A practical method for measuring thermodynamic temperature based on the speed of sound in a constant volume

Abstract

The present application relates to a kind of constant volume practical method based on gas sound velocity measurement thermodynamic temperature.Based on the method, the experimental measurement time of thermodynamic temperature can be greatly reduced, the calculation of thermodynamic temperature can be programmed, the measurement of different thermodynamic temperatures does not require accurate pressure measurement and gas control, therefore the measurement system can be integrated, thus having good practical application prospect.

Description

Technical Field

[0001] This invention belongs to the field of measurement, and in particular relates to a practical method for measuring thermodynamic temperature based on gas sound velocity at constant volume. Background Technology

[0002] On May 20, 2019, the International System of Units (SI) redefined its base units, with the temperature unit Kelvin being redefined using the Boltzmann constant. This will revolutionize temperature measurement technology. Current methods for measuring thermodynamic temperature based on the Boltzmann constant mainly include gas acoustic methods, gas dielectric property (refractive index, dielectric constant) methods, gas Doppler broadening methods, noise methods, and radiation methods. Among these, the gas acoustic method, based on the relationship between gas sound velocity and thermodynamic temperature, has the advantages of low measurement uncertainty and a wide range of applicability.

[0003] Traditional gas acoustic methods measure the speed of sound at a series of different pressures, extrapolate to zero pressure to obtain the speed of sound in an ideal gas, and then obtain the thermodynamic temperature. Although this method has high measurement accuracy, its measurement cycle is long, with measurements at a single temperature point typically exceeding 24 hours, making it difficult to apply in practice.

[0004] Therefore, a practical measurement method that can shorten the time is needed. Summary of the Invention

[0005] The purpose of this invention is to provide a practical thermodynamic temperature measurement invention with a shorter measurement time. This invention relates to a method that uses the constant volume method, that is, a fixed gas specific volume (the derivative of density), to obtain thermodynamic temperature without controlling pressure. It is a practical measurement method that sacrifices some measurement accuracy but can greatly shorten the thermodynamic temperature measurement time and reduce the complexity of the system.

[0006] This invention enables the measurement time of a thermodynamic temperature to be controlled within 10 minutes, which is comparable to the traditional fixed-point thermometer calibration method. It can provide a solution for thermometer calibration under special conditions or direct in-situ thermodynamic temperature measurement.

[0007] This invention provides a practical method for measuring thermodynamic temperature at constant volume based on gas sound velocity, comprising:

[0008] Step 1, at the reference temperature T ref A certain amount of gas is introduced into the acoustic resonance cavity, and the gas pressure p is measured at a reference temperature. The gas density ρ and sound velocity u are calculated from the gas pressure p and the virial coefficient. ref ;

[0009] Step 2: Measure the resonant frequency f of the gas inside the acoustic resonance cavity using the acoustic resonance method. refThe internal dimensions of the cavity are measured using the microwave resonance method, and then the deviation of the actual gas resonance frequency from the ideal resonance frequency is calculated. This deviation mainly includes disturbances to the ideal resonance frequency caused by non-ideal factors such as the thermal boundary layer, viscous boundary layer, inlet duct, and shell vibration. By comparing the difference between the measured resonance frequency and the resonance frequency calculated using sound velocity, the unknown non-ideal factor disturbance Δf, which is difficult to calculate clearly, is obtained. unkn The unknown non-ideal disturbances mentioned are generally mainly shell vibration disturbances.

[0010] Step 3: Change the temperature of the acoustic resonance cavity and the gas. When thermal equilibrium is reached at thermodynamic temperature T, measure the gas acoustic resonance frequency f at this point. T Correction including Δf unkn The non-ideal factors, including those affecting the acoustic resonance cavity, introduce changes in the cavity size due to the thermal expansion α of the cavity, resulting in the density ρ at temperature T. T and the measured speed of sound u T .

[0011] Step 4, combining the virial equation for the speed of sound of gas and the virial coefficient for density, through...

[0012]

[0013] The thermodynamic temperature T is calculated.

