid level measurement with high pressure in the container, the strength of the sealed shell of the sensor is required to be enhanced, and the pressure-bearing sealed plug socket is used to lead out the signal line. The buoyancy probes for cryogenic liquids with large latent heat of vaporization such as liquid nitrogen, liquid oxygen, liquid argon and liquid hydrogen should be made of thin-walled stainless steel pipe fittings, but for liquid helium with relatively small latent heat of vaporization and low density, buoyancy probes should use thermal It is made of epoxy pipe fittings with lower conductivity and small specific gravity, and the connection between the buoyancy probe and the sensor is also made of hollow epoxy pipes.

The liquid level gauge of the present invention is used for liquid level measurement in deep low-temperature liquid tanks (such as liquid nitrogen, liquid oxygen, liquid argon and liquid hydrogen) with positive pressure or negative pressure, and the reading is stable and reliable, and it is resistant to electromagnetic interference. It has been applied to the liquid level measurement of the liquid nitrogen tank in the high-current lead wire thermostat of the large-scale superconducting tokamak nuclear fusion test device EAST. It operates stably and is not disturbed by the magnetic field generated by tens of thousands of amperes of high current.

Description of drawings

Fig. 1 is a structural schematic diagram of the buoyancy liquid level gauge of the present invention.

Fig. 2 is a schematic diagram of a buoyancy-type liquid level gauge installed in a liquid nitrogen tank in a high-current lead cryostat of the superconducting tokamak nuclear fusion experimental device EAST.

Detailed ways

See accompanying drawings 1 and 2.

Labels in the figure: 1. Pull-pressure sensor, 2. Sensor sealing shell, 3. Flange, 4. Connecting rod of buoyancy probe, 5. Anti-decoration piece, 6. Buoyancy probe, 7. Liquid nitrogen tank, 8. Pressure bearing Shell, 9. Sealed socket leading out of sensor signal line, 10. Amplifier/transmitter, 11. Digital display instrument, 12. Data acquisition card.

The anti-swing piece 5 is a limit seat with a hollow, the connecting rod 4 is made of epoxy tube, and the anti-swaying piece 5 prevents the connecting rod 4 from swinging. The buoyancy probe 6 is a hollow stainless steel cylinder, which is fixedly connected with the connecting rod 4 and is located at Inside the liquid nitrogen tank 7, there is a pressure-bearing shell 8 outside the liquid nitrogen tank 7. The upper end of the connecting rod 4 is installed on the tension-compression sensor 1 . On the one hand, the flange 3 installs the shell 2, and on the other hand, it also provides a support for the tension-pressure sensor 1. The tension-pressure sensor 1 is located on the flange 3, and the signal line of the tension-pressure sensor 1 leads out from the sensor signal line to the sealed socket 9 lead out. The signal from the tension-pressure sensor 1 is respectively sent to the digital display instrument 11 and the data acquisition card 12 through the amplifier/transmitter 10 .

The air separation and oxygen production equipment of the superconducting tokamak nuclear fusion experimental device and the refrigeration/cooling system for high-temperature or low-temperature superconducting devices all require deep-low temperature liquid level measurement. Need decompression (ie negative pressure) supercooling. The use of differential pressure liquid level gauges is often unreliable; I have sought solutions for capacitive radio frequency liquid level gauges, but encountered electromagnetic interference; this is why this buoyancy type liquid level gauge was developed.

The buoyancy type liquid level gauge of the present invention has been tested by actual operation for 10 consecutive days. According to the data collected by the computer, the measurement accuracy is ±9mm, reaching the intrinsic resolution of the sensor, and the reading is stable and free from electromagnetic interference. The calibration of the liquid level gauge is simple, and the accuracy of the calibration is checked with water before installation. During use, liquid nitrogen was continuously added to the container until the overflow port, and the liquid level reading at the time of overflow coincided with the height of the overflow port. This shows that the buoyancy type liquid level gauge can fully meet the requirements of the automatic control of the cryogenic system, and it is not affected by the magnetic field generated by the power feeder of the superconducting magnet through thousands to tens of thousands of amperes. In contrast, the measurement of the differential pressure level gauge equipped at the same time not only has a large error, but also has a reading jump of up to 2cm.

Claims (1)

1. The buoyancy-type deep and low temperature liquid level gauge includes a pull-pressure sensor, which is characterized in that the pull-pressure sensor is installed in a sealed cavity connected to the deep low-temperature liquid pool at room temperature, and there is a buoyancy probe in the deep low-temperature liquid pool. The sensor and the buoyancy probe are fixedly connected by a connecting rod. The signal line of the pull-pressure sensor is drawn out from the sealed plug seat on the sealed cavity. When the cryogenic liquid is liquid nitrogen, liquid oxygen, liquid argon, or liquid hydrogen, the buoyancy probe adopts thin-walled stainless steel pipe fittings sealed at both ends; when the deep cryogenic liquid is liquid helium, the buoyancy probe adopts epoxy pipe fittings sealed at both ends.

Images