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UV Comparison.com, your comparison source for factual pond, water garden and aquarium uv equipment information.
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| Ultraviolet
Sterilization is unmatched in its efficiency, simplicity, and dependability when applied as a microorganism disinfectant. UV sterilization is a proven solution to waterborne planktonic algae as well as other harmful pathogen problems. Certain critical UV performance factors greatly affect all UV sterilizers, no matter who's the manufacturer. The information contained in this outline should be considered before purchasing any UV equipment
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Factors
Influencing UV Effectiveness
Whether you choose to label a UV as a clarifier or a sterilizer,
the same design, performance, and operating principals apply.
Successful UV operation destroys the targeted microorganism.
Here are five main factors that will help determine the ability
of a UV sterilizer (or clarifier) to achieve this desired effect.
1. The type of lamp used in the application. (low-pressure or
medium/high-pressure)
2. The length of the lamp being used. (the ARC Length)
3. The physical design of the UV's water exposure chamber.
4. The condition of the water being treated.
5. The water flow rate through the UV's exposure chamber
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The
Science of UV
Let's start at the beginning. Ultraviolet light is a spectrum
of light just below the range visible to the human eye (below
the blue spectrum of visible light in the chart above). UV light
is divided into four distinct spectral areas-Vacuum UV (100
to 200 nanometers), UV-C (200 to 280 nanometers), UV-B (280
to 315 nanometers), and UV-A (315 to 400 nanometers). The UV-C
spectrum (200 to 280 nanometers) is the most lethal range of
wavelengths for microorganisms. This range, with 264 nanometers
being the peak germicidal wavelength, is known as the Germicidal
Spectrum.
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The
Targeted Microorganism
It is critical to first identify the microorganism. Each type
of microorganism requires a specific UV-C radiation exposure
rate to successfully complete the disinfection process. The
targeted microorganism must be directly exposed to the UV-C
radiation long enough for the radiation to penetrate the microorganism's
cell wall. However, it takes only seconds for UV-C light rays
to inactivate waterborne microorganisms by breaking through
the microorganism's cell wall and disrupting their DNA. This
often totally destroys the organism, or at the very least will
impair its ability to reproduce. |
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The
UV Lamp… the Source of UV
UV light
sources primarily come as low-pressure or medium/high-pressure
lamps. Low-pressure lamps produce virtually all of their UV
output at a wavelength of 254 nanometers-very close to the
peak germicidal effectiveness curve of 264 nanometers. These
lamps generally convert up to 38% of their input watts into
usable UV-C watts. This is much higher than other classes
of lamps. (i.e. a 150-watt low-pressure lamp will have approximately
57-watts of UV-C power.) Low-pressure lamps typically run
on low-input power currents of 200 to 1,500 milliamps and
operate at temperatures between 100 and 200 degrees Fahrenheit.
They have a useful life of 8,000 to 12,000 hours depending
on the operating current of the lamp. |
Medium/high-pressure
lamps produce wavelengths widely ranging from 100 nanometers
to greater than 700 nanometers, well into the visible light
spectrum. These lamps are very poor producers of usable germicidal
wavelengths; they generally convert only up to 8% of their input
watts into usable UV-C watts. (i.e. a 400-watt medium-pressure
lamp will have approximately 32-watts of UV-C power. The remaining
368-watts are converted into heat and visible light.) Medium/high-pressure
lamps typically run on high-input power currents of 2,000 to
10,000 milliamps and operate at temperatures between 932 and
1,112 degrees Fahrenheit. They have a useful life of only 1,000
to 2,000 hours depending on the lamp's operating current. As
you can see from these comparisons, low-pressure lamps perform
safely and efficiently. They are the better option for use in
UV sterilization.
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| Low-pressure
UV lamps come in many different styles and lengths. As
a general rule, the longer the lamp, the greater amount
of UV the water will receive because it will be exposed
to the UV source for a longer period of time. |
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UV
Lamp Length + UV-C Output + Useful Lamp Life = Lamp Value
• UV lamp length is a critical performance factor
that helps establish UV exposure.
