A method for manufacturing a carbon dioxide triple point container with variable inner diameter

Abstract

The invention discloses a method for manufacturing a carbon dioxide triple point container with a variable inner diameter, which changes the conventional method of extending the axial length of the container to provide expansion space for the medium into an innovative method of extending the radial length to provide expansion space for the medium, thereby solving the limitation problem of short axial space; the carbon dioxide triple point container with a variable inner diameter can simultaneously meet the immersion depth requirement for calibrating a long-rod standard platinum resistance thermometer, the axial dimension limitation of the container and the pressure safety technical requirements at room temperature.

Description

Technical Field

[0001] The invention belongs to the field of temperature measurement, and in particular relates to a method for manufacturing a carbon dioxide triple point container with a variable inner diameter. Background Art

[0002] The current International Temperature Scale (ITS-90) contains three elements: definition fixed points, interpolation instruments and interpolation formulas. The temperature of the triple point of mercury (Hg) is 234.3156K, which is an important definition fixed point. However, with the prohibition of mercury-containing products, the triple point of mercury is facing the situation of being replaced by the triple point of carbon dioxide (CO2) (216.5920K).

[0003] Normal mercury triple point containers are cylindrical structures. Taking into account the size of the isothermal zone, temperature fluctuation and uniformity of the constant temperature device, the maximum height of the content of the mercury triple point container is about 300mm internationally. In order to replace the mercury triple point without introducing other technical requirements, to ensure that the carbon dioxide triple point container can also meet the isothermal zone size requirements of the constant temperature device and to ensure temperature uniformity, the content height of the carbon dioxide triple point container should still be maintained at 300mm.

[0004] The triple point temperature of carbon dioxide is 216.592K, the pressure is 0.51795MPa, and the density of solid phase CO2 is 1530kg / m 3 , liquid phase density is 1178kg / m 3 The saturated liquid density of carbon dioxide decreases with increasing temperature and is 679.24 kg / m at 300 K. 3 . In the process of heating solid carbon dioxide from 216K to 300K, the solid phase turns into liquid phase, and the volume increases by about 126%. This means that to fill 1 unit volume of solid carbon dioxide, the sealed container needs to provide 2.26 units of volume to accommodate the liquid carbon dioxide that expands due to the temperature rise. At room temperature, the volume of liquid carbon dioxide in a sealed container increases sharply with the increase of temperature. If the pressure of liquid carbon dioxide is maintained at 320K and does not exceed 12MPa, the liquid-solid surface height of carbon dioxide in a cylindrical cavity with a height of 300mm at the triple point is no more than 130mm, which does not meet the 200mm solid-liquid height requirement required for reproduction. This is because the sensing coil length of the long-rod standard platinum resistance thermometer is about 50mm. In order to obtain the highest temperature measurement accuracy, the immersion depth in the container is required to reach 150mm to 180mm, that is, the height from the center of the sensing coil of the long-rod standard platinum resistance thermometer to the solid-liquid interface of the triple point is at least higher than 150mm, that is, the bottom of the thermometer well is at least 175mm away from the solid-liquid interface of the triple point. There is a certain distance between the bottom of the thermometer well and the bottom of the container to ensure that the thermometer well is surrounded by the solid-liquid interface of carbon dioxide during reproduction, so the solid-liquid height at the triple point temperature is at least higher than 200mm.

[0005] It can be seen that in the prior art, the safety restrictions of the container, the technical restrictions of the thermometer and the constant temperature environment are contradictory, making it impossible for the conventional carbon dioxide triple point container to meet the above technical requirements at the same time. Summary of the invention

[0006] The object of the present invention is to provide a carbon dioxide triple point container which can simultaneously meet the requirements of immersion depth for calibrating a long-rod standard platinum resistance thermometer, the limitation of the axial dimension of the container and the technical requirements of pressure safety at room temperature.

[0007] The present invention proposes a method for manufacturing a carbon dioxide triple point container with a variable inner diameter, wherein the triple point container comprises a working section, a transition section and a buffer section; the working section is a space occupied by a carbon dioxide medium, and the transition section is arranged between the working section and the buffer section; wherein,

[0008] Step 1: Determine the upper limit of the design pressure of the triple point container based on the physical properties of carbon dioxide;

[0009] Step 2: Determine the diameter and depth of the thermometer well in the triple point container. The depth of the thermometer well in the working section is greater than or equal to 175 mm.

