Experiment to determine ‘Blocking BTUs’ of double coating Materials.
Assumptions are:1) that there is a constant number of BTU’s blocked and
2) That the 16 Rounds Per Minute (RPM) or the radiometer used represents
a constant number of BTUs regardless of the distance or wattage of the source.
Materials used : 4 Light bulbs, 1 radiometer, tape measure, calculator.
Radiometer has 4 vanes and each vane is 2 square centimeters.
Data collected:
Watts Distance to 16 RPMs Bulb Diameter Distance (D) in Diameters 1/D2
138 25.75 “ 2 3/8” 10.842 1/ 117.5
100 17.25 “ 2 3/8” 7.263 1/ 52.75
75 13.25 “ 2 3/8” 5.579 1/ 31.12
42 8.5 “ 2 1/4" 3.777 1/ 14.27
Calculations:
Since 1 watt is 3.414 BTUs and the bulb diameter that is ‘seen’ by the radiometer
is convecting heat as it radiates, we combine the inverse square law modified by the
diameter as a distance basis instead of the intensity itself. Then we must consider
the incremental reflection of the radiometer glass bulb as it reacts to increased wattage.
The formula : (Watts/5.88 ) * 1/D2 * 3.414 = BTUs arriving at radiometer dome.
Data before adjustment:
Watts BTUs
138 .68
100 1.1
75 1.4
42 1.7
We can see that as expected, the higher intensity radiation shows reduced energy due
to reflection of the glass radiometer dome, which we infer is equal to .3 BTUs for every
25 watts above the 40 watt magnitude. Thus 1.7 becomes 2.0, 1.4 becomes 1.4 + (2*.3)
Data adjusted for spectral reflection:
Watts BTUs
138 2.0
100 2.0
75 2.0
42 2.0
Conclusion:
For vertical orientation with heat sources at 90 degrees to the radiant barrier tested, the
expected BTU blockage is 2.0 BTUs. This is already converted to BTUs per hour
per square foot per degree Fahrenheit by the conversion factor 3.414 times watts.
However, since emissivity is so dominating over the temperature differences present,
the temperature differences of each light bulb can be ignored, as can be the room
temperature. This data is better than table values given in text books for most
vertical position radiant barriers. See NATCAN page
Go to the Natural Convection Notre Dame validation PAGE