A calibration method for low temperature medium density sensor

Abstract

A method for calibrating a low-temperature medium density sensor, step 1, selecting experimental reference low-temperature liquid media A0, A2 according to the medium to be measured A1, the upper limit temperature Tmax, and the lower limit temperature Tmin in use; step 2, measuring the capacitance C0 of the sensor immersed in A0 under approximately standard atmospheric pressure, and the relative dielectric constant ε of A0 under this condition is known A0 ; Step 3: Under approximately standard atmospheric pressure, measure the capacitance C2 of the sensor immersed in A2. The relative dielectric constant ε of A2 under this condition is known. A2 ; Step 4. Substitute C0 and C2 into the formula C=K×ε+Cs to obtain the sensor structure coefficients K and Cs; Step 5. Under approximately standard atmospheric pressure, measure the capacitance C1 of the sensor immersed in A1. Under this condition, the relative dielectric constant ε of A1 is known. A1 ; Step 6: Based on K, Cs, calculate the measured ε′ A1 , and the known ε A1 The present invention uses the liquid phase boiling point temperature with good reproducibility under normal atmospheric pressure as the calibration condition, and can complete the calibration under laboratory conditions, and has the advantages of simplified operation, high efficiency, reliable calibration, etc., and has good application effect.

Description

Technical Field

[0001] The present invention relates to the field of clean energy, and in particular to a calibration method for a low-temperature medium density sensor, which is particularly suitable for occasions of low-temperature medium density detection in the fields of hydrogen energy, aerospace, air separation, etc. Background Art

[0002] The density measurement of cryogenic media is often involved in clean energy applications. Capacitive density sensors are widely recognized for their safety, pollution-free, easy installation, low cost, and fast response. Among them, density sensor calibration is an indispensable and important link in the development and production process. The cryogenic medium density measurement devices used in existing space launch sites and liquid rocket engine test benches adopt the calibration method proposed by the Beijing Institute of Spacecraft Testing Technology, that is, calibrating the sensor under different temperature conditions for the same medium, using the obtained data to establish and solve linear equations to obtain the corresponding structural coefficients. In recent years, domestic research units have further proposed to use cryogenic refrigerators and other equipment to build a calibration system to determine the density of cryogenic media (liquid tritium) based on this method.

[0003] In the existing method, repeated replacement, vacuuming, filling, rewarming, stabilization waiting and other process steps are required. The calibration system is relatively complex, the calibration conditions that need to be met are demanding, the cost is high, and the calibration efficiency is low.

[0004] This technology uses different media and the reproducible liquid boiling point temperature under normal atmospheric pressure as the calibration condition. It can be used to complete the calibration under conventional laboratory conditions without the need for replacement, vacuuming, filling, reheating, stabilization and waiting, which simplifies the operation and is highly efficient. At the same time, the test is carried out in a wider temperature range and the actual application medium verification is added, which expands the validity and credibility of the test results. Summary of the invention

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] Step 1: Select experimental reference low-temperature liquid media A0 and A2 according to the medium to be tested A1, the upper limit temperature Tmax and the lower limit temperature Tmin during use;

[0007] Step 2: Under approximately standard atmospheric pressure, measure the capacitance C0 of the sensor immersed in A0. The relative dielectric constant ε of A0 under this condition is known. A0 ;

[0008] Step 3: Under approximately standard atmospheric pressure, measure the capacitance C2 of the sensor immersed in A2. The relative dielectric constant ε of A2 under this condition is known. A2 ;

[0009] Step 4: Substitute C0 and C2 into the formula C = K×ε + Cs to solve for the sensor structure coefficients K and Cs.

[0010] Step 5: Under approximately standard atmospheric pressure, measure the capacitance C1 of the sensor immersed in A1. The relative dielectric constant ε of A1 under this condition is known. A1 ;

[0011] Step 6: Calculate the measured ε′ based on K and Cs. A1 , compare it with the known ε A1 and calculate the relative deviation.

[0012] Among them, the boiling point temperature TA1 of A1 under standard atmospheric pressure satisfies Tmin < TA1 < Tmax, the boiling point temperature TA0 of A0 under standard atmospheric pressure satisfies 0K < Tmin - TA0 < 20K, and the boiling point temperature TA2 of A2 under standard atmospheric pressure satisfies 0K < TA2 - Tmax < 20K.

[0013] During the calibration process, the range of the condition of approximately standard atmospheric pressure is 91KPa to 110KPa.

[0014] During the calibration process, the low-temperature media A0, A0, and A2 are stored in a low-temperature dewar. Description of the Drawings

[0015] Figure 1 For the steps of the calibration method

[0016] Figure 2 For the schematic diagram of the sensor structure principle Detailed Implementation Manner

[0017] The present invention will be further described below in conjunction with embodiments.

[0018] The theoretical basis of capacitive low-temperature medium density measurement is the Clausius–Mossotti equation derived from the electrostatic field theory.

