[at-l] UV pen

["McBride, James" <mcbride_j@lxe.com>]

David,

> I will beg to differ from you on this. In my experience solar uv works
> quite well to purify water.
> 
This is the kind of unscientific statement I have a problem with. How can
you say that solar uv works well without any testing? Only a before and
after test is valid (test for "bugs" and find them - radiate them - test and
they're gone). I can say the same thing about my PUR filter - in my
experience it works fine. But I haven't tested it so the statement is
meaningles. I've never gotten sick while using it but then maybe I never
filtered any bad water. By the way, I am not saying uv doesn't purify water.
I understand that it does. I'm just questioning the claims from this little
pen.

> Second, how much uv actually gets to bottled water via sunlight depends on
> the container. As you well know many artificial substances block and
> breakdown in uv ... 
> 
Of course. I agree. Actually everything (not just artificial substances)
either absorbs or reflects the uv to some extent. Just some more than
others.

> > we have to assume that the UV water purifier does something
> > differently than sunlight.
> 
> Actually, we do not need to make this assumption: see the above discussion
> in relation to solar powered iced tea and water purification which has
> always worked well for me.
> 
	I stand by the original statement - but I need to clarify. Solar
radiation may purify but it takes a long time. For this small pen to do the
job in a short period of time would take a lot of energy output.

> The unit of "power" for uv purification is defined in terms of milliwatt
> seconds per centimeter squared; the usual dosages, depending upon the
> water
> source and the contaminate treated for ranges between 16 and 40 of these
> units. The steri-pen has a 5 watt light source, which suggests over-kill
> to
> me.
> 
A milliwatt second per centimeter squared is just energy per unit area
(Joules/square meter). Energy is expressed in Joules which are watts/second,
and area is expressed in square meters. 

> Remember that the "power" of light cubicly decreases with
> distance, so stiring a cup of water puts the contaminants darn close to
> the
> light source, while you're considerably more distant from the same source.
> 
You are correct except that the energy from a light source should obey the
inverse square law and therefore decrease as the square of the distance.
Below is a link to HYperPhysics which is an interactive physics website
created by one of my  professors at GSU where I got my physics degree. It
explains the inverse square law. If there is something else going on to
force an inverse cube law please explain.

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html


> As I understand it, the uv light sources are much better tuned to out put
> wave lengths than are standard house hold bulbs.  Moreover, while I agree
> entirely with your valid point here, I wouldn't doubt that tunable light
> sources, including lasers in the uv range are technologically feasible,
> 
	Better but not too much better. UV lasers exist but I believe thay
are gas lasers. Gas lasers a huge monstrosities that take up a lot of space
- definitely not handheld. A solid state laser (semiconductor) could be
possible in the uv range though.

> I have not yet found any independent lab results. I have submitted the
> query, though, to one of the sites mentioned in my post and have given
> them
> the url to the steri pen.
> 
	Keep looking. This is ALWAYS the test to see if a claim is valid. If
a product makes claims that are legitimate then they will get it tested. 

> Again, theoretically, the steripen should work. I continue to believe uv
> is
> a viable alternative means for purifying water, and I have not found any
> valid scientific evidence otherwise. The jury remains out for me, though,
> on the question regarding the steripen itself, since I'm well aware of the
> difference between theory and practice .....
> 
> * From the Appalachian Trail Mailing List |  http://www.backcountry.net  *
> 
Please don't take my comments as being critical of your research or ideas. I
am just a skeptic and I believe in analyzing these things very critically
from a scientific/engineering standpoint. Keep researching and let us know
if you find any independent test results. I will be the first one to swap my
heavy PUR filter for a ster-pen if it really works!

Jim McBride
* From the Appalachian Trail Mailing List |  http://www.backcountry.net  *

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