Radiation Thermometry Research Articles

High-Precision Temperature Measurement System for Magnetic Resonance Imaging

To determine the compliance of an implantable medical device with the safety requirements in magnetic resonance imaging, an experimental assessment of the heating of this device during a study is necessary. The use of traditional methods, such as thermocouple measurements or radiation thermometry, is difficult in the conditions of a magnetic resonance imaging room. A spectrometric system for measuring temperature in conditions of a magnetic resonance imaging room is proposed. The developed system has a sensitivity of 0.01°C and an error of 0.1% in the range of 10–50°C. The temperature sensors used in the system are Fabry–Perot interferometers. The design of the sensors and the method of calibration are described. The system was tested in determining the heating of two passive implants during the study in a magnetic resonance imager with a magnetic field induction of 1.5 T. The compliance of the developed system with the recommendations adopted in magnetic resonance imaging for evaluating the heating of implantable medical devices is demonstrated. The temperature value obtained is comparable with the value found during testing of this implant according to ASTM F 2182. The presented measuring system can be used to assess the magnetic resonance compatibility of implantable medical devices, to develop scanning protocols for patients with metal structures, as well as to confirm or refi ne mathematical models of heat transfer.

The radiative characteristics of inconel 718 superalloy after thermal oxidation

The Inconel 718 is a promising Ni-based alloy for a wide range of high-temperature industrial applications. However, the radiative characteristics of Inconel 718 alloy change due to surface oxidation and cause large deviation in temperature field computation and radiation thermometry. Herein, the Inconel 718 alloy has been oxidized at different temperatures, ranging from 700 K to 1100 K, for different time (up to 100 h) to systematically study the influence of oxidation on radiative properties. The highly oxidized alloy samples have been prepared, whose oxidation degrees are more similar to the engineering applications. The normal spectral emittance of as-prepared and surface-oxidized Inconel 718 alloys has been measured in the range of 2–15 μm at different temperatures (400–1400 K). The spectral data have been integrated to analyze the effect of temperature on total emittance. In the case of as-prepared Inconel 718, the normal spectral emittance increased with decreasing wavelength, whereas the total emittance linearly increased with increasing temperature. By comparing the emittance data of oxidized and as-prepared samples, it is clear that the high-temperature oxidation improves the emittance of Inconel 718 alloy. The effect of oxidation on radiative characteristics of Inconel 718 can be divided into three stages: 1. the spectral emittance slightly increases in short wavelength range; 2. obvious emittance fluctuation occurs due to radiation interference; 3. emittance oscillations disappear and the spectral emittance curves shows more radiative characteristics of the main oxide (Cr2O3). Moreover, the total emittance of Inconel 718 alloy, with compact oxide scales, decreased with increasing temperature, and during the 2nd stage, a knee point is observed on the total emittance curve near the Néel temperature of NiO. To unveil the mechanism of oxidation-induced emittance variations, scanning electron microscope (SEM) and energy disperse spectroscopy (EDS) are employed. The structure of a multilayered oxide scale is demonstrated and the process of oxidation is explained. According to the analysis, the oxidation degree determines the weight of influence of substrate alloy and surface oxides on radiative characteristics of oxidized alloy, and this is also the reason why the influence of oxidation on radiative characteristics shows staged. Moreover, NiO was detected on the surface of oxidized samples, and combined with the emittance results, it can be concluded that the magnetic ordering of oxides will influence the radiative behavior of Inconel 718 alloy.

