SEKUNDARNI AKUSTIČNI TERMOMETER Z VALOVODOM

TAVČAR, ROK (avtor), Beguš, Samo (mentor), Bojkovski, Jovan (komentor)

Cilj doktorske disertacije je izvedba meritev temperature z akustičnimi parametri. Primarni termometri, ki temeljijo na principu merjenja hitrosti zvoka v plinih v akustičnem resonatorju, so eden izmed najbolj točnih termometrov in so bili uporabljeni za novo definicijo enote za termodinamično temperaturo kelvin, zasnovano na Boltzmannovi konstanti. Vendar imajo to pomanjkljivost, da gre za velike, okorne in občutljive naprave. V doktorski disertaciji je opisana izdelava praktične izvedbe akustičnega termometra, ki je primeren za industrijske meritve v območju med –100 in 300 °C. Industrijske meritve temperature se običajno izvajajo s standardiziranimi uporovnimi termometri PT100 ali s termočleni. Kljub njihovi razširjenosti so taki senzorji običajno občutljivi na motnje. Še posebno občutljivi so na lezenje, ki je posledica izpostavljenim zahtevnim okoljem. To so okolja, kjer so prisotni visoka temperatura, vlažnost ali sevanje … Zaradi tega smo raziskali potencial uporabe akustične termometrije z akustičnim valovodom za meritve v industrijskih okoljih. Razlika med primarnim in sekundarnim termometrom je v tem, da je treba sekundarni termometer umeriti z drugim termometrom ali fiksno točko. Sekundarni akustični termometer (v angleški literaturi se pojavlja tudi izraz praktični akustični termometer, PAT) ima pred ostalimi načini merjenja temperature nekaj večjih prednosti: umerjanje v eni temperaturni točki in sledljiva uporaba na širokem temperaturnem območju, od vrelišča uporabljenega plina do tališča ohišja oziroma meje, kjer temperatura povzroči prevelike mehanske spremembe termometra. Pri temperaturah blizu vrelišča plina je treba v termometru vzdrževati tlak, ker pri nizkem tlaku prihaja do prevelikega dušenja zvočnega valovanja. Pri višjih tlakih ni potrebno vzdrževanje točno določenega tlaka, vendar ga je vseeno treba meriti. Odporen je na električne in magnetne motnje, zaradi svojega merilnega principa in enostavne zamenjave merilnega medija (plin v ohišju) je tudi primeren za uporabo v jedrski industriji, kjer je treba zaradi ionizirajočega sevanja ostale vrste klasičnih termometrov pogosto menjavati. Druga dobra lastnost je, da lahko meri povprečno temperaturo v velikem volumnu, kar je primerno za kemijske reaktorje, prav tako pa je mogoče tak termometer vgraditi v napravo, kjer se bodo izvajale meritve, recimo v motor z notranjim izgorevanjem, saj je treba predhodno dodati samo zrakotesno cev, na koncu katere namestimo potrebno elektroniko po končani izdelavi. Glavni cilj doktorske disertacije je bila izdelava sekundarnega akustičnega termometra, ki je primeren za meritve v industrijskem okolju s temperaturnim območjem od –100 do 300 °C, z razširjeno merilno negotovostjo manjšo od 1 ℃, njegovo umerjanje in preizkušanje. Da je izdelan termometer uporaben v industrijskem okolju, je treba zagotoviti naslednje lastnosti: odpornost na zunanje dejavnike, kot so tresljaji, magnetno in električno polje; zanesljivost, točnost, ponovljivost, obnovljivost in enostavna uporaba. Druge motilne dejavnike, na primer hrup in okoliško temperaturo, pa je treba kompenzirati ali zagotoviti odpornost s primerno izvedbo termometra. Pri zanesljivosti je največji poudarek na preizkušanju, pod kakšnimi pogoji bo izdelan termometer prikazal pravilno temperaturo. Točnost in negotovost meritve je treba preveriti na izdelanem termometru, da ugotovimo, kako veliki so posamezni prispevki k skupni negotovosti in če je termometer še posebno občutljiv na kakšen motilni dejavnik. Dobra lastnost je tudi enostavna uporaba, ker je pomembno, da termometer ne potrebuje stalnega vzdrževanja in dodatne opreme za delovanje. V prvem poglavju je opisano meroslovje, ki je znanstvena veda in zajema vsa merjenja. Predstavljena je razdelitev meroslovja na znanstveno meroslovje, zakonsko meroslovje in industrijsko meroslovje. Mednarodni sistem enot SI definira sedem osnovnih enot oziroma konstant in njihovih definicij. Bolj podrobno je predstavljena termodinamska temperatura. Kjer je predstavljena definicija enote in možnost njene realizacije v laboratoriju. Nato so predstavljeni štirje zakoni termodinamike. Prvi zakon je razširjena različica zakona o ohranitvi energije, ki vsebuje tudi toplotno energijo. Drugi zakon pojasnjuje neobrnljivost naravnih procesov in definira količino, imenovano entropija. Tretji zakon poda limito entropije, ko se sistem približuje temperaturi absolutne ničle. Tako imenovani ničti zakon termodinamike definira temperaturo. Na koncu poglavja je predstavljena temperaturna lestvica iz leta 1990 (ITS-90), v kateri so dogovorjene temperature določenih točk (trojne točke, vrelišča, strdišča) čistih materialov. Ta lestvica je pomembna za primarno realizacijo temperature v laboratorijih. Nato sta opisana umerjanje in sledljivost. Umerjanje je postopek, kjer se ugotavlja povezava med vrednostmi veličine in merilno negotovostjo, ki jo daje etalon, ter vrednostjo, ki jo daje merilni