[OneDummHikk]

Adam, the difference the amount of water in the pipe makes is how hard it is on the pump to create the vacuum. Even though the pump has the same pressure on both sides (inlet and outlet), it has to create a difference in pressure in order to draw air. It is that ability/inability to create a difference in pressure.

Take a small powerhead (MJ1200) and put it 4" below the surface of a display tank. Put an air line in the inlet of the pump, it will pull in air. Slowly lower the pump and it will stop drawing air. The pressure is the same on both sides of the venturi since its in a closed type system yet it stops pulling air. Why? Even though the pump can move water under 10' of water pressure, it can't overcome that pressure to create the difference in pressure needed to pull in air.

It changes a little with a skimmer pump since the pump body is outside but the internals of the pump are still submersed. The difference is how far the vacuum has to pull air. If its in 10' of water, it has to overcome that 10' of water pressure to get air to the pump. When its out of water but still has the 10' of water pressure inside the pump, it only has to create a vaccuum.

As Jason said, the Reeflo Dart pump starts losing its ability to draw air at roughly 4' of head pressure so Reeflo says to use the Alita Air pump to get air into it. And that is at only 4' of pressure on a pretty stout pump. It can pump water against a lot more head pressure than that but can't pull air at that depth.

Jason, it may have an impact on Becketts but I don't know for sure. I do know MRC builds a skimmer that is 8' or so tall (commercial unit) and it runs Becketts. The beckett chamber is - typically - above the water level to help try to combat salt creep stopping up the air inlets. The issue may come after the air is in it and trying to get the air to go down the tube and into the skimmer and that will come with water speed more than the pressure. The faster the water moves, the better chance it has of getting it down the tube and into the body. Its the reverse of getting air bubbles in the drain. If you get air bubbles in the drain of a skimmer, the water is moving too fast, slow the water down, no air in the drain. The opposite is true as well. The faster the water, more air out the drain. If you think of the piping after the beckett as the "drain", then its a little easier to picture (to me anyway). The faster the water is moving, the better chance it has of "sucking" the air along with it. And that is where the pressure rated pump comes in.

Lots of tinkering I can do with it if I ever get where I can tinker again. Right now my back isn't allowing it

 

[three90s&125sump]

If I had a way to measure the air I would do a test with my skimmer. All I would have to do is put the other 2 foot tube on the skimmer and measure the differance. I'm sure there would be a differance. One of the laws of physics is that nothing can be lost or gained... Ever, only rearanged. My thinking on this is that if you removed the feed pump from the skimmer and had no flow through the skimmer and you turned on the recirculating pump with a 24inch tall body you would have X percent efficiency. If you add another 24inches of body to it you would have X+ percent efficiency. You would have a gain in efficiency but when you have a gain you have to have a loss somewhere. Since we know its not going to be the water flow through the recirc pump the only other variable would be the air intake. I know its not perfect but I don't have any degrees.

How ever I do know this... Its not nessessarily the speed of the air over the wing that causes lift, it is the differance in the density of the air on top of the wing verses the air on the bottom of the wing. As the air on top of the wing speeds up to cover the larger distance it has to cover to pass the top of the wing its density greatly drops while the air on the bottom of the wings density remains the same. Because of the drop in density above the wing and the same density under the wing there becomes a vacuum above the wing causing the denser air below the wing to push the wing up.

This relates to the venturi on the skimmer in that it takes more vacuum to over come the head presure in the skimmer so the actual size of the air would remain the same while its density would be lower resulting in a reduction in the mass of the air entering the venturi. Once the air is removed from the vacuum its density will be dictated by the head in the skimmer.

Wow that hurt but I think its right.

 

[Alittlefishy]

Well, it seems I had a few errors...that MJ1200 got me remembering that one time I had a seaclone on my 30" deep 140 tank. I did not like seeing the pump, so I extended the connection to put the pump on the bottom of the tank, but it would not draw hardly any air.:nuts:
I did a bit of research and found that the pressure created by the water in the skimmer body does have an effect on the air being pulled in. I did find out why and it is this:

For the pump to move water with out the air venturi, all it has to do is create pressure on one side of the impeller and a vacuum on the other. And it does this by spinning the impeller. On the vacuum side, vacuum is relative to the pressure side, and this is where my earlier thoughts got misguided. That vacuum is not really a vacuum, but an area of pressure that is lower than the output. And contrary to my earlier post, that area of lower pressure can definately exceed the ambient air pressure that is used to get the air down the venturi and into the pump. So to conclude, a taller skimmer would adversely affect the amount of air being drawn in to the pump. Thanks for the guidance. Just remember that it doesn't suck, it aspirates.:nuts::nuts::nuts::nuts: Sorry, bad lab joke.