Development and Realization of Fe–C and Co–C Eutectic Fixed-Point Blackbodies for Radiation Thermometry at CSIR-NPL

A series of metal–carbon (M–C) eutectics with robust reproducible melting phase transitions are evolving as the reference high-temperature fixed-points up to 3000 °C for radiation thermometry. The mise-en-pratique prepared for the realization and dissemination of new kelvin allows the use of set of M-C blackbody fixed-points or a combination of M-C fixed-points along with silver (Ag), gold (Au) or copper (Cu) blackbodies for the direct measurement of thermodynamic temperatures at high-temperature range. The iron–carbon (Fe–C) [1153 °C] and cobalt–carbon (Co–C) [1324 °C] fixed-point cells are the doorway to high temperature above a copper fixed-point. This paper describes the in-house development of Fe–C and Co–C eutectic graphite crucible blackbodies and their realizations by linear spectral pyrometer (LP4, 650 nm) at CSIR-NPL, National Metrology Institute (NMI) of India. The novel design and assembly approaches were employed to fabricate the blackbody crucibles with 120o inner-well cone, 100 mm ingot length and 8 mm aperture diameter with 0.9997 emissivity and robust thermo-mechanical stability. The eutectic ingots were prepared by multiple fillings of the homogeneously ground and mixed powders with 4.2 and 2.6 weight percentage of carbon in Fe and Co, respectively. Repeatability of measured melting plateau and uncertainty of the temperature measurement for both Fe–C and Co–C cells are presented. The ITS-90 temperatures (T90) to these M-C fixed-points were assigned by a spectral radiance ratio measurement (Planck’s Law) relative to the freezing point of Ag [961.78 °C]. The T90 temperatures thus determined for Fe–C and Co–C blackbody fixed-point cells are 1154.05 °C and 1323.93 °C with the expanded uncertainty of 0.37 °C and 0.50 °C, respectively.

Measurement of temperature and wavelength-dependent emissivity distributions using multi-wavelength radiation thermometry

A multi-wavelength radiation thermometry method used to determine the temperature and wavelength-dependent emissivity according to spectroscopic radiation intensities is presented. Over a predetermined temperature range, with a set step size, the method traverses the temperature range, and for each temperature, inversely fits a spectral line according to the intensity in the middle of the wavelength region, based on the Rayleigh approximation. The temperature with the spectral line closest to the measured spectral line is considered to be the optimum temperature at that accuracy level. Then, near that optimum temperature, the temperature range and step size are narrowed, to determine the optimum temperature at higher levels of accuracy. Iterating on this method until reaching the required accuracy level, the temperature can be achieved, and the wavelength-dependent emissivity can also be determined. Simulation verification was conducted showing good anti-noise performance and rapid processing capacity. Temperature and emissivity distributions for six heights of an axisymmetric laminar diffusion flame with 194 mL/min ethylene and 240 L/min air were given. Results show that along the line of sight, the radiation temperatures exhibit an average effect of the local temperatures, while the emissivity exhibits a cumulative effect of the absorption coefficients.

Temperature and emissivity measurements from combustion of pine wood, rice husk and fir wood using flame emission spectrum

Biomass combustion for heat and power generation is one of the best ways for substituting fossil fuels. It is necessary to develop in situ temperature and emissivity measurement technology for biomass volatile flame. In this study, combustion experiments on three biomass pellets of pine wood, rice husk and fir wood were carried out in a laboratory biomass burner at two A/F flowrates. A multi-wavelength radiation thermometry method was proposed to calculate temperature and wavelength-dependent emissivity based on Rayleigh approximation. The results showed that the temperature and emissivity of pine wood combustion were higher than those of rice husk and fir wood volatile flames. Though the emissivity of rice husk was less than that of fir wood, the temperature of rice husk combustion was higher than that of fir wood due to the latter's higher moisture content. The temperatures measured by the spectrometer system agreed well with the thermocouple data. The normalized emissivities of the three different biomass show the same trend, indicating the radiative properties of biomass volatile flames do not change with biomass species and combustion conditions.

Inversion of target radiant temperature and distance via spectrum separation.

