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CO2 Loss Rates

Contents:

  1. CO2 Lossage Tests
    by George Booth
  2. DIY-CO2 and "Tetra Bells"
    by gb-at-dixie.cs.unc.edu (Gary Bishop) (27 Jan 94)
  3. DIY-CO2 and "Tetra Bells"
    by steveb-at-chelm.uucp (Steve Benz) (27 Jan 1994)
  4. DIY-CO2 and "Tetra Bells"
    by niels-at-lofgren.demon.co.uk ("Niels M. Sampath") (Fri, 28 Jan 1994)
  5. DIY-CO2 and "Tetra Bells"
    by booth-at-hplvec.LVLD.HP.COM (George Booth) (Mon, 31 Jan 1994)
  6. CO2 and anabantoids
    by Hoa Nguyen <nguyenh/nosc.mil> (Wed, 27 Oct 1999)
  7. CO2 rich layer above water
    by Zxcvbob/aol.com (Wed, 27 Oct 1999)

George's CO2-Loss Test

The test tank was a "90 gallon" glass tank (48"x18"x24" tall, 79
gallons of water).  It was set up with Dupla heating coils and a Dupla
"DuplaTherm" temperature monitor/controller to maintain the water 
temperature at 79.0 +/- 0.1 degrees F.  A Dupla Reactor "S" was used
to inject CO2 via the sump of a trickle filter.

CO2 concentrations and pH were measured with LaMotte test kits.  Note
that the LaMotte CO2 test kit has a resolution of 1 ppm (mg/l) and an
error of about +/- 2 ppm.  The LaMotte pH test kit has a resolution of
0.1 pH units and uses an "octet color comparator".  The pH error is
about 0.05 pH units based on comparisons with a Sandpoint II pH/ORP
controller and comparisons with a pH/KH/CO2 table.

An AquaClear 802 powerhead was used to circulate water in the tank.
It was placed near the bottom in three tests to provide a gentle
circulation current with little surface turbulence.  A 0.3 ft/sec
surface current was noted, giving a smooth surface pattern that looked
like "heat waves" rising off a highway in the summer.  In a forth
test, the powerhead was placed at the surface and was adjusted to give
vigorous ripples without splashing.

The trickle filter used was an Amiracle "100 gallon" unit with a
bio-media capacity of 3.99 gallons.  The media space is 16.125" long x
7.625" wide x 7.5" high.  The media used was 238 Dupla BioKascade
bio-balls, with the internal slats arranged roughly horizontally to
allow the water to move through the media in a gentle, cascading
manner.

The filtered water is circulated by a Quiet One pump controlled by
ball valves to provide a 400 gallon per hour flow, turning over the
tank five times per hour.  The trickle filter has two water returns.
One is directed across the bottom 1/3 of the tank, providing a flow at
what will eventually be the top of the gravel.  The other return
utilizes a Magnum 330 water return fitting.  To provide surface
turbulence, a Magnum diffuser was used to direct the return flow
across the surface, producing ripples equivalent to the powerhead when
placed at the surface.  For tests without turbulence, the diffuser was
removed, allowing the water to be directed towards the bottom of the
tank.

The tank is bare except for the equipment mentioned - no gravel, no
livestock, no plants.  Lighting is room ambient.  The top is open.

Before we turned on any equipment, we filled the tank with tap water
and adjusted the water hardness by adding 3 tablespoons of calcium
carbonate (CaCO3) and 1 tablespoon of sodium bicarbonate (NaHCO3) to
achieve a GH of 3.5-4 degrees and KH of 7 degrees as measured by a
Tetra test kit.  Note that GH has no bearing on the CO2 measurements,
but a KH of over 3.5 is needed to ensure accurate CO2 test kit
readings.  A KH of 7 was selected to keep pH readings in the range of
the LaMotte test kit.

After letting the water equilibrate for one day we measured dissolved
CO2 at 2-3 ppm.  We then set up a large powerhead to circulate the
water (Project RS-500, ~500 gph) and let it run for a day.  The CO2
remained about 2-3 ppm.  At the end of most of the tests, CO2 again
measured about 2-3 ppm, indicating that this was the equilibrium value
for the experimental conditions (note that the altitude was 5000 feet
above sea level).

After the initial tests, the heating coils, trickle filter and CO2
injection were set up.  The large powerhead was replaced with the
AquaClear 802.  For the first test ("trickle, turbulence"), the
trickle filter was run with the Magnum diffuser producing surface
turbulence and with the powerhead running at the bottom.  For the
second test ("powerhead, turbulence"), the filter was turned off and
the powerhead was placed near the surface.  For the third test
("trickle, quiet"), the trickle filter was run without the diffuser
and with the powerhead running at the bottom.  For the last test
("powerhead, quiet"), just the powerhead was used, running at the
bottom.  In all tests, the trickle filter and reactor were used to get
the CO2 level up to the point were the test started.  At that time,
the CO2 was turned off and the reactor allowed to clear of residual
CO2 before actually starting the test.