• Evaluating UV lamp performance based on input
watts is inaccurate! The "Input -vs- UV-C output
Watts Chart above demonstrates the poor germicidal value
of med-pressure lamps compared to low-pressure type UV
lamps. Low-Pressure UV lamps convert approximately 38%
of their input watts into UV-C output watts while the
medium-pressure UV lamps convert 8%. Low-pressure style
UV lamps offer greater germicidal value than medium pressure
lamps for this reason.
• Knowing when to replace UV lamps is critical to
achieving a consistent UV disinfection dose, but not all
lamps offer the same useful operating life! |
| Design
of the Water Exposure Chamber |
The
design of the water exposure chamber is completely overlooked
by some manufacturers, but it is key to successful operation.
The distance UV light energy has to travel from the surface
of the lamp to the inner wall of the UV's water containment
vessel determines how much UV the water will receive.
This is known as the "UV dose rate." The amount
of water passing through the UV filter ultimately determines
the unit's actual UV dose rate, which is expressed in
microwatt's per second per square centimeter or (u-watts-sec/cm2).
When selecting a UV Sterilizer for your application:
• Make sure the UV lamp is positioned between the
water inlet and outlet ports of the unit's water containment
vessel. Any portion of the UV lamp(s) not located between
the water ports is useless. When calculating the UV's
performance data, only the ARC length located between
the water ports can be applied to the calculation, reducing
its capabilities if portions of the lamp are not between
the ports.
• Select the unit with the largest diameter water
containment vessel in the wattage you are considering.
A unit with a larger diameter will always have a greater
contact time. (For example, a 25-watt model with a 3"
diameter housing will flow more water than a 2" housing
model.) |
Single UV Lamp Array Diagram |
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Multiple UV Lamp Array Diagram |
Open-Channel UV Lamp Array Diagram |
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Make sure the unit you are considering uses a quartz sleeve.
A quartz sleeve isolates the UV lamp from the water to
avoid a short circuit path for the lamp's electrical power.
It also allows the lamp to operate at its optimum temperature
by acting as an insulator.
• Does the manufacturer list water flow rates at
the end of a lamp's life or the beginning? Most UV manufacturers
give a water flow rate, but do not indicate whether it
applies to a new lamp or to one that is at the end of
its useful life. Try to find a manufacturer that includes
the water flow rate in the unit's end of lamp life rating.
The end of lamp life rating takes into account the lamp
losing UV-C output due to age so it is a more realistic
prediction of how the unit will perform.
• Do the manufacturer's water flow rates account
for the reduced effectiveness UV light has when treating
green water? This information should be listed as some
type of percent transmissibility rate or absorption coefficient
(decimal value). Units that account for green water will
have lower water flow rates. |
| UV
transmittance is also largely overlooked, but it is one
of the most critical factors in determining the ability
of a UV sterilizer to treat a given volume of water. Regardless
of the type of UV light source used, any body of water
containing impurities will adsorb UV energy. Green water,
water plagued by algae and microorganisms, will absorb
the UV energy emitted by our UV light source in proportion
to its density (or how green the water is). The greater
the amount of algae in the water, the more of a reduction
in percent transmittance. Percent transmittance is the
ability of a body of water to be effectively treated by
a UV light source. This value indicates the quality of
the water being treated. The higher the percent transmittance,
the easier the UV sterilizer will be able to treat the
water at a given flow rate. A lower percent transmittance
means the UV sterilizer will be less effective in dealing
with the algae problem. If the sterilizer's water flow
rates have not been calculated with a reduced percent
transmittance rate, the unit will have considerable trouble
dealing with an algae bloom. |
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| Water
flow rate through the UV's contact chamber |
| A
sound UV sterilizer design revolves around the careful
selection of lamp type, lamp length, lamp position, and
body diameter. These factors, together with the intended
water flow rate, percent transmittance of the water to
be treated, and UV dose rate needed to kill the targeted
microorganism should be your basis for the selection of
a unit for your pond. When researching which type of UV
sterilizer to purchase, remember the criteria laid out
in this article, read the manufacturer's literature, ask
questions, and most of all ask yourself, does this information
make sense to me? If not, consider another UV manufacturer. |
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