Accurate measurement of temperature and distance is essential for applications in multiple fields such as air defense, antimissile systems, laser damage, and infrared remote sensing. However, ranging currently relies on either laser or infrared radiation characteristics, neither of which fully utilize the substantial information contained in the spectrum. In this study, we realized the simultaneous inversion of target radiant temperature and distance via spectrum separation based on the distance information from the atmospheric spectrum as well as the temperature information from multispectral radiation thermometry (MRT), by measuring the thermal radiation spectrum of distant high-temperature objects. The study initially involved conducting a theoretical analysis of long-distance MRT in the atmosphere by utilizing the least squares and Newton's iteration methods. Once the thermal radiation spectrum and the long-distance atmospheric spectral transmittance were acquired from nonlinear models, highly precise and accurate radiant temperature and distance information was derived via simultaneous inversion. Subsequently, experimental verifications were performed to measure an explosion spectrum at a temperature of approximately 2000 K and a distance of 9 km. The spectral band was 400-900 nm, and the temperature and distance of the explosion calculated by the inversion was 2004.4 K and 9.9 km, respectively. The inversion distance error was 10%, indicating that the simultaneous inversion of radiant temperature and distance was successful and that the proposed method was effective. The method proposed in this study can be applied to the measurement of radiant temperature and distance of long-distance objects.

An accurate instrument for emissivity measurements by direct and indirect methods

Emissivity is a quantity essential to consider when assessing the measurement uncertainty in non-contact temperature measurements. This paper presents a new instrument for measuring emissivity of opaque materials from 200 °C to 450 °C in the spectral range of 2.1 to 2.5 µm. These ranges are ideal for measuring the temperature of metals, such as aluminium, during manufacture or heat-treating processes. The instrument consists of a pair of hemispherical cups coated with Vantablack® and gold respectively, a custom designed radiation thermometer, and a hot plate. This instrument enables both the direct and the indirect methods for measuring emissivity of materials. Use of two identical cups allowed for quantitative analysis of the uncertainty of the instrument to determine the most suitable emissivity measurement range. The expanded uncertainty of the instrument was lower than 0.058 (k = 2) over the entire measuring temperature range. Studies were undertaken using different materials with emissivities ranging from 0.06 to 1. These included aluminium alloy 6082, stainless steel 304, and HiE-Coat 840M paint. Relative uncertainty analysis indicated that the indirect method was more accurate for measuring low-emissivity materials, whereas the direct method was more suitable for all other materials. Our instrument, with experimentally determined measurement uncertainty, aims to offer accurate emissivity references for use in radiation thermometry applications.

Calibration of standard radiation thermometers using two fixed points

Radiation thermometry based on the measurement of two fixed points, the so-called n = 2 scheme, is studied in this work. Verification of the scheme was undertaken for various measurement conditions using seven radiation thermometers with nominal centre wavelengths of 650 nm, 890 nm, 900 nm, and 1600 nm. Comparison between the signals calculated using the n = 2 scheme and the signals measured directly with fixed-point cells was studied. For temperatures between the two reference fixed points, the equivalent temperature difference was less than 0.14 K in the temperature range from the In to Cu fixed points, and less than 0.3 K in the temperature range from the Cu to Re–C fixed points. The maximum normalised error was 0.35, confirming the validity of the n = 2 scheme; the consistency was observed not only in the interpolation temperature range, but also in the extrapolation range. The results also indicate that temperature realisation in the range from 150 °C to 2500 °C can be achieved by maintaining only the Zn, Cu, and Re–C fixed-point facilities. The n = 2 scheme would be of great benefit to developing national metrology institutes in terms of time consumed for scale realisation and cost for maintenance of the measurement system.

Highly sensitive magnetic resonance compatible temperature measurement system

One of the steps in determining the compliance of an implantable medical device with the safety requirements in magnetic resonance imaging (MRI) is the experimental assessment of its heating over the course of the study. However, the application of traditional methods, such as thermocouple measurements or radiation thermometry, is difficult in connection with the conditions of high magnetic fields. A spectrometric system is proposed for measuring temperature in a magnetic resonance imaging cabinet with sensitivity of 0.01 °C and error of 0.1 % in the range of 10–50 °C. Temperature sensors are Fabry–Perot interferometers formed by flat ends of optical fibers located at a distance of about tens of micrometers. A design of the sensor and a calibration method are described. A design of the sensor and a calibration method are described. The system was tested in the process of two passive implants heating in 1,5 T MRI. As a result, compliance with the accepted recommendations for assessing the heating of implantable medical devices in MRI was demonstrated, and the temperature rise value was obtained that was comparable to the manufacturer’s tests of this product according to ASTM F 2182. The presented measurement system can be used to assess the MR-compatibility of implantable medical devices, to develop scanning protocols for patients with metal structures, as well as to confirm or refine mathematical models of heat transfer.

