The aim of the doctoral dissertation is to perform temperature measurements with acoustic parameters. Primary thermometers based on the principle of measuring the speed of sound in gases in an acoustic resonator are one of the most accurate thermometers and have been used for the new definition of thermodynamic temperature based on the Boltzmann constant. However, they have the disadvantage that they are large, cumbersome and sensitive devices. The doctoral dissertation describes the production of a practical version of an acoustic thermometer, which is suitable for industrial measurements in the range between -100 °C and 300 °C. Industrial temperature measurements are usually performed with standardized PT100 resistance thermometers or with thermocouples. Despite their prevalence, such sensors are usually susceptible to interference. They are particularly sensitive to drift, resulting from exposure to harsh environments. These are environments where high temperature, humidity or radiation is present. For this reason, we investigated the potential of using acoustic thermometry with an acoustic waveguide for measurements in industrial environments.
The difference between a primary and a secondary thermometer is that the secondary thermometer needs to be calibrated with another thermometer or a fixed point. The secondary acoustic thermometer (the term practical acoustic thermometer, PAT also appears in the English literature) has some major advantages over other methods of temperature measurement. These advantages are: calibration at one temperature point and traceability over a wide temperature range, from the boiling point of the used gas to the melting point of the housing or when the temperature causes excessive mechanical changes in the thermometer. At temperatures close to the boiling point of the gas, it is necessary to maintain the pressure in the thermometer, as low gas pressure causes too much attenuation of the sound waves. At higher pressures it is not necessary to maintain a specific pressure, but it is still necessary to measure it. Acoustic thermometer is resistant to electrical and magnetic interferences. Due to its measuring principle and easy replacement of the measuring medium (gas in the housing) it is also suitable for use in the nuclear industry where other types of conventional thermometers need to be changed frequently due to ionizing radiation. Another good feature is that it can measure the average temperature in a large volume, which is suitable for chemical reactors. It is also possible to make such a thermometer in the device where measurements will be performed, for example in an internal combustion engine. Adding only an airtight tube at one end where the necessary electronics is installed after production is finished. The main goal of the doctoral dissertation was to develop a secondary acoustic thermometer suitable for measurements in an industrial environment with a temperature range from -100 ℃ to 300 ℃, with an extended measurement uncertainty of less than 1 ℃ and its calibration and testing. In order for the developed thermometer to be usable in an industrial environment, the following properties must be ensured: resistance to external factors, such as: vibrations, magnetic and electric fields; reliability, accuracy, repeatability, reproducibility and ease of use. Other interfering factors, such as noise and ambient temperature, must be compensated for or resistance ensured by a suitable thermometer design. In terms of reliability, the greatest emphasis is on testing the conditions under which the developed thermometer will display the correct temperature. The accuracy and uncertainty of the measurement must be checked on the developed thermometer to determine how large the individual contributions to the total uncertainty are and if the thermometer is particularly sensitive to any interfering factor. Ease of use is also a good feature, because it is important that the thermometer does not require constant maintenance and additional equipment to operate.
The first chapter describes metrology, which is a science that covers all aspects of measurements. The metrology can be divided into scientific metrology, legal metrology and industrial. The International System of Units SI defines seven basic units or constants and their definitions. The thermodynamic temperature is presented in more detail. The definition of the unit and the possibility of its realization in the laboratory are presented in more details. Four laws of thermodynamics are then presented. The temperature scale from 1990 (ITS-90) is also presented, in which are the agreed temperatures of certain points (triple points, boiling points, solidification points) of pure materials. This scale is important for the primary realization of temperature in laboratories. Calibration and traceability are then described. Calibration is a procedure where the connection between the values of the quantity and the measurement uncertainty given by the standard and the value given by the measuring system is established. Traceability is a property of a measurement result that connects a continuous chain of calibrations, each of which contributes to measurement uncertainty. At the end of the chapter, the division of temperature measurements into primary and secondary measurements is presented. Primary and secondary thermometers are described. Among the primary thermometers are: classic gas thermometer, noise thermometer, integrated radiation thermometer and monochromatic radiation thermometer. Of the secondary thermometers, a resistance thermometer, thermocouple and liquid thermometer are presented. For each thermometer is described, a basic physical model and a brief description of the principle of operation, their disadvantages and the applicable temperature ranges.
The second chapter describes the acoustic thermometer in more detail, both the primary and secondary version, which is also at the focus of this doctoral dissertation. First, the current versions of the primary and secondary acoustic thermometer with their advantages and disadvantages are described. Greater emphasis is placed on the secondary acoustic thermometer, which is the subject of this doctoral dissertation. Then, the physical background of sound transmission in different matter phases is presented. Since the theory of sound velocities in gases does not match the exact measured velocities of sound, the determination of the velocity of sound in real gases is also presented. The use of a sound waveguide tube also has a great influence on the speed of sound. Thus, the calculation of the propagation constant of sound travelling in a tube is presented. For accurate temperature measurements, accurate data on the speed of sound is needed. In PAT, argon is used for the measuring gas. For argon, speed of sound is calculated by the equation of state. Finally, the equations for calculating the transport parameters for argon, which are also needed to calculate the propagation constant of sound in tubes, are presented. This chapter concludes the description of the theory of operation. The following chapters describe the performed work.
The third chapter describes the options for selecting the individual parts required for the operation of the thermometer. The selection starts with different tube designs, where the properties of different tube layouts are described, as well as the possibility of using specific algorithms to calculate the speed of sound in each design. Next, the problem of acoustic signal overlap, and thus the deterioration of the sound speed measurement, is described. Two possible solutions are given, the first is by eliminating the unwanted acoustic signal that causes the overlap and the second option is to select the appropriate lengths of the acoustic waveguide to prevent signal overlap on a certain temperature range. The second described problem and its solution is the mixing of gases in the thermometer housing. This problem is solved by vacuuming the housing and filling it with clean gas. The implementation of the pressure measurements needed to calculate the speed of sound is also described here. A comparison of algorithms for calculating the speed of sound with qualitatively determined properties of each algorithm is presented next. The next major choice is the choice of speakers and microphones. Different types of acoustic transducers used in tubes are described. The biggest limitation present is the dimension (outer diameter) of the tube, which is less than 10 mm. The last major choice is the choice of excitation signals and algorithms to determine the speed of sound. Different combinations of signals and their properties in connection with the used algorithms are presented.
The forth chapter describes the developed acoustic thermometer and in its making. Four parts are presented: mechanical part, acoustic part, electrical part, software part and algorithms. In the case of a mechanical part, the thermal expansion of the used steel is presented, followed by a description of the measuring part of the developed thermometer and its housing with all connections. For the acoustic part, an acoustic waveguide and a special attachment for connecting the microphone, speaker and acoustic waveguide are described. The electrical part describes all the used amplifiers and their characteristics, as well as the used ADC and DAC system (sound card). The algorithms describe: signal generation, signal transmission and reception, and signal processing. Signal processing includes signal filtering, acoustic signal delay calculation and finally temperature conversion.
The last, fifth chapter describes all the measurements of the developed thermometer and the results. Various interfering influences and metrological characteristics of the thermometer were tested. Measured metrological characteristics are: sensitivity, resolution, repeatability, reproducibility, linearity, hysteresis and uncertainty. Calibration in a mixture of ice and water is also shown.
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