sistem. Sledljivost je lastnost merilnega rezultata, ki povezuje neprekinjeno verigo umeritev, od katerih vsaka prispeva k merilni negotovosti. Na koncu poglavja je predstavljena razdelitev merjenj temperature na primarna in sekundarna merjenja. Opisani so primarni in sekundarni termometri. Med primarnimi termometri so tako predstavljeni: klasični plinski termometer, šumni termometer, termometer celostnega sevanja in monokromatski sevalni termometer. Od sekundarnih termometrov so predstavljeni uporovni termometer, termočlen in tekočinski termometer. Za vsak opisan termometer so podani osnovni fizikalni model in kratek opis principa delovanja, njihove slabosti in uporabno temperaturno področje. V drugem poglavju je bolj podrobno opisan akustični termometer, tako primarna kot sekundarna različica, ki je tudi v središču te doktorske disertacije. Najprej sta opisani trenutno najnovejši izvedbi primarnega in sekundarnega akustičnega termometra z njunimi dobrimi in slabimi lastnostmi. Večji poudarek je na sekundarnem akustičnem termometru, ki je predmet te doktorske disertacije. Nato je predstavljeno fizikalno ozadje prenosa zvoka v različnih faznih stanjih snovi. Ker se teorija o hitrostih zvoka v plinih ne ujema z izmerjenimi hitrostmi zvoka, je predstavljena tudi določitev hitrosti zvoka v realnih plinih. Velik vpliv na hitrost zvoka ima tudi uporaba cevi za zvočni valovod. Tako je predstavljen izračun konstante širjenja potovanja zvoka v cevi. Za točne meritve temperature potrebujemo točne podatke o hitrosti zvoka. V PAT je za merilni plin uporabljen argon. Za argon to hitrost izračunamo z enačbo stanja. Na koncu so predstavljene še enačbe za izračun transportnih parametrov za argon, ki jih potrebujemo za izračun konstante širjenja zvoka v ceveh. S tem poglavjem je končan opis teorije delovanja. V naslednjih poglavjih sledi opis izvedenih del. V tretjem poglavju so opisane možnosti izbire posameznih sklopov, potrebnih za delovanje termometra. Izbira se začne pri različnih postavitvah cevi, kjer so opisane lastnosti različnih postavitev cevi in možnost uporabe posameznih algoritmov za izračun hitrosti zvoka. Nato je opisan problem prekrivanja akustičnega signala in s tem poslabšanja meritve hitrosti zvoka. Podani sta dve možni rešitvi, prva je z izničenjem neželenega akustičnega signala, ki povzroči prekrivanje, druga predstavljena možnost pa je izbira ustreznih dimenzij akustičnega valovoda, ki preprečijo prekrivanje signalov na določenem temperaturnem področju. Drugi opisan problem in njegova rešitev je mešanje plinov v ohišju termometra. Ta problem rešimo z vakuumiranjem ohišja in polnjenjem s čistim plinom. Tukaj je tudi opisana izvedba meritev tlaka, ki jih potrebujemo za izračun hitrosti zvoka. Sledi primerjava algoritmov za izračun hitrosti zvoka s kvalitativno določenimi lastnostmi posameznega algoritma. Naslednja večja izbira je izbira zvočnikov in mikrofonov. Opisani so različni tipi akustičnih pretvornikov, ki so uporabni v ceveh. Največja prisotna omejitev so ravno dimenzije (zunanji premer) cevi, ki so manjše od 10 mm. Zadnja večja izbira je izbira vzbujevalnih signalov in algoritmov za določitev hitrosti zvoka. Predstavljene so različne kombinacije signalov in njihove lastnosti v povezavi s primernostjo uporabe predstavljenih algoritmov. V četrtem poglavju je opisan izdelan akustični termometer, nekaj besed je tudi namenjenih izdelavi termometra. Predstavljeni so štirje sklopi: mehanski sklop, akustični sklop, električni sklop in programski sklop ter algoritmi. Pri mehanskem sklopu je najprej predstavljeno temperaturno raztezanje uporabljenega jekla. Sledi opis merilnega dela izdelanega termometra in ohišja z vsemi priključki. Pri akustičnem sklopu sta opisana akustični valovod in posebni nastavek, ki povezuje mikrofon, zvočnik in akustični valovod. V električnem sklopu so opisani vsi narejeni ojačevalniki in njihove karakteristike ter uporabljena sistema ADC in DAC (zvočna kartica). Pri algoritmih so opisani: generiranje signalov, oddaja in zajem signalov ter obdelava signalov. Med obdelavo signalov uvrščamo filtriranje signalov, izračun zakasnitve akustičnega signala in na koncu preračun v temperaturo. V zadnjem, petem poglavju so opisane vse meritve izdelanega termometra in rezultati. Preizkušeni so bili različni motilni vplivi in meroslovne značilnosti termometra. Preizkušene meroslovne značilnosti so: občutljivost, ločljivost, ponovljivost, obnovljivost, linearnost, histereza in negotovost. Prikazano je tudi umerjanje v mešanici ledu in vode.

• Jezik: Slovenski jezik
• Ključne besede: temperatura, akustika, termometer, hitrost zvoka, akustični valovod
• Vrsta gradiva: Doktorsko delo/naloga
• Organizacija: FE - Fakulteta za elektrotehniko
• Leto izida: 2020
• PID: 20.500.12556/RUL-121988
• Datum objave v RUL: 13.11.2020

Sekundarni jezik

• Jezik: Angleški jezik
• Naslov: SECONDARY ACOUSTIC THERMOMETER WITH A WAVEGUIDE

Izvleček

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.

• Ključne besede: temperature, acoustics, thermometer, speed of sound, acoustic waveguide