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Re: Proposed CO2 setup for equal addition of CO2 to multiple tanks

I apologize for the length of my reply in advance. I also apologize for
using so many words to say so little.

On Mon, 14 Dec 1998, James Purchase wrote:

> Eric Olson very kindly sent me photographs and a detailed explanation of his
> CO2 setup which feeds multiple tanks. However, Eric's setup is not
> necessarily designed (as I understand it) to provide an "equal" amount of
> CO2 to each tank. He uses individual ARO NO-1 Needle Valves on each line.
> This would allow him to fine tune (within the limits of the NO-1 valve) the
> amount of CO2 which enters each aquarium. For a regular setup, it looks to
> me to be very well designed and flexible.

I do not beleive there is *any* way to insure equal delivery of CO2 to
each tank by a means other than adjusting valves or relying on similarity
of manufacturing.  I'll say it all over this post. 

The line to each tank must have its flow independently reduced in order to
overcome the differences in backpressure of the water in each tank.

If you are using a traditional reactor (I like the term "counter-current"
that was used here today), each line must have a valve to accomplish this.

If you are using a sintered glass diffuser, my understanding (from Dave's
posts) is that the diffuser acts as the resistive element, producing the
equivalent flow of a bubble per second at 15 PSI... I don't have one, so
those of you in the know, please correct me here!

I think Steve posted a third possibility today with the daily manual
measured injection into inverted jars (aka the old "Tetra Bells" method).

> Dr. Dave suggested that I set up the system as a closed "loop", wherein an
> equal pressure of CO2 is provided to each tank. He described a system which
> he used during his PhD studies using just such a closed loop with air pumps
> to provide equal aeration to multiple tanks. Several people mentioned in
> archived postings of the APD that pressure and flow are not the same thing,
> but to my mind (again - if I'm mistaken, please jump in!), if I am using
> aquariums of equal size and reactors of the same type and size within each
> tank, an equal pressure of CO2 _should_ provide an equal amount of the gas
> to each tank. The diagram I have included with this posting is based upon my
> interpretation of this idea.

I've mentioned this in e-mail to James that I beleive this is not a
solution. Now I think I understand, Dr. Dave used this for AERATION
studies, where he was pushing air through the system at a rate fast
enought that it probably overcame the backpressure of the water. But in
the system we are describing here, we are trying to push a VERY SMALL
amount of CO2 through the system very slowly, so we have the back-pressure
from the water in the tanks contributing.

In the system I use at home, the needle valves and/or glass diffusers are
the resistive element for each branch (i.e. I have four needle valves).
There is no need for a special loop; it will not help at this low flow
rate.  However, there is no way getting around the problem that the
equality of dosing is going to be dependent on the similarity of the
needle valve settings (or the manufacturing repeatability of the
diffusers).  Read on...

> My only query here is the fact that the tanks will be on differnt levels (2
> tanks side by side over 2 tanks side by side). Would the height difference
> change the amount of CO2 which would flow to the tanks of different levels,
> or would equalizing the tubing length to each tank comensate for this
> difference in height???

If you have separate resistive elements (needle valve &/or diffuser) for
each tank, the height will not contribute effectively to the system,
because there is no way for the backpressure to push upstream of the
needle valve or diffuser.

But now, reading ahead...you are proposing

Cylinder -> Regulator -> Needle Valve -> Ring of T Valves each going to a

Oh my, this is the same trap I fell into when I first tried this...I was
too cheap to shell out $40 a pop for multiple Nupro valves... you are
proposing to use a SINGLE needle valve to reduce the flow and then hoping
it gets split evenly into each tank.  I can tell you from experience, this
just doesn't work.  It's going to be VERY dependent on water
back-pressure. At this low a flow rate, the tubing length and geometry
don't even ENTER the picture, the loop doesn't contribute either. The
bubbles will ultimately emerge from one tank only, the one which has the
least resistance.

Now, that said, I will add that the presence of the check valves and
T-valves will allow you to provide a little bit of resistance in each
branch, and possibly direct flow into more than one tank. But now you're
back to square one, except using cheap T-valves instead of the needle
valves... far less control than even a bank of NO-1's!

[And now some miscellaneous comments]

> From the various "needle valve" disucssions in the archive and on George
> Booth's site, I gather that the NO-1 is now obsolete and has been replaced
> by other models. 

I think this was posted by one person a few years ago, and never
confirmed. When I buy my valves (most recently about 6 months back), I
purchase the NO-1 (or its 1/4" NPT equivalent the NO-2).  The guy at the
fitting store says it's not obsolete, and restocks them often.

> Eric has suggested that I stick with (if I can get them) Swagelok "T"'s
> and their associated compression fittings. I have located several local
> suppliers of Swagelok valves (Swagelok's WWW site has lins to local dealers
> all over the place). Swagelok is apparently the owner/manufacturer of Nupro
> valves.

Basically, if you use the nupro valve, use the swagelok fittings --- they
go together.  If you use the NO1, use NPT fittings --- they go together
too.  You'll be able to buy the appropriate fittings where you get the

   - Erik

Erik Olson
erik at thekrib dot com