UVC, Watts, Microwatts

 

Revised: 1/2/18

UVC, Watts, Microwatts, Joules, & Light Penetration

This article/post is intended to give some basic understanding between the relationship of watts, microwatts, joules of UVC energy and how this translates to UV Sterilizer effectiveness.

Please keep in mind that the diagram in this article is based on air penetration, so some extrapolation is necessary for use in water applications (which is the primary intention of this article, although the principles apply to UVC air sterilization devices as well)
UVC Air Sterilization Devices

What is a Watt/Microwatt?

One Joule of energy = 1,000 milliWatt seconds = 1,000,000 microWatt seconds
One joule is the amount of energy required to perform the following actions:

• The work done by a force of one newton traveling through a distance of one meter (a newton is the unit of force equal to the amount of force required to accelerate a mass of one kilogram at a rate of one meter per second per second);
• The work required to move an electric charge of one coulomb* (the amount of electric charge transported by a current of 1 ampere in 1 second) through an electrical potential difference of one volt; or one coulomb volt, with the symbol C•V;
• The work done to produce power of one watt continuously for one second; or one watt second (compare kilowatt hour), with the symbol W•s. Thus a kilowatt hour is 3,600,000 joules or 3.6 megajoules;
• The kinetic energy of a 2 kg mass moving at a velocity of 1 m/s. The energy is linear in the mass but quadratic in the velocity, being given by E = ½mv²;
we measure UV-C intensity in Micro-Watts that strike one square centimeter of surface area.
Reference: What is a coulomb?

---

ADVERTISEMENT:

---

UVC Penetration;
A quote from an advanced sterilization article:
UV Sterilization; How a UV Sterilizer Works

"The emission or light intensity of a UVC germicidal light bulb is usually expressed in a term called "microwatts per square centimeter" (Mw/cm2). The maximum intensity provided by a single UV-C Bulb is at its surface.
So, if we calculate the surface area of the UVC lamp and only use that area which effectively emits UVC light rays, the effective area of UVC transmission will be established. Basic mathematics will show that the surface area of a cylindrical tube is ‘pie’ D L.

Next extrapolate this effective area of UVC transmission as having a screen with squares 1 centimeter in size. Each of these cm2 areas now, for measurement purposes, emits a UVC lamp intensity measured in microwatts, in other words; the term microwatts/cm2. UVC light intensity decreasingly varies as the distance from the UVC light increases.

Put more simply (a non scientific analogy); The amount of wattage will also increase penetration, as a higher watt UV-C bulb will generally have more Mw/cm2."
See this product link for high output Straight Tube UV Bulbs:
Premium HO Straight Tube UV Lamps

"In my own experiments I have used 15 watt and 25 watt UVC bulbs in exactly the same unit (both were 18”), if wattage were only considered there would be a 60% increase in effectiveness, however I only observed a about a 25% increase.
When I used a 30 Watt UVC bulb in a unit with over twice the exposure as the 15 Watt, the kill rate more than doubled. From my experience, if you increase wattage (and Mw/cm2) you need to also increase the volume of water to maximize the higher watt bulb.
Experiments can also be safely conducted with standard household light bulbs to correlate penetration. For this start with a 7 watt clear bulb (such as a Christmas bulb) and place varying thicknesses of paper/ cardboard in front of the bulb and measure when penetration stops. Continue this with higher and higher wattage bulbs."

The Diagram to the left can give a rough comparison of distance as per UVC energy as expressed by MW/cm2 in Air transmission.
The dose applied by an UV-C lamp installation is a function of the lamp output, the intensity factor, and time. As an equation; Intensity x Exposure time= microwatt seconds/cm2.

As an example, a 9 watt UVC lamp at one inch from the lamp is found by this formula:
9 x 127 = 1143 mW/cm2.
Since many bacteria such as Vibrio require a UVC exposure of 6500 mW/cm2 or more, this means an exposure time of 5.68 seconds is required to kill this pathogen


Now let me point out that even though I have published this diagram, please use this as a rough guide only, as I have found inaccuracies in it. To be more blunt; I have found the distance, wattage, and flow rate to be the MOST IMPORTANT factors in determining exposure/effectiveness. This diagram is STATIC and does NOT take into consideration the dynamics of UVC radiation penetration for which I have yet to find a good formula to demonstrate this (even in University studies).

What is often missing in any equations I have seen is the dynamics of water flow geometry, actual water flow, and wattage. The bottom line is to use this table and others you might find elsewhere with “a grain of salt” noting that these are static and even then are flawed when true output via wattage is taken into consideration.

Further Reading, References, Product Resources:

*UV Sterilization; How it works for Aquariums, Ponds, & more

*Harmful Pathogens: The Threat, mJ/cm2

*http://www.solartanic.com/index_022.htm


UV Bulb; Replacement Lamps
The very best UV Replacement Lamps/Bulbs at competitive prices!

Aquarium UV Sterilizers; Misinformation

Updated 7/8/23 (Original Article, April of 2007)

I continue to follow the threads on forums, aquarium articles, and blogs about UV Sterilization.
It still amazes me how much misinformation is spread about this, some masked in science. As I have stated many times before, a UV Sterilizer is not essential for any aquarium, however even for goldfish I have maintained tanks with true UVs, and many other variables, I have noted a differences because of the sterilizer (especially Category A UVs such as the TMC/AAP Vecton UV Sterilizers). Many times this has lead me to a thriving tanks.
For a better aquarium forum: Everything Aquatic, Aquarium Forum, pros that care

What I also find interesting, is that many of these same persons who trash UVs, will then state a protein skimmer as essential.
I have maintained tanks with and without Skimmers (again every other possible variable the same or as close as possible), and noted the changes in measurable parameters and found that yes they are useful, however often not by much and that Mud Filters, DIY DSB filters or similar can often replace them.