[0010] Step 3, determining the inner diameter and height of the working section of the carbon dioxide triple point container, and calculating the volume of the working section of the carbon dioxide triple point container;

[0011] Step 4: Calculate the mass of carbon dioxide filled into the carbon dioxide triple point container and determine the upper limit of the density of the medium in the carbon dioxide triple point container;

[0012] Step 5. After calculating the lower limit of the volume of the carbon dioxide triple point container, determine the size of the buffer section and transition section of the carbon dioxide triple point container, and calculate the volume of the carbon dioxide triple point container to meet the design requirements of the volume.

[0013] Wherein, the transition section is a truncated cone structure.

[0014] Wherein, the working section and the buffer section are cylindrical structures.

[0015] Wherein, the frustum has a first inclination angle, and the first inclination angle is between 65 degrees and 72 degrees.

[0016] Wherein, the working section is a cylindrical structure with a first diameter; the buffer section is a cylindrical structure with a second diameter; and the second diameter is greater than the first diameter.

[0017] The method of the present invention fully considers the yield stress of the material and the material properties of the medium. The present invention innovatively adopts the concept of providing phase change space in the radial direction. Compared with the design concept of providing phase change space in the axial direction of the cylindrical cavity of the commonly used mercury triple point container, the problem of the limitation of short axial space requirement is solved; the variable inner diameter carbon dioxide triple point container can simultaneously meet the immersion depth requirements of the calibration long rod standard platinum resistance thermometer, the axial dimension limit of the container and the pressure safety technical requirements at room temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic structural diagram of a carbon dioxide triple point container with a variable inner diameter according to the present invention. DETAILED DESCRIPTION

[0019] In order to facilitate the understanding of the present invention, embodiments of the present invention are described below in conjunction with the accompanying drawings. Those skilled in the art should understand that the following description is only for the convenience of explaining the invention and is not intended to be a specific limitation on the scope of the invention.

[0020] In order to meet the metrological test requirements of the immersion depth of the thermometer between 150mm-180mm, that is, the length from the center point of the temperature sensing coil of the long rod standard platinum resistance thermometer to the upper surface of the liquid fixed point medium.

[0021] The present invention designs a triple point container with a cylindrical structure and a variable inner diameter, and the triple point container includes a working section 1, a transition section 2 and a buffer section 3. The working section 1 refers to the space occupied by the medium during the phase change of the medium, and the working section 1 is a cylindrical structure with a first diameter; the buffer section 3 is a cylindrical structure with a second diameter; the transition section 2 is arranged between the working section 1 and the buffer section 3, and preferably the transition section 2 is a truncated cone structure, and the transition section 2 connects the cylindrical structure of the first diameter and the cylindrical structure of the second diameter, wherein the second diameter is greater than the first diameter. The cylinder of the working section 1 is connected to the cylindrical structure of the buffer section 3 through a truncated cone, so that, through the connection of the transition section 2, a more sufficient radial space is provided in the buffer section 3 to provide expansion space for the medium. Compared with the conventional carbon dioxide triple point cylindrical container, providing expansion space for the medium from the axial direction has obvious advantages.

[0022] like Figure 1The structure diagram of the variable inner diameter carbon dioxide triple point container of the present application is shown. The carbon dioxide triple point container is a closed pressure-resistant container. The upper limit of the design pressure of the container is determined according to the physical properties of carbon dioxide. At room temperature of 25°C, the saturated vapor pressure of carbon dioxide is about 6MPa. Twice this pressure is taken as the upper limit of the design pressure, that is, 12MPa. This value can be modified as needed. Preferably, the pressure resistance of the carbon dioxide triple point container is 12MPa, and high-pressure carbon dioxide is loaded to reproduce the triple point temperature and calibrate the long rod or sleeve type platinum resistance thermometer. The carbon dioxide triple point container of the present invention adopts a closed cavity with an overall non-uniform size. The buffer section 3 of the carbon dioxide triple point container has a first height H1, and preferably the first height H1 accounts for 1 / 6 of the overall height of the triple point container; the working section 1 of the triple point container has a second height H2, and preferably the second height H2 accounts for 2 / 3 of the overall height of the triple point container. The buffer section 3 and the working section 1 are both cylindrical in design, wherein the inner diameter of the buffer section 3 of the triple point container is twice the inner diameter of the working section 1 of the container; the working section 1 and the buffer section 3 are connected by a truncated cone, and the truncated cone has a first inclination angle, and the first inclination angle is between 65 degrees and 72 degrees, and preferably the inclination angle is 65 degrees, 66 degrees, 68 degrees, 68.2 degrees, 69 degrees, 70 degrees, 71 degrees or 72 degrees.