Detection method of combustion oscillation characteristics under strong noise background

After the lean fuel premixed combustion technology is applied to aero engines, severe combustion oscillations will be cased and led to hidden safety hazards such as engine vibration, further energy waste and other problems. Therefore, it is increasingly important to actively control combustion oscillations. In this paper, a multispectral radiation thermometry (MRT) is used to analyze the hydroxyl group, which is a measurable research object in the combustion chamber of an aero engine, and to fit the functional relationship between the radiation intensity ratio and the temperature in different bands. The theoretical value of the error is <2%. At the same time, in order to solve the problem of weak detection signal and excessive interference signal, an improved frequency domain filtering method based on fast Fourier transform is designed. Besides, the FPGA platform is used to ensure the real-time performance of the temperature measurement system, and simulations and experiments are performed. An oscillating signal with an oscillation frequency of 315 Hz is obtained on the established test platform, and the error is only 1.42%.

Validation of phosphor thermometry for industrial surface temperature measurements

Surface temperature measurements are required by the aerospace and automotive industries to guarantee high-quality products and optimize production processes. Accurate and reliable measurement of surface temperature is very challenging in an industrial environment. Surface contact probes are widely used but poorly characterized, while non-contact infrared thermometry is severely hampered by the unknown emissivity of the surface and by problems caused by stray radiation from the background. An alternative approach to the above techniques is phosphor thermometry, used here in a hybrid contact/non-contact approach.In this work, the development of a lifetime-based phosphor thermometer, its application to industrial surface temperature measurement and its validation are reported in a metrologically sound manner. The phosphor thermometer was initially calibrated by contact on a reference calibrator system at the Istituto Nazionale di Ricerca Metrologica to provide SI traceability to the measurements at the industrial level; the system was later validated by exploiting a metal phase-change method. The robustness of the approach against a strong radiative background was also investigated. A comprehensive uncertainty analysis was carried out, resulting in an expanded uncertainty (k = 2) lower than 1.4 °C over the temperature range from the ambient to 450 °C.The phosphor-based thermometer was then tested at industrial manufacturing premises to measure the surface temperature of aluminium alloy billets during the pre-heating phase before forging. The phosphor-based approach was compared with radiation and contact thermometry in both static and dynamic measurement conditions. The experimental results proved that phosphor thermometry, besides being a valid alternative to conventional techniques, may offer better performance in an industrial setting.

InGaAs avalanche photodiode thermometry

The infrared detector is the most important component within any radiation thermometry-based system, with its choice determining the wavelength, response time and, ultimately, the temperature measurement capabilities of the instrument. To improve upon the existing generation of radiation thermometers, more sensitive detector technologies are required. In this work, we demonstrate a direct comparison between an indium gallium arsenide (InGaAs) photodiode and an InGaAs avalanche photodiode (APD) for 1.6 µm radiation thermometry. The high internal gain of the InGaAs APD increases the sensitivity of the radiation thermometer, enabling the measurement of a target temperature more than 50 °C lower than is typical with commercially available InGaAs photodiode thermometers. The more sensitive InGaAs APD provides faster response time measurements, hence improving the thermometer’s temporal resolution. Finally, the InGaAs APD is shown to produce a quantitative thermal image with lower measured temperature fluctuation across the scene when incorporated within a highly aperture limited scanning system.

