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[at-l] UV by the numbers



By the numbers:

1 joule/square meter = 16.68 mw-s/10,000 sq cm  =  .001668 mw-s/sq cm

 
UV-A with wavelengths 315 - 400 nm. 

 
UV-B with wavelengths 280 - 315 nm. 

 
UV-C with wavelengths less than 280 nm. 

Sunlight:

for a list of articles with abstracts measuring the in situ effect of solar
UV light on plankton in high altitude lakes see the following address:
http://i115srv.vu-wien.ac.at/uv/newsletter/uvnl_con.htm 

The atmosphere filters out most of the solar UV-C. Almost every measure of
solar UV Iíve found appears to involve either or both UV-A & UV-B or both.
Ozone absorbs a particular wavelength of UV-B, and ozone depletion seems to
correspond with the amount this wave length UV-B arriving at the surface of
the earth. [The steripen uses the more energetic UV-C radiation,
specifically 254nm. See steripen specs below.] So direct comparison of the
steripen with solar UV radiation would appear inappropriate.
See:
http://www.tor.ec.gc.ca/woudc/reports/uv_guidev51.htm - offers a uv index
for meteorological forecasts to the public. So the US. Hereís how the US
calculates the index: http://www.epa.gov/docs/ozone/uvindex/uvcalc.html 
Also See: http://iwin.nws.noaa.gov/iwin/us/ultraviolet.html 
http://www.epa.gov/docs/ozone/uvindex/uvwhat.html 
http://www.epa.gov/uvnet/ which provides measurements for UV-A & -B and
defines irradiance as follows: Irradiance The power transferred to a unit
area of a surface by radiation from all directions within a hemisphere,
measured in watts per square meter (W/m≤); Irradiation 
The energy transferred to a unit area of surface by radiation from all
directions within a hemisphere during a specified period of time, (measured
in Joules per square meter (J/m2)) 
This site provides access to data from a large number of north american
observation sites.

The only data I can find appears to measure UV-B radiation in terms of
joules/meter squared.
Seasonal variation in intensity in the USA appears to range from 1000 to
5000, with the northern latitudes receiving less and southern latitudes
receiving more.

J/m2	mw-s/cm2	
800	1.3344	
1000	1.668	
2000	3.336	
3000	5.004	
4000	6.672	
5000	8.34	

Higher elevations, latitudes closer to the Equator, and clearer skies will
allow more UV light of all kinds to reach the surface.

The literature on the effect of solar UV-B light on in situ bacteria and
viruses appears to support the idea that long exposure times to solar UV-B
will harm these creatures. Most of the studies appear to observe lakes to
varying depths. Aerated water flowing over stones at higher elevations
should harm these pathogens more and sufficiently to make it safe. 

3,600 seconds lapse in every hour. Thus when solar UV-B reaches its peak
around 5000 j/m2, an hourís exposure will create a dose of just over 30,000
mw-s/cm2 of UV-B radiation. Human skin will likely burn within 15-30
minutes at this dose.



What is necessary in a UV-C lamp to kill pathogens:

From http://www.sparklingwaters.com/UVarticlef.html 

Due to individual cell makeup, different levels of UV energy are required
for destruction. UV lamps emit about 90% of their radiated energy at 253.7
nm, which, by coincidence, is very close to the peak germicidal
effectiveness of 265 nm.

The degree of microbial destruction is a product of both time, which is the
actual residence, or contact time the water is within the sterilization
chamber, and intensity, which is the amount of energy per unit area
(calculated by dividing the output in watts by the surface area of the
lamp). This product of intensity and time is known as the Dose and is
expressed in micro watt seconds per centimeter squared (micro w sec/cm1).