[0023] Preferably, the second height H2 of the cylinder in the working section 1 is greater than 200 mm, providing a sufficiently high solid-liquid phase change interface to meet the immersion depth requirement. A blind tube 4 with a length greater than 200 mm and a diameter of 12 mm is provided at the axis of the container. The blind tube 4 is used as a thermometer well, which is used to calibrate long-rod platinum resistance thermometers and sleeve-type platinum resistance thermometers. A first metal tube 5 and a second metal tube 6 are provided at the top of the triple point container. The first metal tube 5 is used to connect a pressure gauge, and the second metal tube 6 is used to connect a valve for exhaust or filling.

[0024] The cylinder of the buffer section 3 of the carbon dioxide triple point container has a first radius, preferably the first radius is 40 mm, and the first height H1 is preferably 50 mm; the cylinder of the working section 1 has a second radius, preferably the second radius is 20 mm, and the second height H2 is preferably 200 mm; the two cylinders are transitioned through a frustum, and the frustum has a third height, preferably the third height H3 is 50 mm, the upper end surface of the frustum has a first radius, and the lower end surface of the frustum has a second radius.

[0025] The carbon dioxide triple point container of the present invention is preferably made of 316L stainless steel, which has good acid corrosion resistance. The upper end cover, the lower end cover, the large cylinder as the buffer section 3, the truncated cone as the transition section 2, the small cylinder as the working section 1, and the thermometer well and other components can be processed in sections and then welded; then the above components are fixedly connected; as a further preference, the rod can also be milled as a whole, and then the upper end cover is fixedly connected. The inner surface of the cavity of the carbon dioxide triple point container and the outer surface of the thermometer well are smooth surfaces, and the smooth surface can reduce the adhesion of gas impurities. Preferably, the surface roughness of the surface is less than 0.8μm. In order to remove impurities such as surface oil, appropriate baking temperature and duration are used to remove water, which is a very necessary auxiliary process.

[0026] The carbon dioxide triple point container of the present invention can be reproduced using the existing thermostatic equipment for mercury triple point reproduction, which can save the cost of purchasing new equipment. The carbon dioxide triple point container of the present invention has a height similar to that of the conventional mercury triple point container, and has a longitudinal axisymmetric structure as a whole, which can be reproduced by continuous heating. The laboratory can use the original thermostatic bath to reproduce the fixed point without having to replace the thermostatic bath with a larger temperature equalization depth, which can save equipment purchase funds for the institution using the present invention.

[0027] When the carbon dioxide triple point container of the present invention is designed, the following steps are adopted:

[0028] Step 1: Determine the upper limit of the design pressure of the container based on the physical properties of carbon dioxide. At room temperature of 25°C, the saturated vapor pressure of carbon dioxide is about 6MPa. Take twice this pressure as the upper limit of the design pressure, that is, 12MPa. This value can be modified as needed.

[0029] Step 2: Determine the diameter and depth of the thermometer well in the triple point container. First, determine the inner and outer diameters of the thermometer well. The inner diameter can be designed to be compatible with the calibration sleeve-type platinum resistance thermometer and the long-rod platinum resistance thermometer, that is, an inner diameter of 12 mm; if the design pressure upper limit of 12 MPa is adopted and the thermometer well is made of 316L stainless steel, calculate the pressure bearing capacity of the thermometer well. It is recommended to calculate the pressure bearing capacity of the thermometer well according to the requirements of the national standard "Pressure Vessel" (GB150-2011). If it is safe under this wall thickness, the wall thickness can be set to 3 mm, and the outer diameter of the thermometer well is 18 mm. Other materials or pressures need to be calculated; secondly, determine the depth of the thermometer well. The length of the temperature sensing coil of a general long-rod standard platinum resistance thermometer is about 50 mm, the half length is 25 mm, and the immersion depth is taken as the lower limit of 150 mm. The depth of the working section thermometer well should be greater than or equal to 175 mm.

