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Steve's DO bubble debate
http://byap.backyardmagazines.com/forum/viewtopic.php?f=1&t=1866		 Page 1 of 5
Author:  steve [ Jul 29th, '07, 20:51 ]
Post subject:  Steve's DO bubble debate
Was recently having a discussion with VB over the amount of (regurgitated) info on the web on how apparently most oxygen is dissolved from bubbles when they are either created or more popularly by the surface disturbace when they pop.

I personally don't believe this. I have seen all too often how the web is a collection of cut-and pastes copied over and over again.

I found this while looking up wheat grass once, the same spelling error was on about 6 different sites! :shock:

Anyway, here is a collection of quotes and references to the websites thay were taken from that i'm using to support my view that it is the size of the bubble and the rise time that governs how much O2 is dissolved.
-------------------------------------------------------------------------------------
The bubble size affects the oxygen transfer efficiency. Smaller bubbles have more surface area per unit volume. This provides more area through which oxygen can diffuse and thereby increase overall transfer efficiency.. Also, since fine bubbles provide larger total surface area, they create more friction and rise slower than coarse bubbles. The combination of more transfer area and a greater contact time enhances transfer efficiency.
http://www.lagoonsonline.com/finebubble.htm

Agitation is agitation and whether it is caused by bubbles or a power head,
the diffusion of gasses in and out of the water is a function of the
surface area exposed to the gas and the agitation in that surface. Aeration
adds the additional area of the bubble surface to the diffusion area.
Estimate the size of the bubbles and the number of bubbles that exist in
the bubble column at one time and figure out the surface area.
400 1/8 inch diameter bubbles have a surface area of 19.625 inches.
"Figures don't lie, but liars can figure!" Mark Twain
http://fins.actwin.com/aquatic-plants/m ... 00409.html

Major considerations in aeration.

1. The gentleness and direction of water flow
2. The size and amount of the air bubbles
3. The temperature of the water
http://www.arkansasstripers.com/oxygen-and-shad.htm

Coarse bubble diffusers typically provide half the mass transfer of oxygen as compared to fine bubble diffusers given the same air volume.
http://en.wikipedia.org/wiki/Coarse_bubble_diffusers

It is important to remember that pure compressed oxygen contains five times the oxygen content as air does. Thus, proportionately less (about 1/5 less) gas flows (bubbling rates) are needed from a pure oxygen supply, compared with an air supply, to adequately oxygenate water for fish.
Very small (micro-size) oxygen bubbles should dissolve faster than larger bubbles because of their greater surface to volume ratio
http://www.oxyedge-chum.com/diffuser,_o ... ubbles.htm

And from my best friend, the wiki ;)

On a given volume of air or liquid, the surface area changes proportionally with drop or bubble size, the very surface area where exchange can occur. Utilizing extremely small bubbles or drops increases the rate of gas transfer (aeration) due to the higher contact surface area.

http://en.wikipedia.org/wiki/Aeration
Author:  bundaberg kid [ Jul 29th, '07, 21:06 ]
Post subject: 
I wouldda thought that dissolving anything would relate to the size...ie smaller dissolves faster than bigger.......smaller is more easily dissolved...therefore an amount of O2 injected into water in a smaller bubble form would dissolve into the water better than the same amount of O2 injected in a bigger bubble size...mostly because it floats to the top and you lose it to atmosphere?
Author:  johnnie7au [ Jul 29th, '07, 21:24 ]
Post subject: 
So you are saying that it is more to do with the bubbles time spent rising upwards?

High surface area (small bubbles), high pressure (ie. injection at depth), low water temperature and maximum contact time would be the criteria IMHO.
Author:  steve [ Jul 29th, '07, 21:31 ]
Post subject: 
correct.

Small bubbles = high surface area
small bubbles = longer rise time = more contact time

am basically trying to argue the "aeration only happens because of the surface agitation caused by bubbles" statement.
Author:  Hex [ Jul 29th, '07, 21:42 ]
Post subject: 
Nice topic Steve
I wonder why everyone goes for bubbles?
I did a quick calculation based on the area of 400 x 1/8" bubbles and compared it to a 2m x 1m x 0.3m deep growbed filled with 10mm gravel
The surface area of the gravel in contact with the water during every fill cycle equates to at least 2000 sq ft!

You`d need to generate about 6 million bubbles in the same time frame..or at least 30 million in the case of EB`s 5 bed turnkey AP system :wink:

Just for the heck of it: To generate 6 million bubbles that size you`d need just over 5 litres of air.. EB`s system would need at least 30L.
Author:  aquamad [ Jul 29th, '07, 21:42 ]
Post subject: 
Quote:
"aeration only happens because of the surface agitation caused by bubbles" statement.

I would never believe that...
if that were the case then your water return would do the same as a bubbler... and I know this is not so just by observation of the barra in the one system - as soon as the bubbler was switched off (it delivered a MASS of very fine bubbles) the fish moved up toward the surface and towards the water returns - presumably for wate with a higher DO... As soon as the bubbler was again in action they roamed the tank and stayed at the bottom of the tank...
Author:  steve [ Jul 29th, '07, 21:56 ]
Post subject: 
Hex, you may have mis understood me :) i mean to argue AGAINST the "aeration only happens because of the surface agitation caused by bubbles" statement ;)
Author:  steve [ Jul 29th, '07, 21:57 ]
Post subject: 
BTW, i agree with the gravel beds supplying huge air exchange.
Author:  earthbound [ Jul 29th, '07, 22:01 ]
Post subject: 
I'd never heard of the "aeration only happens because of the surface agitation caused by bubbles" statement.

I guess I haven't done that much in the way of online research on the subject... Go the small bubbles...... :)
Author:  steve [ Jul 29th, '07, 22:03 ]
Post subject: 
to be fair i think the statement used is "most of the exchange happens.........."
Author:  RupertofOZ [ Jul 29th, '07, 22:04 ]
Post subject: 
Now I'mm going to have to retrace my steps and go back a couple of nights and find the article I read.....

Basically it was a reference to how fish could be "poisoned" by oxygen saturation.....

Caused by super-saturation of water by bubbles below a certain size.....

If I remember correctly the arguement went along the lines that the fish ingested some much oxygen into it's blood stream via the gills due to the small sized bubbles that then expanded out into the fishs blood stream....

An effect like the "bends" in divers

I kid you not.... will find the article tomorrow....
Author:  Hex [ Jul 29th, '07, 22:04 ]
Post subject: 
It`s sunday..i`m awake but the brain must be having a lay-in :wink:

Getting the same contact time during the dry phase of a FD cycle would be impossible with bubbles..unless you used a mile deep well for a fishtank perhaps :lol:
Author:  steve [ Jul 29th, '07, 22:06 ]
Post subject: 
Rupe, one of those links i posted refers to it. i read it a few nights ago too!

but they're talking about TIIIIIINY bubbles that turn your water milky!

and most of the problem is with too much nitrogen dissolving and the micro bubbles getting caught in the fish gills / eyes

and also when using pure o2 and elevated partial pressures
Author:  steve [ Jul 29th, '07, 22:08 ]
Post subject: 
http://www.oxyedge-chum.com/diffuser,_o ... ubbles.htm

there was another with pictures, but i've lost it
Author:  RupertofOZ [ Jul 29th, '07, 22:09 ]
Post subject: 
Ah, wondered where it had come up...... good you guys were quick... hadn't even finished editing :wink:
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