DOSE = output (watts) x time (sec)
------------------------------------------
area (cm1) = micro W sec / cm1

Microorganism Destruction Levels
(Ultraviolet energy at 253.7 nm wavelength required for 99.9% destruction
of various microorganisms - in micro w sec/cm2)
Bacillus anthracis	8,700 	Shigella dysentariae (dysentery) 	4,200 	
Corynebacterium diphtheria 	6,500 	Shigella flexneri (dysentery) 	3,400 	
Dysentary bacilli (diarrhea) 	4,200 	Staphylococcus epidermidis 	5,800 	
Esoherichia coil (diarrhea) 	7,000 	Streptococcus faecaelis 	10,000 	
L.gionelia pneumophlila 	3,800 	Vibro commo (cholera) 	6,500 	
Mycobacterium, tuberculosis 	10,000 	Bacteriophage (E. coli) 	6,500 	
Pseudomonas aeruginom 	31,900 	Hepatitis 	8,000 	
Salmonella (food poisoning) 	10,000 	Influenza 	6,800 	
Salmonella pamfyphi (enteric fever) 	6,100 	Polloviws (poliomyelitis) 
7,000 	
Salmonella typhosa (typhoid fever) 	7,000 	Baker's yeast 	8,800 	

steripen specs: see http://www.hydro-photon.com/hydropgs/techpgs.html 

Battery - AA size lithium-metal, 3.2 volts, 900mA hrs., rechargable, 250+
charge cycles.

Lamp Assembly - Lamp body and outer jacket are made from UV grade fused
silica (quartz). lamp assembly is .51" diameter and 1.925" in overall
length.

Lamp Output - Lamp is a 5 watt low pressure fluorescent with a primary
UV-"C" output of 254 nano-meters.

Electronics - Circuit provides 850 VAC start-up voltage and 270 VAC
operating voltage. Nominal operating current is 16mA. An ultra-low power
micro-controller provides features including: automatic variable on/off
timing, low voltage warning, low voltage shut-down, and "child-safe"
switching.

Battery Charging - The LT1510CS on board charging IC is designed
specifically for use with Lithium batteries. The LT1510C allows for DC
input voltages between 8 and 29 VDC.

We donít need to believe the specs to see why the steripen should sterilize
a cup of water in 30 seconds.
Iím willing to guess, but not bet, that the steripen inventor called up
this company to design his own lamp for the steripen:
http://www.uvp.com/html/light_sources.html. Doubtlessly if he didnít call
this one he called a similar one.

Go elsewhere at this companyís web site to find the methods it uses to
determine the bulbís out put. You wonít find the steri-penís lamp in its
stock inventory but you can review the spec sheets for its stock inventory
and will find that these lamps send 90% of their output in the 254nm
wavelength, so they are very well tuned lamps. The company measures the
output intensity as follows: The output is measured at a distance of 0.75"
(19.1mm). Emission intensities follow the inverse square law. Thus, UV-C
light output from the pen should form a cylinder measuring 1.5 inches (+
what the lamp displaces) in diameter producing the intensity specs
indicated. I canít tell from the spec sheet how long the cylinder might be,
but it claims nearly 2 inches. 12 oz = your basic coke can which measures
approx 2.5 inches in diameter 5 inches tall.

If you assume this tuned lamp produces only 25% of its claimed 5 watts at
the 254 nm wave length, you have 1,250 mw-s/cm^2 to swish around in a
volume of clear water not a whole lot larger than the pen itself. Do it for
30 seconds and youíve doubtlessly exposed whatever is in that water to
37,500 dose units. Pseudomonas aeruginom requires a dose of 31,500 to knock
out.

If the lamp operates as this manufacturer claims it should, at 90%
efficiency, youíve got real over-kill going on: 135,000 of the dose units
@30 seconds.

The other thing to notice when comparing solar radiation to UV-C lamps:
these relatively low power lamps produce significantly more UV-C than ever
reaches the ground from the sun; the less germicidal UV-A & -B measured for
sunburn purposes by meteorologists produces significantly less power than
the steri pen lamp which would appear, given the inverse square law, quite
safe for anyone to use.

If Iíve missed something here, please explain it to me. But it seems to me
that these estimates Ė and thatís all they are Ė would explain why the
steripen should work to sterilize water, and why we can in some places Iíve
previously described assume that solar UV has purified a mountain streamís
water.


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