[0030] Step 3, determine the inner diameter and height of the working section of the carbon dioxide triple point container. Preferably, the inner diameter of the carbon dioxide triple point container is 40 mm, and other sizes can also be used as needed. On the premise of ensuring that the distance from the outer wall of the blind end of the thermometer well to the inner wall of the bottom of the triple point container is greater than the distance from the outer wall of the thermometer well to the inner wall of the triple point container, the recommended working section height of the container is 200 mm (the depth of the thermometer well is 175 mm), and other sizes can also be used as needed.

[0031] Step 4: Calculate the volume of the working section of the carbon dioxide triple point container. First, calculate the volume of the working section cylinder. The working section is calculated as a cylinder with a diameter of 40 mm (d1) and a height of 200 mm (h1). The volume is v1 = πh1d1 2 / 4=251327mm 3 ; Secondly, calculate the volume of the thermometer well in the working section. The volume is calculated as a cylinder with a diameter of 18 mm (d2) and a height of 178 mm (h2). The volume is v2 = πh2d2 2 / 4=45295mm 3 ; Working section volume v = v1-v2 = 206032mm 3 .

[0032] Step 5: Calculate the mass of carbon dioxide filled into the carbon dioxide triple point container. Take the density of solid carbon dioxide at the triple point temperature as ρ = 1530 kg / m 3 , then the mass of solid carbon dioxide at the triple point temperature filled in the working section is m=ρv=0.3152kg.

[0033] Step 6: Determine the upper density limit ρ0 of the medium in the carbon dioxide triple point container. According to the state equation of Span1996, when the storage upper limit temperature is 320K and the design pressure is 12MPa, the upper limit of carbon dioxide density is 632kg / m 3 , the upper limit of carbon dioxide density corresponding to 330K and 12MPa is 476kg / m 3 Therefore, the design density of the container should be less than the upper limit of density under the design temperature and pressure. The recommended density is ρ0 = 550 kg / m 3

[0034] Step 7: Calculate the lower limit v' of the volume of the carbon dioxide triple point container cell According to the design density and filling mass, v' can be obtained cell v cell =m / ρ0=573090mm 3 . The designed container volume needs to be larger than this value.

[0035] Step 8: Determine the size of the buffer section (non-working section) of the carbon dioxide triple point container. First, design and calculate the volume of the buffer section cylinder. The buffer section is calculated as a cylinder with a diameter of 80 mm (d3) and a height of 50 mm (h3). After deducting the volume of the thermometer well, the volume is v3 = πh3d3 2 / 4-πh3d2 2 / 4=251327-12723=238604mm 3 Secondly, design and calculate the volume of the transition section. Assuming that the height of the transition section is 50 mm (h4), the volume of the truncated cone with end diameters of 40 mm and 80 mm respectively minus the volume of the thermometer well is v4 = πh4 (d3 2 +d3 d1+d1 2 ) / 12-πh4d2 2 / 4=146608-12723=133885mm 3 .

[0036] Step 9. Calculate the volume v of the carbon dioxide triple point container cell .v cell =v+v3+v4=206032+238604+133885=578521mm 3 >v' cell Therefore, the design is feasible. If v cell <v’ cell , the height h3 of the buffer section cylinder can be modified to meet the design requirements of the volume.

[0037] Step 10. Carbon dioxide triple point container Calculate the wall thickness of the triple point container. It is recommended to calculate the side wall thickness and end face thickness of the triple point container according to the requirements of the national standard "Pressure Vessel" (GB150-2011). If the design pressure is 12MPa and the triple point container is made of 316L stainless steel, the side wall thickness can be 5mm and the end face thickness can be 15mm.

