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8h.6 Black vs White Helmet - Thermal Test




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This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.

8h.6 Black vs White Helmet - Thermal Test

From: terry morse <tmorse@terrymorse.com>
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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