I have saved these basic rules and found them helpful over the years--
First rule: Flow rate
Flow rate through the skimmer. The formula for figureing it out is Hours=9.2 purity coefficient (which means that 99.9% of the water has been treated (gallons/gph) so this formula gives you an idea on how to see how many times a day the tank water runs through the skimmer in order to treat all the tank water completely. So an example of say my skimmer would be:I have a pump pushing 600 gph and my tank has about 1000 gallons of water and I want to have a treatment of 9.2 coefficient (which means 99.99% of the water is treated in the tank). Or (1000/600)9.2=15.3 So all my water will hae seen the skimmer every 15.3 hours. This formula works for all equipment like skimmers, uv, ozone and so on.
2nd Rule: Bombardment rate
Which is the number of times an assending air bubble hits a descending water drop with in the skimmer. This concept deals with the time the tank water spends in the skimmer and the diameter of the skimmer. Ok the formula for figuring this out is Bombardment rate (R) = the dwell time of the water/the dwell time of the air. So an example: my skimmer holds 30 gallons of water and is feed by that 600 gph so that means that the water completely passes through the skimmer every 180 seconds So my dwell time for the water is 180sec. The dwell time for air is a long formula (like about a page of math) and I go cross eyeed just looking at it so here is my simpler method. I turned the air pump super low so thier was only a few bubbles in the skimmer. With the bubbles coming in and hitting the water flow in the skimmer I timed it out at a very approximate but it came out to 12 seconds, so my bombardment rate is 180/12= 15 bombardment rate. 10 is concidered to be perfect.
Rule 3: Volume of air
This is the amount of air as compared to the amount of water in the skimmer. The forumla is that the air volume should be 20% of the total water volume. So if you have a skimmer that holds 5 gallons of water, you should only have 1 gallon of air in thier. If you have more air then this figure in your skimmer the bubbles will join togehter and be to large and thus will not hold as much of the protiens/organics on their surface. Also it will reduce your bombardment rate as thier wont be as many bubbles hitting the water moles. An easy way to figure out what your percentage is, is to run the skimmer normal with air being injected (or however you get air into the skimmer) and make a mark on the skimmer body with some tape. Then kill the air and make another mark, the water level should go down the cylinder by 20%
Rule 4: Configuration.
The injection points for both water and air are also critical for the skimmer to work efficiently. Example of this would be, if the water enters the top of the skimmer and exits the bottom and that your air enters the bottom and leave via the top, it would mean that all water must pass through the air bubbles in order to leave the skimmer. This method is the best. If the water and the air are injected at the same point, then your bombardment rate is reduced as the air bubble will travel with the same water mole all the way to its exit, thus it only hits a few water moles instead of many. Now I know some are going to get their undies in a not on this one (Skimmy I see that strained look ) So I will say that the water and air can be injected through the same point, but I have yet to seen the math to prove its not less effective or as good.
Rule 5: Contact time.
Since the function of a protien skimmer is to first mechanically strip the protien/organics from the water mole and then to chemically attract the protien/organic to the surface of the air bubble, it is important for the organic/protien to spend as much time in contact with the air bubbles. Now with some protiens it take a long time for them to actually bond to the surface of the bubbles, so the more the better. We have to remember that some protiens dont want to let go of their bond and thus some can take minutes to be mechanically seperated.
Ok those are the basic laws of skimming, but their are some other thoughts that should be concidered. A big one is the amount of violence with in the skimmer, if the water and air mixture is so violent, then the air bubbles will break and will thus release their chemical bond on the protien mole and put you back to square one. Or they will join together and form a bigger bubble which will then reduce the surface area available for protiens to attach.
Another is the what I call the freight train effect. This is where the [ump is so big and moves water through the skimmer so fast that it basically acts as a frieght train and takes much more out of your water then just the protiens we are looking to remove.