An Accurate Device for Apparent Emissivity Characterization in Controlled Atmospheric Conditions Up To 1423 K

Emissivity is a material property that must be measured before an accurate noncontact temperature measurement can be made. We have developed a novel instrument for measuring apparent emissivity under a controlled atmosphere, providing data for applications in radiation thermometry. Our instrument employs a split furnace, a sample-blackbody component, two custom-designed radiometers, and a controlled atmospheric system. We measure across the temperature range from 973 to 1423 K and the spectral range from 0.85 to 1.1 μm; this range is matched to the majority of high-temperature radiation thermometers. The sample and reference approximate blackbody are heated and maintained in the thermal equilibrium, with a temperature difference of better than 1 K at 1423 K. The combined standard uncertainty of the system is lower than 0.0590 (at k = 2) over the whole temperature range. Apparent emissivity of type 304 stainless steel (SS304) was studied under different oxidizing procedures. Nitrogen and compressed air were input into the system to control the oxidization process. We elucidated the relationship between the apparent emissivity variations and the surface composition changes of SS304 during oxidization. This article aims toward accurate and traceable apparent emissivity data, with well-investigated uncertainty, for use in radiation thermometry.

Coupled effects of reflection and absorptive gas mixture on surface temperature determined by single color pyrometer

To investigate the coupling effects of reflection and absorptive gas mixture on surface temperature determined by single color pyrometer, an analytical radiation thermometry model combined with a mathematical radiative heat transfer model, which simultaneously considering both the reflected ambient radiation and the gas absorption and emission of radiation, was established in this paper. The temperature errors caused by the combination of intervening gas mixture and reflected ambient radiation were quantitatively analyzed under three practicable measurement strategies depending on the relative target and gas temperatures. As the results indicate, the investigated wavelength ranges can be roughly divided into three parts depending on the relative importance of reflection and gas mixture. Correcting for the hot/cool gas mixture effect through use of a low/high temperature reflector is usually effective because these two effects will cancel each other out. The selection methodology of spectral pass-band of radiation thermometer to minimize the influences of reflection and gas mixture absorption and emission under different conditions was presented. Sensitivity analysis of the derived temperature error to the variations in ambient temperature, target surface emissivity, gas mixture temperature and concentration, and viewing path length was investigated. Considering the case T1 = 1200 K, T2 = 1400 K, Tg = 2000 K, ε1 = 0.85, L = 100 cm, XH2O = 0.2, XCO2 = 0.1, p = 1 atm, the most influential factor is gas mixture temperature for wavelength longer than 1.33 µm, followed by other factors. For wavelength shorter than 1.33 µm, the high ambient temperature is the main contributor to the imprecision in the proposed model output. The analyses in the paper may provide the necessary theoretical supports for the design and application of a radiation thermometer in the presence of intervening gas.

Single-crystal-Ga2O3/polycrystalline-SiC bonded substrate with low thermal and electrical resistances at the heterointerface

A single-crystal β-Ga2O3 substrate was directly attached to a polycrystalline SiC (poly-SiC) substrate using a surface-activated-bonding method to enhance heat extraction from β-Ga2O3 devices. The effective thermal conductivity of the n+-Ga2O3/n+-poly-SiC bonded substrate and the electrical resistance at the heterointerface were characterized by using periodic heating radiation thermometry and analyzing vertical current–voltage characteristics, respectively. Small thermal and electrical resistances at the bonded interface demonstrated the strong prospects of the bonded substrates for applications to high-power vertical Ga2O3 devices.

Effect of hot water vapor on strand surface temperature measurement in steel continuous casting