I am glad this was brought up, because I would've been uber pissed if I had built the monster 10' tall skimmer and it would not pull air!:thumbup: Please continue on

 

[OneDummHikk]

Jason, its right. We are saying the same thing, just different ways/analogies of trying to say it.

Adam, you are right about vacuum vs low pressure. When I refer to a vacuum in a pump it is relative. If the pressure on the inlet side is X and the pressure on the outlet is X/2, then to me there is a vacuum on the outlet and will "aspirate". But, it is the difference in pressure that causes the air to be pulled in and the height of the skimmer isn't what causes the issue. The issue is caused by water pressure. And the water pressure that you need to be concerned with is whats in the pump itself and on each side of the venturi valve.

Go back to your Seaclone. It pulled air when the pump was at the surface of the aquarium. Yet when you moved the pump to the bottom of the aquarium, it stopped pulling air. The volume of water in the skimmer didn't change. What changed was the head pressure on the pump itself and the water it was pumping against. But its still a little different there because you weren't in a recirc setup, it was pumping and pulling air/water against the head pressure of the water column in the tank itself. And that brings up an interesting point about it. That's why skimmer manufacturers have a "preferred water depth" is so that the pump has the amount of head pressure needed to pull air properly. It has nothing to do with water level in the skimmer, which many people wrongly assume is the reason for the water depth criteria.

Assume that you have two identical skimmers. Same pumps, sizes, everything is identical. Both are recirculating skimmers. You set them up in two different scenarios. One is setup where the pump is in 12" of water. The other is setup where the pump is in 8' of water. The recirc pumps, not the skimmers. Which one will pull more air? The water volume in both skimmer bodies will be the same. The "head pressure" from the skimmer in both situations will be the same. The only difference would be how much water it has to pull air through to get it to the pump. The result might surprise you.

 

[Alittlefishy]

Yeah, my example with the seaclone is off a little from our current discussion.

So from our discussion, the pump in 8 feet of water would pull less air, right??? because of the 8' of water over the pump???

 

[three90s&125sump]

Right but you could buy a high pressure air pump and force feed it.

 

[OneDummHikk]

Would it draw less air? Sketch a picture of both scenarios, then really think about it. How would the 8' of water add any pressure to the inside of the venturi where the low pressure area needs to be? And, how would the water above the pump play any part in the air if an airline went all the way to the surface of the water? It is all about the pressure inside the venturi, inside the pump, and very little to do with whats going on outside the pump unless it has an effect on whats going on inside the pump.

Water depth outside the skimmer is totally different than water depth inside the skimmer.

 

[Alittlefishy]

Randy, So any water outside of the recirc loop and skimmer has no effect on the air draw, but the more water in the skimmer that's above the recirc pump does adversely effect the air draw.

Another ?...so why do "they" say that skimmers work better in 6-8" of water?

Jason, I have an Aqualifter pump if I did have to inject air, but I would rather not since that is just one more part to potentially break

 

[OneDummHikk]

Randy, So any water outside of the recirc loop and skimmer has no effect on the air draw, but the more water in the skimmer that's above the recirc pump does adversely effect the air draw.

With regards to the recirculation pump, yes. Not for the feed pump though. It is affected by both the water column in the skimmer and the sump.

What matters is the depth of the water column(s) that the pump is operating in. Whether that column of water is a single column in the skimmer or its a dual column - one in the skimmer and one in the sump.

Then think about an external pump setup

Another ?...so why do "they" say that skimmers work better in 6-8" of water?

If you look at them, most of what "they" say is about non-recirculating skimmers. Then when it gets to a re-circ, its easier to give you a depth of water than it is to try to tell you that it doesn't matter. Or, they simply don't know it doesn't matter.

In addition to why they say that, I have been trying to figure out how they rate a skimmer and tell you what size tank it can handle. Especially when the tank size has absolutely nothing to do with a skimmers capacity

Jason, I have an Aqualifter pump if I did have to inject air, but I would rather not since that is just one more part to potentially break

If you don't exceed a 30" body, which is very doubtful without a wet room, then you shouldn't have an issue pulling air into the skimmer.

