Development and calibration of high precision pressure sensors for extreme environments

A pressure sensor which can be used in high temperature, high pressure and strong corrosive environment is designed to meet the needs of scientific research in extreme environment. The metal with high yield strength, high temperature and high pressure resistance, corrosion resistance and high yield strength are used as the sensor material, and the combined structure and special circular diaphragm patch and bridge technology are adopted to develop the pressure sensor suitable for extreme environment work. The multi-dimensional linear interpolation algorithm is applied to calibrate the sensor to improve the detection accuracy. The experimental test results show that the pressure sensor is at 60MP It can work normally in acidic or alkaline solutions under a, 400C environment, and the detection precision can reach 0.%FS to meet the needs of high-precision pressure detection in extreme environments.

The discovery of modern submarine hydrothermal system is of great significance to a series of disciplines such as geology, geochemistry and life science, and has aroused strong interest of scientists. Related research has produced many new discoveries and challenged traditional theories [24 Field detection and experimental simulation of hydrothermal system are conducted. It is a necessary method for the study of biological mineralization, origin of life and other scientific issues. However, due to its particularity in the extreme environment of the deep sea, extremely high requirements are put forward for detection technology. The particularity of the extreme environment of the hydrothermal system is mainly manifested as [1.56]. The depth may be more than 6 000 m. Second, the hydrothermal corrosion is strong; third, there are microorganisms growing around the hydrothermal solution, all of which bring great difficulties to the detection. The distribution of biological resources is closely related to the vertical depth [7-0] Accurate depth measurement is a prerequisite for the research and simulation test of life science geochemistry and is one of the important guarantee technologies for scientific exploration.

The self-contained high-precision CTD agent level meter developed in China cannot meet the special requirements of extreme environment with its maximum working depth of 3000m. The silicon pressure sensors currently used in the market are mainly diffused silicon pressure sensors. When the working temperature exceeds 120C, the leakage between the strain resistance and the bottom of the village is intensified. The maximum detection pressure of polycrystalline silicon pressure sensor, monocrystal silicon pressure sensor and silicon on sapphire pressure sensor developed by new technology and new materials cannot reach 60MPa, and the silicon material sensor is very easy to corrod under alkaline environment. Not suitable for testing in extreme environments Common metal materials or alloy sensors may dissolve into monomers in hydrothermal systems and have a toxic effect on microorganisms. 10-13]

This paper introduces a kind of high-precision pressure sensor that can corrod acid and alkali under high temperature and high pressure environment. It adopts combined structure and uses industrial pure iron as sensor processing material. Iron metal has stable physical and chemical properties, high temperature resistance, corrosion resistance, high yield strength, high pressure resistance, and no biological toxicity. The sensor adopts multi-dimension calibration method to compensate the drift error caused by temperature change and improve the detection accuracy.

1. Sensor design

1.1 Material selection

The particularity of the extreme environment of the hydrothermal system puts forward extremely high requirements for the strength of the sensor material, so the primary difficulty of the sensor development and design is the selection of the material, which must meet the following conditions: resistance to high temperature and high pressure; @ Anticorrosion 3 has high detection accuracy; @ Testing is not dirty and not natural.

The metal material TC4 selected in this paper as the processing material of the sensor has the following advantages: 1. It has stable physical and chemical properties at high temperature and can withstand high temperature, and it has high yield strength and tensile strength and can withstand high pressure through experiments; @ Chemical properties are very stable, can resist acid and alkali corrosion: has a good elastic modulus, to meet the requirements of high-precision detection; With biological inert, no biological toxicity, no pollution to the detection environment; Therefore, the material can fully meet the harsh requirements of extreme environments, and is the best choice for pressure detection sensor materials in extreme environments.

1.2 Structural Design

The sensor adopts a combined structure (as shown in Figure 1) and the front end connector adopts internal thread to butt with the pipeline of the measured flow body to ensure safe and reliable connection under high pressure; Sealing ring prevents high pressure liquid overflow at bayonet joint; The elastic element of the pressure sensor is a circular diaphragm fixed around the periphery, which is affixed to the metal base to avoid direct contact with the hot liquid; The conductor plug is fixed by the flange structure to avoid the failure of the sensor caused by the force of the conductor falling off.

