Vol.1 No.1 2008

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Research paper : Improving the reliability of temperature measurements taken with clinical infrared ear thermometers (J. Ishii)−53−Synthesiology - English edition Vol.1 No.1 (2008) (3)-2 Effect of temperature distribution of cavity wall(4) Stability and reproducibility of the systemCompared to conventional type of industrial infrared radiation thermometer, ear thermometer has much wider field-of-view, so accurate calibration cannot be achieved when ordinary shape of cavity designed for thermometers with small view angle is used. Therefore, a shape of cavity with sufficiently high effective emissivity for ear thermometer having wide-angle was newly designed based on the Monte Carlo simulation[8]. Figure 5 is a schematic diagram of the Monte Carlo simulation. Light ray bundles were entered into the cavity from outside of aperture opening, the reflection on wall surface was simulated using random numbers, and the effective absorptivity (corresponds to effective emissivity) of the cavity was calculated from the probability of absorption of the incident ray bundle in the cavity. The reflective property of the cavity wall was expressed by a model composed from perfect diffuse reflection and specular reflection. Then, the effect of temperature distribution on the cavity wall was also quantitatively evaluated by inputting measured temperature distribution in the water bath to the simulation.Intrinsic emissivity of coating material of cavity wall was measured with a Fourier transform infrared spectrometer system developed at AIST. The measurement uncertainty of spectral emissivity data was evaluated at 1 % or less[9], and was taken into account as an uncertainty of intrinsic emissivity of cavity wall. Measured emissivity data was used as parameter for Monte Carlo simulation to evaluate the effective emissivity of cavity. Also, effect of heat loss by convection of air and thermal radiation in the blackbody cavity was evaluated using high-resolution infrared radiation thermometer[10].Table 1 shows the performance (uncertainty budget) of standard BBR, which is the national standard for radiance temperature scale developed by AIST. For reference thermometer, about 5 mK uncertainty was maintained by calibrating the standard platinum resistant thermometer against AIST’s national standard temperature fixed point cells, and the reference temperature of the cavity was measured at around 5 mK level of uncertainty in the thermostatic water bath shown in Figure 4. Effective emissivity of 0.9995 or more was achieved for the cavity by designing the cavity shape suitable for ear thermometers with wide view angles, and uncertainty of radiance temperature resulting from this was set at 20 mK or less. With these technological developments, radiance temperature scale traceable to the international temperature scale (SI unit for temperature) at approximately 0.03 °C uncertainty (95 % confidence level ) was achieved for human body temperature range (32 °C~42 °C)[11].The technology developed for standard BBR was also employed as recommended specification for standard equipment in JIS for ear thermometers which will be explained later[12]. Currently, it is commercially available as working standard BBR for body temperature range from a manufacturer which participated in this joint development[13].6 Calibration of working standard BBR in industry against the national standard BBR at AISTNext, specific calibration method was reviewed to link between the national standard BBR of AIST and working standard BBR of manufacturers in this new measurement management system. As mentioned in Section 5, AIST developed the standard BBR as national primary standard for radiance temperature scale, and the actual calibration will be done by either transporting the manufacturer’s working standard BBR to AIST to be calibrated against national standard BBR or by transporting standard BBR of AIST to the manufacturer to calibrate the working standard BBR.6.1 Uncertainty estimation for calibration of working standard BBRSimulated calibration experiment was conducted to estimate the uncertainty when working standard BBR was calibrated against national standard BBR at AIST. Aside from standard BBR, which is the national standard, a BBR for simulated calibration was prepared at AIST. Four types (about three thermometers for each type) of high-resolution (displayed temperature resolution 0.01 °C) ear thermometers provided by several manufacturers were used for direct comparison measurement of radiance temperature of the blackbody cavities. In the experiment, AIST standard blackbody furnace and BBR for simulated calibration were run simultaneously, and were stabilized at nearly identical temperature for calibrating the radiance temperature (e.g. 37.0 °C). While monitoring the temperature of each reference thermometer, the difference in radiance temperature between Table 1. Uncertainty budget of standard BBR developed by AIST32 ℃37 ℃42 ℃851216211141822283625<1mKmKmKmKmKmKmKmKTemperature of blackbody cavityUnitUncertaintyComponent of uncertaintyCalibration of reference thermometerTemperature measurement by reference thermometer (including stability of water bath)Heat loss inside cavityEffective emissivity of isothermal cavityEffect of temperature distribution of cavity wallEffect of change in ambient temperature(Tambient=23±2 ℃)Combined standard uncertaintyExtended uncertainty (95 % confidencelevel )

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