Andy Hipkiss

Question for you ....

I was just doing a bit of surfing and noticed in an FAQ that a skimmer should be adjusted for dry foam. Fine, conventional wisdom there, but it occurred to me that I have no idea *why* it is more desirable.

The downside of wet foam is the need to top up salinity more frequently than dry foam, but apart from that I can't come up with reasons I'm comfortable with.

Maybe,

1. excessive removal of xxxxxxx .... but then feeding more is probably a better idea anyway.
2. greater potential of overflow when using epoxy ... reasonable I guess
3. the real reason????

Anybody help me out here ...?

Ken Stockman

I use a large counter current airstone skimmer (4" in diameter and 7 feet tall) and I produce about 1-2 gallons of skimmate per week. It all depends on how the air is adjusted. I have noticed no ill effects. I think that it is a concentration thing. When I produce really wet stuff, I can fill the collection cup (1/2 gallon) in a couple of hours. In essence, I have filled the cup but have not taken a lot of organics with it. For me it is a balance. I try fill the collection cup every 2 days or so. That produces really smelly stuff.

Henry Martinez

I have a large diy skimmer (5 feet tall --6 inches in diameter) The air imput is via a beckett pond head so there is potential for lots of air input. I have the collection cup hooked to a 5 gal bucket which fills every 4 days or so. Many would say this is overskimming. I look at what collects in the bucket and would rather have it in the bucket than in the tank. It may not be thick foam but it looks gunky none the less. This means I have to top off more often and keep an eye on the salinity but so be it. The inhabitants of my 220 gal dont seem to mind the results.

Andy Hipkiss

Thanks for your thoughts guys.

So what you are both saying (I think!) is that the desirability of dry foam is over stated (the point about rather having a weak skimmate with low concentration of organics is better than the organics being in the tank is particularly important I believe).

As I originally posted, this kinda fits in my original thoughts. But then why do we see in almost every book and article on the subject that wet foam is bad. OK then, so there's a case for proposing that most books/articles merely copy what has gone before and suddenly an "idea" becomes "fact", but is there more?

If anybody else can help me out?

Donna Stimetz

You're right. It's really the concentration of removed organics in wet foam versus dry. With wet foam you are removing MUCH FEWER organics and a lot more plain old tank water, so in essence the skimmer is not really working efficiently and you're not pulling out enough of what you bought the darn thing for in the first place.

William Zech

Sounds like you have done measurements or have access to measurements that will show wet foam has "much fewer organics". I for one would like to see this documentation/test results. Please share the site were this maybe found.

IMO the amount of organics will be around the same for both wet or dry foam over a set time period. The concentration will be greater with dry only because there is less water. I think the problem will be in salinity changes and the possible change/loss of trace elements with wet foam. Again related to more water leaving the system. The other negative wouldbe in monitoring the waste collection cup, which will fill much faster with wet foam. Again, this is my opinion, I for one have done no measurements to verify this assumption.

Donna Stimetz

No I haven't done any tests on this, but I will tell you is how a skimmer works this is from TRA vol, 1, "At the top of the foam riser tube, the heavy concentration of surfactants collecting there causes the bubbles to group together and become larger. As this top layber rises and collapses on itself, excess water drains back downwards, allowing the formation of thick, stable, dry foam on top of a wetter foam layer. Wilkens (1973) discusses two basic types of foam that develop in effective skimmers. The first layer of wet foam is referred to as "standard" scum and the second as "protein" scum. It is the second layer, the protein scum, that concentrates harmful organic substances from solution."

So if you foam is constantly wet - you're not pulling out the amount of organics that you should be.

yz183@yahoo.com

Well I would have to agree that dry foam probly pulls more organics just from my own observations. When I pull wet foam it's like the tea colored water in the cup and I have to empty the cup twice a day (small cup I guess) If I pull dry foam it goes into the cup as foam and after a little it turns to liquid and is like a dark green/black color. I'm probly pulling a lot of plankton with the dry foam but I have a light bio-load also so there probly ain't alot to remove. I would try messing with the air and water and see what works best for you. I found that the more water I pull through the counter current the more dry stuff it produces the less water goes through it the more wet foam it pulls.

Dallas Warren

With wet foam you are removing MUCH FEWER organics and a lot more plain old tank water, so in essence the skimmer is not really working

Be careful how you are quantifying things here. Are you talking about total amount of organic material that has collected in the collection cup in a given time period, or the concentration of the organic in the liquid contained in the collection cup? They are two very different things.

I personally go for a skimate that is a medium tea colour and keep the skimmer foaming constantly. Any darker and you don't actually collect very much, it forms as a scum on the inside of the foam riser.

It also depends on how people measure how "dry" or "wet" to what everyone is talking about here. For me a dry foam is one that is very stable and you can pick it up on your hand and it wont collapse too fast. A wet foam on the other hand will quickly run off or through your hand, leaving only a few bubbles left as the others burst quickly. If then you make wet 10 and dry 1, I would say mine is around 6-7. Leaves a lot to personal interpretation though.

Scott Wilson

I class as dry when I can put my fingertip into the foam, remove it and rub against another finger. If finger feels wet then foam is wet if finger feels dry is dry. Can still use your scale at a guess mine would on average be around 3 sometimes dropping to 2 and yes get a lot of gunk build up around the riser, fortunately this is part of collection cup and is easy cleaned when i empty the cup. Get about 250ml - 300ml every 3 days of dark thick liquid in the cup as well as the gunk in the riser.