CN104020358B - A kind of measure dielectric permittivity and the method for liquid level in LNG gas tank - Google Patents

Abstract

The invention relates to a method for measuring the dielectric constant and liquid level of a medium in an LNG gas tank, comprising the following steps: (1) setting a top capacitor, a middle capacitor and a bottom capacitor at the upper end, the middle part and the lower end of the LNG gas tank respectively. (2) Measure the capacitance values in the air, which are C _H1 , C _M1 and C _L1 . (3) According to the formula Calculate the dielectric constant ε ₁ of the liquid. (4) According to the formula The dielectric constant ε ₂ of the gas is calculated. (5) by formula Calculate the liquid level height H _x in the LNG gas tank. The three-segment capacitive sensor can be used as a tool for verifying and calibrating other sensors, and it can also be used as a high-precision liquid level sensor itself. When measuring the liquid level, it can overcome the measurement errors caused by factors such as temperature, pressure changes and magazines.

Description

A method for measuring dielectric constant and liquid level of medium in LNG gas tank

technical field

The utility model relates to the technical field of measurement, in particular to a method for measuring the dielectric constant and liquid level of a medium in an LNG gas tank.

Background technique

Liquefied Natural Gas (LNG) has been widely used in automobiles. LNG for automobiles is stored in thermally insulated gas tanks, and a liquid level detector must be equipped to display the LNG liquid level in real time. The liquid level detector generally includes: a liquid level sensor installed in the gas tank, a liquid level transmitter and a liquid level indicator for sensor signal acquisition and processing.

The automotive LNG liquid level sensor is installed in the inner cavity of the gas tank and is a key component of the entire detector, generally a capacitive sensor. At present, the detection methods of most liquid level detectors are: (1) first detect the no-load capacitance C ₀ of the capacitive sensor in the air, and record it as the liquid level when it is no-load; (2) record the capacitance when it is filled with LNG , which is the full-load capacitance C ₁ , and recorded as the liquid level at full load; (3) Assuming that the dielectric constant of gas and liquid is fixed, the capacitance value of the capacitive sensor and the change of the liquid level are linear, so the liquid level changes The transmitter detects the sensor capacitance, according to the formula Calculate the liquid level h and send the liquid level information to the liquid level indicator.

The above-mentioned liquid level detection method has the following disadvantages: (1) The other gas components contained in the liquefied natural gas will vary due to factors such as the place of origin, and the different components will affect the dielectric constant of LNG, so the full-load capacitance may change, so It will affect the accuracy of liquid level measurement, and the liquid level transmitter needs to be calibrated after each refueling; (2) The pressure range of liquefied natural gas for automobiles in the gas tank is 0.4-1.5MPa, and the storage temperature is close to -162°C. Over time, changes in temperature and pressure affect the dielectric constant of liquids and gases in the tank.

Contents of the invention

In order to overcome the shortcomings of the above technologies, the utility model provides a method for measuring the dielectric constant of the medium in the LNG gas tank with high measurement accuracy and a method for measuring the liquid level without being affected by the change of the dielectric constant.

The technical solution adopted by the utility model to overcome its technical problems is:

The method for measuring the dielectric constant and liquid level of the medium in the LNG gas tank comprises the following steps:

a) Top capacitors, middle capacitors and bottom capacitors are respectively arranged at the upper end, middle part and lower end of the LNG gas tank, and the top capacitors, middle capacitors and bottom capacitors are all inner tubes, outer tubes are circular, and have the same radius A coaxial capacitor, the inner tube radius of the top capacitor, the middle capacitor and the bottom capacitor is r, and the outer tube radius is R;

b) The top capacitance, the middle capacitance and the bottom capacitance respectively measure their capacitance values in the air, _which are C _H1 , C _M1 and C _{L1 respectively .} The output cable III is transmitted to the signal processing unit;

c) When liquefied natural gas is injected into the LNG tank, and the liquid level of the liquefied natural gas is higher than the bottom capacitor, the capacitance value measured by the bottom capacitor is C _L2 , and is transmitted to the signal processing unit through the output cable III, and the signal processing unit is based on the formula Calculate the dielectric constant ε ₁ of LNG in the LNG tank, where H ₁ is the height of the bottom capacitor, and ε ₀ is the dielectric constant of air;

d) When the liquid level of liquefied natural gas in the LNG tank is lower than the bottom end of the top capacitor, the top capacitor is in the gas, and the capacitance value measured by the top capacitor is _{CH 2} , which is transmitted to the signal processing unit through the output cable I, and the signal processing unit according to the formula Calculate the dielectric constant ε ₂ of the gas in the LNG tank, where H ₂ is the height of the top capacitor, and ε ₀ is the dielectric constant of air;

e) When the liquid level of liquefied natural gas is lower than the top capacitance and higher than the bottom capacitance, the dielectric constants ε1 and _{ε2 of} the liquid and gas calculated by the signal processing unit are calculated by the formula Calculate the liquid level height H _x in the LNG gas tank, where H _M is the height of the intermediate capacitor.