Due to some difficulty in getting the CO2 to the same starting point
for each of the tests (actually, a lack of patience on our part), the
CO2 readings were normalized for the table below.  To normalize the
readings, the raw data was plotted with the CO2 concentration on a log
scale.  A best-fit line was drawn by eye through the data points.  The
numbers in the table below were then read from the plotted lines at
hourly intervals.  Just CO2 data in the range of 10-33 ppm is shown,
since we consider that range to be the most relevant for planted
tanks.  The raw data is shown at the end of this note.

                      CO2 concentration (ppm)
                      -----------------------

               trickle,   powerhead,   trickle,   powerhead,
Time (hrs)    turbulence  turbulence    quiet      quiet
-------------------------------------------------------------
   0             33          33           33        33
   1             21          24           28        28.5
   2             13.5        17           24        25
   3              -          12.5         20        21.5
   4              -           -           18        19
   5              -           -           14.5      16
   6              -           -           12.5      14
   7              -           -           10.5      12
   8              -           -            -        10.5

At KH=7, the following table relates CO2 to pH:

   CO2 (ppm)  pH
   -------------
   42        6.7
   33        6.8
   26        6.9
   21        7.0
   17        7.1
   13        7.2
   10.5      7.3
    8        7.4

What surprised us was the fact the trickle filter itself was not
responsible for much CO2 loss (compare the last two tests).  It should
be noted that air was not pumped into the media chamber during the
tests.  We suspect that any out-gassing of CO2 by the media will
quickly produce a concentration of CO2 in the chamber such that it
reaches equilibrium with the CO2 in the water. 

In our other trickle-filtered tank, we have noted very high CO2 loss
(we go through a 10 pound tank in 6-7 weeks).  We now suspect that the
loss is caused by the Ehiem canister filter spray-bar return.  We plan
to run further experiments on that tank to verify this conjecture.

Although some authorities recommend pumping air into the media chamber
of a trickle filter, we have found no evidence of a need for this.
Thriving plants will provide plenty of oxygen for the aerobic bacteria
colonies during the day and we have noticed no problems at night when
the plants are at rest.  We ran a long term test on another tank using
a Sandpoint II pH/ORP controller and found no difference in ORP with
the air pump on or off.  We also noted that less CO2 was used with 
the air pump off (longer intervals between CO2 bottle refills). 
  
From the table, it would appear that a 10 pound CO2 tank will last
about 5 months when we finally get the new tank set up.  We will have
a KH of about 5 and will regulate the pH to be 6.8 +/- 0.5.  This is a
CO2 concentration swing of 5 mg/l (27 mg/l to 22 mg/l) times 300
liters and should occur within 1.25 hours for a usage of 29 grams per
day.  Of course, the usage by the plants will increase this by some
amount, but that's another experiment!


CO2 test raw data
-----------------

                 CO2 concentration in ppm and measured pH ()

 Clock          trickle,   powerhead,   trickle,   powerhead,
 Time          turbulence  turbulence    quiet       quiet
-------------------------------------------------------------
 6:00 pm          -           -           23 (7.0)  47 (6.65)
 7:00 pm          -           -           20        37 (6.75)
 7:30 pm          -          33 (6.8)      -         -  
 8:00 pm         27 (6.9)    27 (6.9)     17        31 (6.8)
 8:30 pm         20 (7.0)    24 (7.0)     -          -  
 9:00 pm         17 (7.1)    19 (7.0)     14        27 (6.9)
10:00 pm         11 (7.3)    14 (7.2)     10        23 (7.0)
11:00 pm          8 (7.4)    11 (7.3)     10        21 (7.0)
12:00 am          -           -            9        17 (7.1)
 3:00 am          -           -            -        15     
 8:00 am          2           2            -         8 (7.4)  
12:00 pm          -           -            -         5       


DIY-CO2 and "Tetra Bells"

by gb-at-dixie.cs.unc.edu (Gary Bishop)
Date: 27 Jan 94
Newsgroup: rec.aquaria

In article <759686598snz-at-lofgren.demon.co.uk> ("Niels M. Sampath") writes:

>In article <NARTEN.94Jan27074239-at-percival.albany.edu> narten-at-percival.albany.edu writes:

>>I would not think that turning the CO2 on/off on a daily basis is a
>>good idea.  As the CO2 concentration drops, pH will have a tendancy to
>>rise.  Besides, what is the problem with leaving the CO2 concentration
>>constant throughout the night?
>>
>>If you had a CO2 tank, one might argue that the tank will last longer,
>>but I would think the pH fluctuation would still be a bigger concern.

>>>  Plants produce CO2 at night.. hence no need to add CO2 then. 
>>>  Precisely because it night then cause pH to drop/fluctuate.

The amount of CO2 consumed in the day and released at night by the
plants is small compared to that lost to the atmosphere at the surface
of the water.  pH is most stable if you leave the CO2 on all the time.
I carefully measured the pH of my CO2 injected aquarium (before the
controller) and found leaving the CO2 on all the time bubbling at a
constant rate was best of pH stability over night.

gb

DIY-CO2 and "Tetra Bells"

by steveb-at-chelm.uucp (Steve Benz)
Date: 27 Jan 1994
Newsgroup: rec.aquaria

In article <NARTEN.94Jan27074239-at-percival.albany.edu> narten-at-percival.albany.edu (Thomas Narten) writes:
>I would not think that turning the CO2 on/off on a daily basis is a
>good idea.  As the CO2 concentration drops, pH will have a tendancy to
>rise.  Besides, what is the problem with leaving the CO2 concentration
>constant throughout the night?