[0014] Among them, u0 2 Let be the speed of sound for an ideal gas; γ0 be the specific heat ratio of an ideal gas; M be the relative atomic mass of the gas; and R = k. B ·N A k is the universal gas constant. B N is the Boltzmann constant. A β is Avogadro's constant. a γ a These are the second and third acoustic virial coefficients, which are only related to temperature. B and C are the second and third density virial coefficients, respectively.

[0015] The gas used is a gas with a known virial coefficient, such as a monatomic gas, as the working fluid.

[0016] The acoustic resonance cavity is connected to a pressure gauge via a first conduit and a valve. The pressure gauge is used to measure the pressure p of the gas at a reference temperature.

[0017] The method of this invention, as a practical approach to obtaining thermodynamic temperature after the redefinition of the SI temperature unit Kelvin, is of great significance for implementing the new Kelvin. Based on this method, the experimental measurement time for thermodynamic temperature can be greatly reduced, thermodynamic temperature calculation can be programmed, and measurements of different thermodynamic temperatures do not require precise pressure measurement and gas control. Therefore, the system can be integrated, thus showing good prospects for practical applications. For laboratories where periodic traceability is difficult to achieve, a standard device established based on this method can realize direct thermodynamic temperature measurement. In the long run, with the continuous improvement of measurement uncertainty, compared with traditional fixed-point methods, thermodynamic temperature measurement devices established based on this method can achieve continuous temperature measurement over a wider temperature range on a single device, eliminating the need for multiple fixed-point temperature devices, which is of great significance to the field of temperature metrology. Attached Figure Description

[0018] Figure 1 This is a graph showing the relationship between the square of the sound velocity of argon gas, gas pressure, and thermodynamic temperature under constant volume conditions according to the present invention.

[0019] Figure 2 This is a schematic diagram of the constant-volume thermodynamic temperature measurement process based on gas sound velocity according to the present invention.

[0020] Figure 3 This is a schematic diagram illustrating the implementation scheme of the constant-volume practical thermodynamic temperature measurement method of the present invention. Detailed Implementation

[0021] To facilitate understanding of the present invention, embodiments of the present invention will be described below with reference to the accompanying drawings. Those skilled in the art should understand that the following description is only for the purpose of explaining the invention and is not intended to limit its scope.

[0022] For real gases, the relationship between the speed of sound u, gas pressure p, and thermodynamic temperature T, when expanded using Taylor, can be expressed in the form of the virial equation for the speed of sound:

[0023]

[0024] Among them, u0 2 For the speed of sound of an ideal gas, β a γ a These are the second and third acoustic virial coefficients, which are only related to temperature; γ0 is the specific heat ratio of an ideal gas, k B N is the Boltzmann constant. A R is Avogadro's constant, M is the relative atomic weight of the gas, and R = k B ·N A This is the universal gas constant.

[0025] When the gas pressure is not very high, the relationship between actual gas pressure and density can be expressed by the second-order virial equation:

[0026] p=ρRT(1+Bρ+Cρ 2 (2)

[0027] Where ρ is the gas density, B is the second virial coefficient of density, and C is the third virial coefficient of density.

[0028] Substituting equation (2) into equation (1), we can obtain the relationship between sound speed and density and thermodynamic temperature:

[0029]

[0030] As can be seen from equation (3), under the condition of fixed density, the square of the sound velocity has a first-order approximate linear relationship with the thermodynamic temperature. In practical applications, it is generally only necessary to consider the influence of the second-order and third-order virial coefficients, so as to obtain a higher measurement level. Figure 1 The figure shows the relationship between the square of the sound velocity, pressure, and thermodynamic temperature of argon, a monatomic gas, at a certain fixed density.

[0031] Based on the above physical relationships, the present invention can form as follows: Figure 2 The basic procedure for constant-volume thermodynamic temperature measurement shown is as follows: Figure 3 The apparatus shown is for measurement purposes only, and those skilled in the art will understand that it may include structures or components other than those shown.