Now admittedly Protein Skimmer have come a long ways since I started in this business over in the late 1970s with vastly improved, more efficient, and with less hassles Protein Skimmer such as the TMC V2 Skim Protein Skimmers and similar high end skimmers.
My point is not to trash Protein Skimmers (which have improved), but to only point out the hypocrisy of many in the marine keeping hobby that will insist on a Protein Skimmer while trashing a UV Sterilizer, despite the science behind their use.

Back to the subject at hand, here is a thread I will comment on.
In this thread one person who offered proof (albeit not of the perfect scientific nature, but as good as it can often be within this hobby) was attacked dog pile style without other backing up their statements other than criticizing his methods:

Author #1:

“If you read some scientific documents about UV sterilization of water you will know they talk usually about a standard long, 40W mercury UV tube in a 3" diameter lamp and water passing at the speed about 500 gph or less. This kind of setup delivers dosage of about 18,000 mW-sec/cm2. In order to increase this dosage you need to lower the water flow.
For example, 250 gph flow will in above mentioned lamp would be exposed to the almost double dosage of 34,000 mW-sec/cm2. This dosage is enough to kill bacteria, yeast, some mold spores, viruses and microalgae. And some aquarium grade lamps achieve this effect with unicellular algea.
To kill protozoa (like a well known from school "Paramecium" or our ich-causing "Cryptocaryon irritans") you would have to increase the dosage to the range of 200,000 mW-sec/cm2 !!!!!
This would mean that if you had 3" diameter lab-grade UV lamp with a 40W mercury tube you could give it maximum water flow of 40 gph... How about that!”

My Comment:

First, most studies I have read show under 10,000 mW per se/cm2 for bacteria. (References below)

Second, I have read articles that have shown 30,000 + mW-sec/cm2 with 90 watt bulbs at a rate of 200 + gallons per MINUTE (which even I find hard to believe). My point is it often depends on whose data you look at.

Third, there is more to treatment of Cryptocaryon than just out-right sterilization (I would not want it that high in my reef tanks!!!), there is also the Redox potential which Sterilizers improve and also general over all water quality that is aided by the addition of an Ultraviolet Sterilizer.
Finally, each unit is different and there are many variables from age of the bulb (even a 3 month old bulb has lost 25%), to temperature, design of the unit, pre-filtration, and to flow rate which I have found to be best at 20 gph (or less for most good units in aquarium applications).
See: Filter Max Pre-Filters

References:

• UV Disinfection
• Triangular Wave
• UV sterilization

Author #2 (edited):
I started keeping marine fish in 1975. I first added a UV sterilizer to my tank about 1977 or '78. It was an 8 watt unit. In the early '90s, I added another. In 2005, I went back to an 8 watt unit.

Without a hospital tank, adding a new fish was always hazardous.
Back when I had no UV on the tank, once one of the fish started showing signs, the life expectancy of that fish was usually less than 3 days; they could not take the combined stress of the parasites and the copper treatment. Other fish in the tank would invariably show signs of infestation later, but would usually make it through.

Once the first sterilizer was added, the rate at which the disease spread decreased; my theory is that some of the parasites were being killed in their free-swimming stage.
After that, the first fish to show signs usually survived, and some of the other fish would never show any signs at all. Once I added the second sterilizer, I never lost a fish to Ich, though I did get the occasional infestation.
In short, there was a direct relationship to the number of watts of UV and the intensity and rate of spread of the disease.

Since I was not religious in changing the UV Bulbs (and the bulbs get less effective with time), I was also in a position to notice that an outbreak was worse when the sterilizer bulb was pretty old. Again, this didn't happen once I had two units running.
Here is a source for premium highest UVC output bulbs: Aquarium/Pond UVC Replacement Lamps

My Comment:

This person most likely had decent units with low flow rates, pre filtration, and useful discernment.
Without elaborating too much (my expanded articles have much about this), as stated earlier there are other aspects of UV Sterilizers such Redox Potential, this is not a hard test to perform; simply test the Redox without a UV and then run one for a week and test again, easy!
So many who trash persons like this have not considered all the variables if they have failed, myself I have seen UV Sterilizers not do a damn thing. These were often 8 watt units running off a sump with a flow rate of 800 gph, a calcified quartz sleeve and poor pre filtration. This is not to say a 8 watt unit (or less) cannot work, just not under those conditions.

• The "15 Watt Custom UV Sterilizer" I made has performed well in most every application I have placed it in.
• The standard TMC Vecton & Titan UV Sterilizer are the best when all types are considered, especially the ultra premium TMC Titan UV, which is far superior to every other brand at any price point when compared apples to apples.

What a UV Sterilizer is, is a tool for good aquarium husbandry along with water changes, filtration, protein skimmers and more. In many of my side by side applications with and without, the UV was just one more piece of the puzzle to improved water quality in let’s face it, an artificial environment (unless we want to kid ourselves).

I can think of one experiment in particular with several goldfish tanks at a large service customer of mine. All these tanks in particular had canister filters, cleaning and the same feeding schedule and bio load; the only difference was UV Sterilizers.
The difference in amount of diseased goldfish, growth and color was very noticeable (the vice president did not kno we were performing this experiment and even commented about why some tanks did better than others, after which she had us install UV Sterilizers on all).

Copyright; Carl Strohmeyer

Please read this article about UV Sterilization for a MUCH more in depth understanding of How a UV Sterilizer Truly works:

ULTRAVIOLET STERILIZATION (How it Works)