In order to ensure that the dielectric constant of the liquid is measured, the height H ₁ of the bottom capacitance does not exceed (H ₁ +H _M +H ₂ )×10%.

In order to ensure that the dielectric constant of the gas is measured, the height H ₂ of the top capacitor does not exceed (H ₁ +H _M +H ₂ )×10%.

For the sake of simple structure and convenient installation, the above-mentioned top capacitors, middle capacitors and bottom capacitors are installed coaxially and on the same vertical line.

The beneficial effect of the utility model is that: the dielectric constant measured by the top capacitance and the bottom capacitance is real-time, and can provide a reference dielectric constant for common capacitive liquid level gauges. The method can also measure the liquid level of LNG in real time, because the dielectric constant measured in real time is more accurate and is affected by changes in temperature and pressure. Using this method to measure the height of the liquid level is also more accurate than other capacitive liquid level gauges. The three-stage capacitive sensor can be used as a tool for verifying and calibrating other sensors, and it can also be used as a high-precision liquid level sensor. When measuring the liquid level, it can overcome the measurement errors caused by factors such as temperature, pressure changes and magazines.

Description of drawings

Fig. 1 is the capacitive sensor installation structure schematic diagram of the present invention;

In the figure, 1. Output cable Ⅰ 2. Top capacitor 3. LNG gas tank 4. Middle capacitor 5. Bottom capacitor 6. Output cable Ⅱ 7. Output cable Ⅲ 8. Signal processing unit.

detailed description

Below in conjunction with accompanying drawing 1 the utility model is described further.

The method for measuring the dielectric constant and liquid level of the medium in the LNG gas tank comprises the following steps:

(1) The top capacitor 2, the middle capacitor 4 and the bottom capacitor 5 are respectively arranged at the upper end, the middle part and the lower end of the LNG gas tank 3, and the top capacitor 2, the middle capacitor 4 and the bottom capacitor 5 are all inner tubes and the outer tubes are For coaxial capacitors that are circular and have the same radius, the radius of the inner tube of the top capacitor 2 , the middle capacitor 4 and the bottom capacitor 5 is r, and the radius of the outer tube is R. (2) The top capacitor 2, the middle capacitor 4 and the bottom capacitor 5 respectively measure their capacitance values in the air, _which are C _H1 , C _M1 and C _{L1 respectively .} The output cable II6 and the output cable III7 are transmitted to the signal processing unit 8 . (3) When liquefied natural gas is injected into the LNG gas tank 3, and the liquid level of the liquefied natural gas is higher than the bottom capacitor 5, the capacitance value measured by the bottom capacitor 5 is C _L2 , and is transmitted to the signal processing unit 8 through the output cable III7, and the signal processing Unit 8 according to the formula The dielectric constant ε ₁ of the LNG in the LNG gas tank 3 is calculated, where H ₁ is the height of the bottom capacitor 5 and ε ₀ is the dielectric constant of air. (4) When the liquid level of liquefied natural gas in the LNG gas tank 3 is lower than the bottom end of the top capacitor 2, the top capacitor 2 is in the gas, and the capacitance value measured by the top capacitor 2 is _CH2 , which is transmitted to the signal processing through the output cable I1 Unit 8, signal processing unit 8 according to the formula The dielectric constant ε ₂ of the gas in the LNG gas tank 3 is calculated, where H ₂ is the height of the top capacitor 2, and ε ₀ is the dielectric constant of air. (5) When the liquid level of liquefied natural gas is lower than the top capacitor ₂ and higher than the bottom capacitor 5, the dielectric constants ε1 and _ε2 of the liquid and gas calculated according to the signal processing unit 8 are calculated by the formula The liquid level height H _x in the LNG gas tank 3 is calculated, where H _M is the height of the intermediate capacitor 4 . The temperature and pressure in the LNG gas tank 3 usually vary in a large range, and the dielectric constant of natural gas liquid and gas changes due to the influence of temperature and pressure, so the three capacitive liquid level sensors can clearly understand the gas tank The dielectric constant of liquids and gases. The dielectric constants measured by the top capacitor 2 and the bottom capacitor 5 in this method are real-time, and can provide reference dielectric constants for common capacitive liquid level gauges. Since the dielectric constant measured in real time is more accurate, this method is more accurate than other capacitive liquid level gauges in measuring liquid level height. The three-stage capacitive sensor can be used as a tool for verifying and calibrating other sensors, and its own It can also be used as a high-precision liquid level sensor. When measuring the liquid level, it can overcome the measurement errors caused by factors such as temperature, pressure changes and magazines.