[0032] like Figure 3 As shown, the device includes: an acoustic resonance cavity 1, which has a certain accommodating space. Preferably, the acoustic resonance cavity is a regular-shaped cavity with fixed dimensions, and the cavity can be a cylinder, a sphere, or a cuboid. The top of the cavity has at least a first hole and a second hole. A first conduit is connected to the acoustic resonance cavity 1 through the first hole, and a second conduit is connected to the acoustic resonance cavity 1 through the second hole. The first conduit is connected to a valve 3. A measuring component 5 measures the acoustic and microwave resonant frequencies within the acoustic resonance cavity 1 through the conduit and related sensors.

[0033] Step 1, at a reference temperature T ref Preferably, the reference temperature is the triple point temperature of water. A certain amount of gas 2 is introduced into the acoustic resonance cavity 1. The gas 2 is preferably a monatomic gas as the working fluid. The acoustic resonance cavity 1 is connected to a pressure gauge 4 through a first conduit and a valve 3. The pressure gauge 4 is used to measure the gas pressure p at the reference temperature. When thermodynamic temperature measurements are carried out at other temperatures T, the valve 3 is closed, and the mass of the gas sealed in the cavity 1 remains unchanged, which can be approximated as the gas density remaining unchanged.

[0034] According to step 1, the reference temperature T ref Using the pressure p and virial coefficient of gas 2, the density ρ and speed of sound u of the gas are calculated. ref ;

[0035] Step 2: Measure the resonant frequency f of the gas 2 inside the acoustic resonance cavity 1 using the acoustic resonance method. ref The internal dimensions of the cavity were measured using the microwave resonance method, and the deviation of the actual gas resonance frequency from the ideal resonance frequency was calculated. This deviation mainly includes disturbances to the ideal resonance frequency caused by non-ideal factors such as the thermal boundary layer, viscous boundary layer, inlet duct, and shell vibration. Among these, shell vibration disturbance is a factor with relatively poor theoretical calculation accuracy and is density-dependent, being a weak function of temperature. Therefore, by comparing the difference between the measured resonance frequency and the resonance frequency calculated using sound velocity, the disturbance Δf caused by non-ideal factors can be obtained. unkn .

[0036] Step 3: Change the temperature of acoustic resonance cavity 1 and gas 2. When they reach thermal equilibrium at thermodynamic temperature T, measure the gas acoustic resonance frequency f at this point. T Correction including Δf unkn Considering the non-ideal factors, including the thermal expansion α of the acoustic resonance cavity 1, which introduces changes in the cavity size, the density ρ at temperature T can be obtained. T and the measured speed of sound u T .

[0037] Step 4: Combining the gas sound speed virial equation and the density virial coefficient, through equation (3)

[0038]

[0039] The thermodynamic temperature T is calculated. The relationship between the speed of sound and the density virial coefficient with temperature is generally known. For monatomic gases such as argon and helium, theoretical calculations based on quantum mechanics from scratch can be used. The calculation of thermodynamic temperature T is generally performed using iterative solutions.

[0040] The method of this invention can be used for the calibration, standardization, or direct in-situ measurement of thermodynamic temperature of a thermometer. When used for the calibration or standardization of a thermometer, an opening can be made in the wall of the acoustic resonance cavity 1 to ensure good thermal contact between the thermometer being calibrated and the cavity. When thermal equilibrium is reached, the thermodynamic temperature of the gas corresponds to the temperature value of the thermometer being calibrated. When used for direct in-situ measurement of thermodynamic temperature, a cavity of appropriate size can be designed according to the site environment, and a gaseous working fluid suitable for the corresponding environment can be used.

[0041] This invention enables the measurement time of a thermodynamic temperature to be controlled within ten minutes, which is comparable to the traditional fixed-point thermometer calibration method. It can provide a solution for thermometer calibration under special conditions or direct in-situ thermodynamic temperature measurement.