[0038] The phase change interface height of the present invention is about 200 mm, which makes the immersion depth of the thermometer ( Figure 1 h) is greater than 150 mm, ensuring a longer temperature range for the long-rod platinum resistance thermometer, reducing the influence of heat conduction, and improving the accuracy of calibration. If a 300 mm high cylindrical design is adopted, the phase change interface height is only 130 mm. Under the same axial height and the same medium density, the solid-liquid interface height of the present invention is 70 mm higher than that of the cylindrical design, an increase of 54%.

[0039] When the same mass of solid carbon dioxide melts into liquid phase, the liquid level rises, and the pressure at the temperature measuring point rises. The pressure increase brings about the change of solid-liquid phase transition temperature at the temperature measuring point, which directly affects the calibration level. This effect is called static pressure effect. The static pressure effect introduced by the present invention is smaller than that introduced by the conventional cylindrical design, and the temperature measurement performance of the triple point is improved. Figure 1 According to the calculation of 200mm in H3, when all liquid phases are transformed into liquid phase at 216K, the liquid level height of the present invention is 230mm, which is only increased by 30mm, and the temperature rise is about 0.08mK; while with the conventional cylindrical design, the full liquid phase height is 260mm, which is increased by 60mm, and the temperature rise is about 0.16mK. It can be seen that the present invention greatly reduces the static pressure effect of the liquid level rise, improves the flatness of the temperature plateau, and improves the uncertainty level of the triple point.

[0040] The present invention fills a container with high-purity carbon dioxide gas, which can achieve a high level of fixed point reproduction, a long duration of the triple point melting temperature plateau, and good temperature plateau reproducibility. When the purity of carbon dioxide gas reaches 99.9999%, the temperature of the thermostatic bath is 15mK higher than the carbon dioxide temperature plateau, and the triple point temperature plateau of the present invention can last for more than 7 days. Using the internal melting technology of F=0.1, the temperature plateau variation range within 7 days is within 0.6mK, and the temperature plateau temperature variation range of the melting coefficient F=0.3-0.7 is less than 0.2mK. The temperature repeatability of the five-repeated melting coefficient F=0.5 can reach within 0.05mK. The phase change temperature measured by the present invention is consistent with the international recommended value within the uncertainty range.

[0041] The present invention innovatively adopts the concept of providing phase change space radially. Compared with the design concept of providing phase change space axially in the cylindrical cavity of the commonly used mercury triple point container, the problem of short axial space limitation is solved. The variable inner diameter carbon dioxide triple point container can simultaneously meet the immersion depth requirements for calibrating long-rod standard platinum resistance thermometers, the axial dimension limitations of the container, and the pressure safety technical requirements at room temperature.

[0042] It is to be understood that, although the present invention has been disclosed as a preferred embodiment, the above embodiment is not intended to limit the present invention. For any person skilled in the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or modified into equivalent embodiments of equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of protection of the technical solution of the present invention.

Claims

1. A method for manufacturing a carbon dioxide triple point container with a variable inner diameter, the triple point container comprising a working section, a transition section and a buffer section; the working section is a space occupied by a carbon dioxide medium, the transition section is arranged between the working section and the buffer section; the transition section is a truncated cone structure; the working section is a cylindrical structure with a first diameter; the buffer section is a cylindrical structure with a second diameter; the second diameter is greater than the first diameter; characterized in that: Step 1: Determine the upper limit of the design pressure of the triple point container based on the physical properties of carbon dioxide; Step 2: Determine the diameter and depth of the thermometer well in the triple point container. The depth of the thermometer well in the working section is greater than or equal to 175 mm. Step 3, determining the inner diameter and height of the working section of the carbon dioxide triple point container, and calculating the volume of the working section of the carbon dioxide triple point container; Step 4: Calculate the mass of carbon dioxide filled into the carbon dioxide triple point container and determine the upper limit of the density of the medium in the carbon dioxide triple point container; Step 5. After calculating the lower limit of the volume of the carbon dioxide triple point container, determine the size of the buffer section and transition section of the carbon dioxide triple point container, and calculate the volume of the carbon dioxide triple point container to meet the design requirements of the volume.

2. The method for manufacturing a carbon dioxide triple point container with a variable inner diameter as claimed in claim 1, characterized in that: The truncated cone has a first inclination angle, and the first inclination angle is between 65 degrees and 72 degrees.

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