The height H ₁ of the bottom capacitor 5 does not exceed (H ₁ +H _M +H ₂ )×10%, so the bottom capacitor 5 is guaranteed to be as short as possible to ensure that the liquid level of the natural gas liquid in the LNG gas tank 3 is still low. can be measured.

The height H ₂ of the top capacitor 2 does not exceed (H ₁ +H _M +H ₂ )×10%, which can ensure that the natural gas in the LNG tank 3 can be measured as much as possible.

The top capacitor 2 , the middle capacitor 4 and the bottom capacitor 5 can be installed coaxially and on the same vertical line, so the arrangement structure is simple, the cost is low, the installation is easy, and the signal processing unit is simple.

Claims (4)

1. A method for measuring medium dielectric constant and liquid level in an LNG gas tank, characterized in that: comprise the steps: a) The top capacitor (2), the middle capacitor (4) and the bottom capacitor (5) are respectively arranged at the upper end, the middle part and the lower end of the LNG gas tank (3), and the top capacitor (2), the middle capacitor (4) and the The bottom capacitors (5) are coaxial capacitors with an inner tube and a circular outer tube with the same radius, and the inner tube radius of the top capacitor (2), the middle capacitor (4) and the bottom capacitor (5) is r, The radius of the outer tube is R; b) The top capacitance (2), the middle capacitance (4) and the bottom capacitance (5) respectively measure their capacitance values in the air, which are C _H1 , C _M1 and C _L1 respectively, and the capacitance values C _H1 , C _M1 and C _L1 are respectively Transmission to the signal processing unit (8) through output cable I (1), output cable II (6) and output cable III (7); c) When liquefied natural gas is injected into the LNG gas tank (3) and the liquid level of the liquefied natural gas is higher than the bottom capacitor (5), the capacitance value measured by the bottom capacitor (5) is C _L2 and is transmitted through the output cable III (7) To the signal processing unit (8), the signal processing unit (8) according to the formula Calculate the dielectric constant ε ₁ of the LNG in the LNG gas tank (3), where H ₁ is the height of the bottom capacitor (5), and ε ₀ is the dielectric constant of air; d) When the liquefied natural gas liquid level in the LNG tank (3) is lower than the bottom of the top capacitor (2), the top capacitor (2) is in the gas, and the capacitance value measured by the top capacitor (2) is _CH2 , and passed The output cable I (1) is transmitted to the signal processing unit (8), and the signal processing unit (8) according to the formula Calculate the dielectric constant ε ₂ of the gas in the LNG gas tank (3), where H ₂ is the height of the top capacitor (2), and ε ₀ is the dielectric constant of air; e) When the liquid level of liquefied natural gas is lower than the top capacitor (2) and higher than the bottom capacitor ( ₅ ), the dielectric constants ε1 and _ε2 of the liquid and gas calculated according to the signal processing unit (8) are calculated by the formula Calculate the liquid level height H _x in the LNG tank (3), where H _M is the height of the intermediate capacitor (4), and C _M2 is the intermediate capacitor (4) measured after the natural gas is filled into the LNG tank (3) measured value. 2. The method for measuring the dielectric constant and liquid level of the medium in the LNG gas tank according to claim 1, characterized in that: the height H ₁ of the bottom capacitor (5) does not exceed (H ₁ +H _M +H ₂ )× 10%. 3. The method for measuring the dielectric constant and liquid level of the medium in the LNG gas tank according to claim 1, characterized in that: the height H of the top capacitor ( ₂ ) does not exceed (H ₁ +H _M +H ₂ )× 10%. 4. according to claim 1 or 2 or the method for measuring medium dielectric constant and liquid level in the LNG gas tank described in claim 3, it is characterized in that: described top capacitor (2), intermediate capacitor (4) and bottom capacitor (5 ) to be coaxial and on the same vertical line during installation.