A novel thermometer having multiple spherical thermocouples is developed.
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The design adopts energy-balance and heat-transfer theory for a spherical surface.
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Radiation effects are eliminated without using an aspirated radiation shield.
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Optimum combination of sphere diameters with highest accuracy was 0.25, 1 and 4 mm.
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The root-mean-square error of the measured air temperature is 0.13 °C.
Abstract
A simple formula is proposed to calculate the air temperature from the temperatures of multiple spheres of different size and thus eliminate the effects of radiation on the measurement of air temperature. The formula is derived from energy balance and heat transfer theory for a spherical surface in external flow. Using this formula, we developed an instrument with which to obtain the air temperature from multiple thermocouples with spherical tips (multiple globes) without the need for a radiation shield and ventilation. Field experiments were conducted during summer and winter. Five globes having different diameters (d = 0.25, 0.5, 1, 2, 4 mm) were set in the field, and a standard thermometer having a threefold radiation shield and aspiration of 5 m/s was used as a reference. All globe temperatures were higher than the air temperature during the day; the maximum difference was 3.3 °C for d = 4 mm. The air temperature calculated using the proposed formula agrees well with measurements made using a standard thermometer. In the formula, the optimum value of the coefficient m relating to the surface geometry that minimizes error in the calculated air temperature was m = 0.5. This value is consistent with the results of previous studies on heat transfer theory and experiments on spherical surfaces. The best combination of globe diameters providing the highest accuracy was 0.25, 1, and 4 mm. The root-mean-square error of this combination for all summer and winter data was 0.13 °C. The results demonstrate the sufficient accuracy of the proposed thermometer. The proposed thermometer can be used on farmland and in forests where mains power is unavailable and will thus help clarify the actual temperature and microclimate in rural areas.