Because it won't stay constant.  During the day, photosynthesis is the
process that's keeping your CO2 levels from going up to the saturation
point.  But come nighttime, photosynthesis stops -- worse yet, plant
respiration (O2->CO2) begins.  Eventually, CO2 concentrations could
reach a point where the fish begin to suffocate, or worse yet, your
Ph could fall.

Of course, this assumes that the number one cause of CO2 loss in your
setup is the plants.  If I recall correctly, your setup has a `waterfall'
filter, which is probably keeping your CO2 concentration down at managable
levels through the night.

					- Steve

DIY-CO2 and "Tetra Bells"

by niels-at-lofgren.demon.co.uk ("Niels M. Sampath")
Date: Fri, 28 Jan 1994
Newsgroup: rec.aquaria

In article <GB.94Jan27152129-at-dixie.cs.unc.edu> gb-at-dixie.cs.unc.edu writes:
>The amount of CO2 consumed in the day and released at night by the
>plants is small compared to that lost to the atmosphere at the surface
>of the water.  pH is most stable if you leave the CO2 on all the time.
>I carefully measured the pH of my CO2 injected aquarium (before the
>controller) and found leaving the CO2 on all the time bubbling at a
>constant rate was best of pH stability over night.
>
>gb
>
>  Well, perhaps different tanks just have different qualities
re: surface exchange. Mine is covered in floating plants and a glass
hood just a couple mm's above the water surface. If I leave the CO2
on all night my fish are gasping in the a.m. Must be the differentials
between dissolved CO2 and O2?

-- 
Niels M. Sampath      Internet: niels-at-lofgren.demon.co.uk

                      CompuServe: 100020.2050-at-CompuServe.com
Oxford, England       fax: (0830-0030 GMT)(UK/0)(865)513202

DIY-CO2 and "Tetra Bells"

by booth-at-hplvec.LVLD.HP.COM (George Booth)
Date: Mon, 31 Jan 1994
Newsgroup: rec.aquaria

In rec.aquaria, steveb-at-chelm.uucp (Steve Benz) writes:

    Because it won't stay constant.  During the day, photosynthesis is the
    process that's keeping your CO2 levels from going up to the saturation
    point.  But come nighttime, photosynthesis stops -- worse yet, plant
    respiration (O2->CO2) begins.  Eventually, CO2 concentrations could
    reach a point where the fish begin to suffocate, or worse yet, your
    Ph could fall.

In our manual CO2 setup, the CO2 runs all the time, day and night.  The
pH varies from a minimum of 6.8 (early morning) to 7.1 (maximum 
photosynthesis).  This correslates to a CO2 range of 22 pmm to 11 ppm. 
I would consider 30 ppm to be the beginning of the "dangerous" region. 

-------
George

CO2 and anabantoids

by Hoa Nguyen <nguyenh/nosc.mil>
Date: Wed, 27 Oct 1999

I found this discussion very interesting.  I'm not a physicist either and I 
hope some physicist will step in and enlighten us all.  But after reading 
some of the arguments, I went back to a physics text book (actually, the 
Feynman lectures) and did some digging.  I think, if I interpreted what I 
read correctly, there would not be distinct layers of CO2 and air in the 
closed container due to the difference in molecular weights.  What happens 
is that there will be a gradient, more of the heavier gas at the bottom, 
more of the lighter gas at the top.  The slope of this gradient depends on 
the temperature.

My personal belief is that at room temperature, and especially at the 
elevated temperature found at the top of the tank (due to the lights), this 
gradient is very small, i.e., the gases will pretty much mix 
uniformly.  And if there is any leak at all in the seal, the CO2 will 
readily escape.

Hoa

___________________________________________________________________________
Hoa G. Nguyen
Freshwater Planted Aquarium: http://www.geocities.com/Heartland/Hills/2637/


CO2 rich layer above water

by Zxcvbob/aol.com
Date: Wed, 27 Oct 1999

I'm sorry, but I just could not stand it anymore.  The partial pressures law 
is a steady state equation  It take a finite amount of time to reach 
equilibrium.  So if you are constantly adding CO2 to the water, the air at 
the surface of the tank *will* have a higher concentration of CO2 that the 
ambient atmosphere.  If the CO2 injection rate is minimal, and the top of the 
tank is uncovered, the amount of excess CO2 at the surface will be 
insignificant.  But, if the top of the tank is covered (doesn't have to be 
tight, just drastically reduce the area of the boundary between the outside 
air and that trapped behind the cover), and then especially if the CO2 
injection rate is kind of high (that's a technical term :-), the air right 
above the tank will be far richer in CO2 than the air in the room -- because 
it cannot dissipate as fast as it is being added.

I feel better now.

regards,
bob


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This page was last updated 25 December 1999