1.3 Stress analysis of circular diaphragm

When the circular flat diaphragm is subjected to uniform pressure under the condition of peripheral support, if the deflection is much less than the thickness of the diaphragm, it can be considered to meet the small deflection theory, which can be obtained from the small deflection theory of elastic thin plate 5].

The circular flat diaphragm is subject to uniform pressure, and the radius of the diaphragm is set as a and the thickness is t. The two straight stresses on the unit A are the radius stress and the tangential stress o of the half square straight respectively at any point from the center of the circle 0. The distribution diagram is shown in Figure 2. The maximum pressure 9 can reach 60 MPa. owx=358MPa and oro=-551MPa can be obtained by calculating the radius of the sensor's circular diaphragm a=7mm and the thickness t=2mm. It can be seen from the experimental detection of iron and metal materials that Poisson's ratio v=03, tensile elastic limit R =710MPa, and yield strength Rpo-920 MPa meet the requirements of extreme environment detection.

Figure 2. Circular diaphragm force diagram

1.4 Strain of circular diaphragm and bridge construction technology

According to strain distribution calculation formula 15] :

FIG. 3. Technology of wafer strain gauge mounting and bridge assembly

2. Sensor calibration

The concentrations of free and decomposed amino acids and other nutrients were distributed with the star index of seawater depth. In the thermocline, the relationship between sea water temperature and depth is also approximately exponential. Therefore, the accuracy of pressure will have an important impact on the results of other Marine scientific experiments. The extreme environmental pressure sensor developed in this paper has a wide range of operating environment temperature changes, while the performance of iron metal materials and circular film strain gauges will vary slightly at high temperatures, resulting in measurement errors. In this paper, a multi-dimensional piecewise linear interpolation algorithm is adopted to improve the accuracy [16].

Under different set temperatures of hydrothermal solution, this paper first obtained the original output voltage by calibrating the specific standard pressure, and the original output voltage dot plot was obtained.

The original lattice data is discontinuous, and the output voltage measured outside the standard pressure cannot be calibrated. In this paper, piecewise linear interpolation algorithm is used to obtain the corresponding relationship between different output voltages and detection pressures under the same temperature condition, as shown in Figure 5.

As can be seen from the linear relationship between the output voltage and the detected pressure obtained after a linear interpolation, the corresponding relationship between the output voltage and the detected force is still incomplete, and only a limited number of voltage values measured on the known temperature curves can be calculated to obtain the detected pressure value. Therefore, it is necessary to carry out piecewise linear interpolation again to obtain the three-dimensional relationship diagram between temperature, measured voltage and detected pressure (as shown in Figure 6). After the three-dimensional relationship diagram is established, as long as the temperature and output voltage values are obtained, the measured target pressure value can be obtained according to the diagram.

3. Detection accuracy

According to the test conducted by the National Second-class Metrology Test Center of Guangdong Academy of Metrology, * when the ambient temperature is 25C, the error between the measured pressure value and the standard value is less than 02%FS, as shown in Table 1.) Laboratory test shows that under other different temperature conditions, the error between the measured pressure value and the standard value is less than 0.2%FS.

Table 1 Comparison table between standard pressure value and measured pressure value

To test means to note the number; RZD20044455

4. Summary and prospect

This paper introduces a pressure sensor suitable for extreme environment. The sensor takes iron metal as raw material, can withstand high temperature and high pressure, and can detect pressure up to 60 MPa under 400C hydrothermal environment. Besides, it has strong acid and alkali corrosion resistance, no biological toxicity, durability, and no pollution in the detection environment. Taxis can reach full capacity by 0.2%. The successful development of the sensor provides technical support for major scientific research such as the origin of biological mineralization in extreme environments, but its detection technology still has defects, that is, it must be jointly tested and calibrated with temperature detection instruments to ensure accuracy to achieve independent pressure detection high-precision pressure sensor is the future research and development direction.

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