[0019] The Clausius–Mossotti equation reveals the relationship between the density and dielectric constant of a substance:

[0020]

[0021] In the formula: ρ is the density, with the unit kg / m 3 ; P is the specific polarization, with the unit m 3 / kg; ε L is the relative dielectric constant of the medium.

[0022] The relationship between the relative dielectric constant of the medium between the electrodes of an ideal capacitive sensor and the capacitance C of the capacitor is as follows:

[0023] C = A TC ·εr (2)

[0024] Where A TC is the structural coefficient, which already includes the term of the vacuum permittivity ε0. Since ε0 is a dimensionless unit, A TC has the same unit as C and is expressed in the unit of pF magnitude here. C is the measured ideal capacitance value, which is only proportional to the relative permittivity ε r between the plates.

[0025] The above is only an ideal situation. In the actual measurement of the capacitive sensor, non-ideal effects caused by the edge effect of the electrostatic field, additional structures such as insulation supports, and imperfect local shielding are inevitable. It is equivalent to a parallel combination of an ideal capacitance that varies proportionally with the permittivity and a relatively fixed parasitic capacitance CS, which is reflected as a superposition relationship and can be expressed by the following formula:

[0026] C = A TC ·ε r + CS S (3)

[0027] Based on the above, a method for indirectly obtaining the medium density through capacitance measurement can be obtained. ATC can also be represented by K in the present invention.

[0028] The actual medium A1 to be measured is liquid hydrogen, and its boiling point temperature under standard conditions is 20.3K. The upper limit temperature Tmax required during use is 23K, and the lower limit temperature Tmin is 18K. According to the present invention, the cryogenic medium A0 is liquid helium, and its boiling point TA0 under standard conditions is 4.2K, satisfying 0K < Tmin - TA0 < 20K. The cryogenic medium A2 is selected as liquid neon, and its boiling point TA2 under standard conditions is approximately 27K, satisfying 0K < TA2 - Tmax < 20K.

[0029] The density sensor to be calibrated adopts a multi-layer coaxial capacitive structure, and the electrode material is 316L stainless steel, and the electrode thickness is 0.5 - 0.8mm. As Figure 2 shown, in the order from the inside to the outside, the electrodes are named 1 - 6 respectively. The odd electrodes 1, 3, and 5 are cascaded, and the even electrodes 2, 4, and 6 are cascaded to form a parallel-type capacitance to be measured to improve the detection sensitivity.

[0030] Under standard conditions, the relative permittivities ε A0 , ε A1 , ε A2 are 1.050, 1.230, and 1.186 respectively.

[0031] Under the atmospheric environment of experimental calibration, the capacitance C0 of the sensor immersed in liquid helium is measured to be 38.670pF.

[0032] In the same way, the capacitance C2 of the sensor immersed in liquid neon is measured to be 43.443pF.

[0033] Therefore, substituting into the formula C=K×ε+Cs, the solution is: the structural coefficient K is 35.096pF, and Cs is 1.8196pF.

[0034] In the same way, the capacitance C1 of the sensor immersed in liquid hydrogen is measured to be 44.917pF. Substituting the K and Cs obtained above, the relative dielectric constant ε′ of A1 is solved. A2 is 1.228.

[0035] The calculated relative error is (1.228-1.230) / 1.230=-0.16%. It can be seen that the method of the present invention can achieve better calibration accuracy.

[0036] The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiment. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, if some improvements and modifications are made without departing from the principle of the present invention, such improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for calibrating a low-temperature medium density sensor, characterized in that: Work according to the following steps: Step 1: Select experimental reference cryogenic liquid media A0 and A2 according to the medium A1 to be measured, the upper limit temperature Tmax and the lower limit temperature Tmin during use; the boiling point temperature TA1 of A1 under standard atmospheric pressure satisfies Tmin < TA1 < Tmax, the boiling point temperature TA0 of A0 under standard atmospheric pressure satisfies 0K < Tmin - TA0 < 20K, and the boiling point temperature TA2 of A2 under standard atmospheric pressure satisfies 0K < TA2 - Tmax < 20K; Step 2: Under approximately standard atmospheric pressure, measure the capacitance C0 of the sensor immersed in A0. The relative dielectric constant ε of A0 under this condition is known. A0 ; The condition range of approximate standard atmospheric pressure is 91KPa~110Kpa; Step 3: Under approximately standard atmospheric pressure, measure the capacitance C2 of the sensor immersed in A2. The relative dielectric constant ε of A2 under this condition is known. A2 ; Step 4: Substitute C0 and C2 into the formula C = K×ε + Cs to solve for the sensor structure coefficient K and the parasitic capacitance Cs; where C is the measured capacitance value; Step 5: Under approximately standard atmospheric pressure, measure the capacitance C1 of the sensor immersed in A1. The relative dielectric constant ε of A1 under this condition is known. A1 ; Step 6: Calculate the measured εˊ based on K and Cs A1 , and the known ε A1 Compare and calculate the relative deviation.

2. According to A0, A0, and A2 described in claim 1, they are stored in a cryogenic dewar during the calibration process.

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