CALIBRATION OF IRT 



We are using the IRT Barnes Model IT-2. The first calibration was made indoors, 

 using a water bath of 15 cm. diameter and 50 cm. depth agitated by a small motor-driven 

 propeller. The room temperature was almost constant from 75.8° to 77.0°F. The IRT 

 sensor was mounted on a tripod about one meter above the water surface. Water tempera- 

 ture was measured with a mercury thermometer recently calibrated by the Bureau of Stand- 

 ards. The comparison of the readings of IRT and water temperature measured by this 

 thermometer for a range of water temperature from 32.0° to 88.0°F is shown in curves A 

 and B of Figure 2. It is noted that the IRT readings are higher than the true temperature 

 by almost 2.0°F for lower temperatures but the difference becomes very small as the water 

 temperature becomes close to air temperature. This suggests that the air temperature be- 

 tween the sensor and the water has some influence on the IRT readings. The second cali- 

 bration was made at the rooftop using the same set-up. The air temperature changes from 

 31.5° to 33.0^ F. Curve C of Fig. 2 indicates that the effect of the air temperature caused 

 IRT readings lower than the true values by 4.0°F in the range of 90.0° to 100. 0°F and by 

 2.0°F in the range of 60.0° to 80.0°F and has 1.0° to 2.0°F in the range of 40.0° to 50.0°F 

 water temperature. It was also recognized in this experiment that the wind gusts chained 

 the readings of the IRT by 1.0° to 2.0° F. 



In order to study further the effect of the air between the sensor and the water, a 

 series of experiments were made by changing the vertical distance between the sensor and 

 the water surface. A basin of 1.2 meters diameter was filled with hot water about 10 cm. 

 deep. The water temperature was changed by melting snow. It was kept almost constant 



I I I I I I I M I I I I I I I I I I I I I I I I I I I I I I 



90 100 no 



Figure 2, Calibration of IRT with water temperatures ranging from 32.0°F 

 to 88° F; A and B, indoor tests; C, outdoor test 



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