The surface temperature of continuous casting slab in water spraying zone is a key parameter of production control. However, the accuracy of radiation temperature measurement is greatly affected by water vapor at high temperature. In this paper, a mathematical radiative heat transfer model combined with an analytical radiation thermometry model was developed. The hot water vapor absorption coefficient at a spectral resolution of 25 cm−1 as an input parameter to the radiative heat transfer model is obtained by employing the statistical narrow-band model along with gray-band approximation on the basis of the updated database HITEMP 2010. The proposed model was applied to quantitatively calculate the surface temperature, obtained by ratio and single waveband thermometer for infrared waveband from 50 cm−1 to 11250 cm−1, of assumed gray strand covered by hot water vapor. Detailed simulation studies were performed considering the following parameters: concentration of the hot water vapor, reflectivity of the target surface, background temperature of the surrounding surfaces and distance between target and thermometer. A sensitivity analysis was put forward trying to ascertain how the proposed model depends on those parameters. As the results indicate, both hot water vapor and reflected ambient radiation greatly affect the measurement accuracy of the radiation thermometry. The temperature errors caused by hot water vapor are spectrally selective. The most suitable spectral regions for ratio and single waveband thermometer operating and a criterion for choosing the optimal wavelength pairs for ratio thermometry were put forward. The analyses in the paper may provide necessary theoretical supports for the design and application of a radiation thermometer in the presence of hot water vapor.

EMISSIVITY MODEL OF STEEL 316L AT 800–1100 K DURING OXIDE LAYER GROWTH ON THE SPECIMEN SURFACE

This work strived to study the variation in the spectral emissivity of steel 316L specimens with wavelength at a certain temperature. The spectral emissivity and radiances from the specimen surface were measured by multispectral radiation thermometry at wavelengths of 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, and 2.1 μm and at temperatures from 800 to 1100 K in increments of 20 K. The specimen was heated and kept at a certain temperature in air for 6 h. The temperature of the specimen surface was measured by two thermocouples, which were symmetrically welded onto the front surface of the specimens near the measuring area for accurate monitoring of the surface temperature. The average of their readings was regarded as the real temperature. Variation in the spectral emissivity with wavelength was studied at different temperatures and different heating times. Effect of surface oxidation on the fitting accuracy of the emissivity models was evaluated. Two favorite models were determined. With the radiant values measured by thermometry and in combination with the least-squares fitting technique, two favorite models were examined. On the whole, both the five-parameter LLWE and LWE models could yield the best overall temperature prediction and could farthest relieve the effect of surface oxidation on the accuracy of temperature prediction over the present wavelength and temperature ranges.

Design and realization of a wide field of view infrared scanning system with an integrated micro-electromechanical system mirror.

We present a wide field of view (FOV) infrared scanning system, designed for single-pixel near-infrared thermal imaging. The scanning system consisted of a two-axis micro-electromechanical system (MEMS) mirror that was incorporated within the lens. The optical system consisted of two groups of lenses and a silicon avalanche photodiode. The system was designed for both the production of thermal images and also to utilize the techniques of radiation thermometry to measure the absolute temperature of targets from 500°C to 1100°C. Our system has the potential for real-time image acquisition, with improved data acquisition electronics. The FOV of our scanning system was ±30° when fully utilizing the MEMS mirror's scanning angle of ±5°. The pixel FOV (calculated from the distance to target size ratio) was 100:1. The image quality was analyzed, including the modulation transfer function, spot diagrams, ray fan plots, lateral chromatic aberrations, distortion, relative illumination, and size-of-source effect. The instrument was fabricated in our laboratory, and one of the thermal images, which was taken with the new lens, is presented as an example of the instrument optical performance.

The Importance of Establishing a Very High-Temperature Radiation Thermometry Measurement Capability at the National Metrology Institute of South Africa (NMISA)

The ITS-90 scale above the Silver (Ag) point is realized using a standard linear radiation thermometer by measurement at a reference point which is an Ag, Gold (Au), or Copper (Cu) point. The fixed...

Uncertainty propagation through integrated quantities for radiation thermometry

Uncertainty formulae are derived for propagating both correlated and uncorrelated components of uncertainty in spectral responsivity measurements directly through the Planck integrals for the radiation thermometry range of the international temperature scale of 1990 (ITS-90) and for absolute radiometry. These formulae are compared to those obtained using the Sakuma–Hattori equation as an approximation to the Planck integrals. It is shown that for typical spectral responsivity bandwidths, the differences are negligible, confirming that the Sakuma–Hattori approximation provides an excellent model for the basis of uncertainty propagation for radiation thermometry, resulting in simpler and more intuitive uncertainty equations.