 

[Jadinop]

With a recirc skimmer the depth of water affects performance but IME it is not affected by small changes in water levels like a non-recirc skimmer can be. I run one of my G3's recirc'd and since I don't have my ATO running yet the sump water level varys slightly by an inch or two, maybe even 3" sometimes. A recirc skimmer is not as sensitve to the small variations in sump water levels and maintains the same internal water level regardless.

In a recirc skimmer, I would say that water level is a determining performance factor but it's just not as important in maintaining consistent performance like it is on other types of skimmers.

 

[three90s&125sump]

I put the extra 2 foot on my skimmer and I don't need an air meter to tell there is a differance. It makes a huge differance in the amount of air. In light of this I am going to redesign the lower part of the skimmer.

 

[OneDummHikk]

Jason, is yours runnin recirc or normal operation?

 

[Alittlefishy]

Jason, can we get a pic? You say you are going to revise the bottom section, does that mean you will move the recirc pump higher up the tower?

 

[OneDummHikk]

Jason, can we get a pic?

http://www.thereeftank.com/forums/f6/pump-test-on-a-little-diy-skimmer-127587.html#post1492478

From the pics, it isn't recirc although there may be some hidden plumbing that is hiding the recirc aspects.

 

[Alittlefishy]

OIC very good then, cant wait to see it in action.

 

[three90s&125sump]

It is a recirc. Its being fed with a mag 12 right now. The orca pump is recirculating at a rate of about 2600GPH and the mag 12 is doing about 1000GPH. I did all the mods I'm going to do to it right now. I didn't have any pvc so I couldn't raise the pump up any to see if that will help any with the higher head set up. I might do it this week tho.

 

[OneDummHikk]

I missed the plumbing for the Mag12. I am on my laptop and its a bear to type on (I hate it) so I will get on my PC in a little while. I have some thoughts/info on feed pumps/recirc pumps and the effects they can have on one another. Yes, they are related and have an impact on one another. You didn't think skimmers were simple did you? Heck, all they are is a "tube full of water and bubbles"

 

[Alittlefishy]

SOOOO, to revive this discussion, if I had a 6" skimmer with a 24" tall reaction chamber, and I wanted it to be recirc, can the recirc pump be too big?

ie a sedra 5000 on the ASM G-3 and G-4 vs the sedra 9000 on the G-4x. The G-3 has the 6" body and the G-4's both have an 8" body, but the G-4x is about 6" taller. I use the ASM only as an example of pump size vs skimmer reaction chamber size.

The other side of this is that IF I use a sedra 5000, it gets about 700lph on average and that corresponds to a 3.5" neck and the sedra 9000 is around 100lph and would let me use a 4" neck. The 4" could be cool cause it would only use one adapter for the neck. On the other hand, if I stick with the sedra 5000, it would be overkill for a 3" neck and I could use that 4way drain on the randystacie link I posted.

 

[OneDummHikk]

SOOOO, to revive this discussion, if I had a 6" skimmer with a 24" tall reaction chamber, and I wanted it to be recirc, can the recirc pump be too big?

Yes, it can be too big. The neck size helps determine the recirc pump size. Unless the feed pump is pumping air as well (odd idea), then the recirc pump is what is injecting the air and airflow is what determines neck size. Oversized pump = too much air = poorly performing skimmer.

ie a sedra 5000 on the ASM G-3 and G-4 vs the sedra 9000 on the G-4x. The G-3 has the 6" body and the G-4's both have an 8" body, but the G-4x is about 6" taller. I use the ASM only as an example of pump size vs skimmer reaction chamber size.

Adding 6" to the body height helps a little but it won't change the air draw. It helps in recirc mode a little. It does let you run a larger pump but you lose some of that pump due to head pressure.

The other side of this is that IF I use a sedra 5000, it gets about 700lph on average and that corresponds to a 3.5" neck and the sedra 9000 is around 100lph and would let me use a 4" neck. The 4" could be cool cause it would only use one adapter for the neck. On the other hand, if I stick with the sedra 5000, it would be overkill for a 3" neck and I could use that 4way drain on the randystacie link I posted.

Use the sedra 5000 on a 4" neck. You would rather have the neck a little larger than a little smaller. Smaller hurts worse than it being a little too large. And, if you make it a little larger, it gives you room for improvement by tinkering with the pump.

 

[Alittlefishy]

so a sedra 5000 on a 6"x24" body. cool I need to get some 3" clear pipe for the neck just to make tuning easier. I have a question about uniseals are they pretty leakproof? like for an external skimmer? I haven't decided whether I want it in the sump or not.