Date:    Fri, 19 May 2000 10:20:57 -0700

At the encouragement of others, I ran a more elaborate test to see 
how black and white helmets react thermally in sunlight under forced 
air cooling. This new test aims to answer the question of whether or 
not a black helmet is hotter than a white one when worn in direct 
sunlight, both while at rest and while moving.

First of all, many thanks to Mike of Chain Reaction Bicycles 
<http://www.chainreactionbicycles.com/> for the loan of two Trek 
Vapor helmets for the test. Mike: I'll be returning the helmets (none 
the worse for wear) very shortly.

Test equiment:
1 regular household fan
1 150W halogen lamp
1 styrofoam head (from a wig store)
1 handheld anemometer
2 Trek Vapor helmets, size large (1 white, 1 black)
1 digital thermometer
1 stopwatch
( photo: <http://www.terrymorse.com/bike/imgs/thtest1.jpg> )

Procedure:
Place the temperature probe at the crown of the styrofoam head, and 
put the helmet on the head. Hang the lamp 5" above the helmet, turn 
the fan on high speed (6.5 mph), record the temperature every minute 
until it stops changing. Set the fan on low speed (5.0 mph), record 
the temperature every minute until it stops changing. Turn off the 
fan, record the temperature until you can no longer stand it. Repeat 
test for the black helmet, white helmet, and bare head.

Black helmet test photo:
<http://www.terrymorse.com/bike/imgs/thtest3.jpg>

Bare head test photo:
<http://www.terrymorse.com/bike/imgs/thtest2.jpg>

Results:

Complete Results: <http://www.terrymorse.com/bike/imgs/temps1.jpg>
Air-Cooled Detail: <http://www.terrymorse.com/bike/imgs/temps2.jpg>

Air Speed | Delta T:  Black Hemlet   White Helmet   Bare Head
----------|--------------------------------------------------
  6.5 mph  |              1.4 F           1.1           0.6
  5.0      |              2.5             1.5           1.0
  0.0 (*)  |             20.4            21.1          29.3
----------|--------------------------------------------------
(*) 16 minutes after turning off fan

As I had expected, there is a measurable difference between the black 
and the white helmets at these air speeds and radiant levels.

The temperature rose quickly when the fan was turned off, and it 
continued to climb for several minutes. There was no significant 
difference between the white and black helmet in this "no air" 
sequence, as the temperature increased at basically the same rate for 
both. The small difference between the two might have been caused by 
a slight shift in the ambient temperature during the test run. One 
might conclude that the black surface got hotter and promoted free 
convection, which made the black helmet wearer slightly cooler. But I 
would hate to conclude that from these small temperature differences.

The bare head test had the greatest and fastest temperature rise in 
the "no-air" test, even though I had surrounded the temperature probe 
with a radiation shield (aluminum foil). While styrofoam certainly is 
not thermally equivalent to the human head, this result add credence 
to the old adage of wearing a hat on a sunny day (at least when 
you're not moving).

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