[0042] The method of this invention, as a practical approach to obtaining thermodynamic temperature after the redefinition of the SI temperature unit Kelvin, is of great significance for implementing the new Kelvin. Based on this method, the experimental measurement time for thermodynamic temperature can be greatly reduced, thermodynamic temperature calculation can be programmed, and measurements of different thermodynamic temperatures do not require precise pressure measurement and gas control. Therefore, the system can be integrated, thus showing good prospects for practical applications. For laboratories where periodic traceability is difficult to achieve, a standard device established based on this method can realize direct thermodynamic temperature measurement. In the long run, with the continuous improvement of measurement uncertainty, compared with traditional fixed-point methods, thermodynamic temperature measurement devices established based on this method can achieve continuous temperature measurement over a wider temperature range on a single device, eliminating the need for multiple fixed-point temperature devices, which is of great significance to the field of temperature metrology.

[0043] It is understood that although the present invention has been disclosed above with reference to preferred embodiments, these embodiments are not intended to limit the present invention. For any person skilled in the art, many possible variations and modifications can be made to the technical solutions of the present invention based on the disclosed technical content, or equivalent embodiments can be modified accordingly, without departing from the scope of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A practical method for measuring thermodynamic temperature based on gas sound velocity at constant volume, wherein the acoustic resonance cavity is a regularly shaped cavity with fixed dimensions, and at least a first hole and a second hole are provided at the top of the acoustic resonance cavity; a first conduit is connected to the acoustic resonance cavity through the first hole, a second conduit is connected to the acoustic resonance cavity through the second hole, the first conduit is connected to a valve, and a measuring component measures the acoustic and microwave resonant frequencies within the acoustic resonance cavity through the conduit and related sensors; comprising: Step 1, at the reference temperature T ref The reference temperature is the triple point temperature of water. A certain amount of gas is introduced into the acoustic resonance cavity, using monatomic gas as the working fluid. The acoustic resonance cavity is connected to a pressure gauge via a first conduit and a valve. The pressure gauge is used to measure the gas pressure at the reference temperature. p， When performing thermodynamic temperature measurements at other temperatures T, the valve should be closed; the pressure of the gas... p The density of the gas is obtained by calculating the virial coefficient. ρ and speed of sound u ref ; Step 2: Measure the resonant frequency of the gas inside the acoustic resonance cavity using the acoustic resonance method. f ref The internal dimensions of the cavity are measured using the microwave resonance method, and then the deviation of the actual gas resonance frequency from the ideal resonance frequency is calculated. This deviation includes disturbances to the ideal resonance frequency caused by non-ideal factors such as the thermal boundary layer, viscous boundary layer, air inlet duct, and shell vibration. By comparing the difference between the measured resonance frequency and the resonance frequency calculated using sound velocity, the unknown non-ideal factor disturbance Δ, which is difficult to calculate clearly, is obtained. f unkn This is due to shell vibration disturbance; Step 3: Change the temperature of the acoustic resonance cavity and the gas until thermal equilibrium is reached at the thermodynamic temperature. T At this time, the gas acoustic resonance frequency is measured. f T Correction including Δ f unkn Non-ideal factors, including those affecting the acoustic resonance cavity, will increase the thermal expansion of the cavity. α The temperature is obtained by introducing changes in the cavity size. T Density below ρ T and measured speed of sound u T ; Step 4, combining the virial equation for the speed of sound of gas and the virial coefficient for density, through... ， Thermodynamic temperature was calculated T； in, γ 0 represents the specific heat ratio of an ideal gas. M The relative atomic weights of gases are... R = k B · N A This is the universal gas constant. k B Boltzmann's constant, N A Let Avogadro's constant be 1. β a , γ a These are the second and third acoustic virial coefficients, which are only related to temperature. B and C These are the second and third virial coefficients of density, respectively; when used for the calibration or standardization of thermometers, holes are made in the wall of the acoustic resonance cavity to ensure good thermal contact between the thermometer being calibrated and the cavity.

2. The practical method for measuring thermodynamic temperature based on gas sound velocity at constant volume as described in claim 1, characterized in that, The gas used is one with a known virial coefficient.

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