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[APD] CO2 measurement revolution in the plant tank hobby

CO2 ppm measurement methods have been increasingly subjected to
critical assessment in the Aquarium plant tank hobby. During
this phase, many hobbyists have noted wide ranging difference in
their measured CO2 ppm concentrations vs the actual tank
observations from both plants and fish. Some report ranges of
upwards of 200ppm of CO2 based on the pH/KH/CO2 cart. But fish
health and plant growth was normal, when they lower the CO2
assumed concentration to 60-120ppm, they had BBA outbreaks. The
hobbyistys had pH monitors that where calibrated. KH test have
some issues potentially though(see below). Such readings
underscore the issues resolving the CO2 concentration
determination with any accuracy that we can have some trust and
faith in being correct.

The chart *assumes* that all the influences are due to KH and
CO2 concentrations.

There is nothing inherently wrong with the chart itself.
What is problematic for aquarist is the application in our tanks
under a wide range of conditions.

Non bicarbonate alkalinity, test kits that measure total
alkalinity rather than specifically bicarbonate alkalinty,
issues with pH: non CO2 acids depressing pH lower giving false
positive readings, erroring to skew the pH/KH/CO2 chart in
showing there is a lot more CO2 than is really there.
Many folks are using ADA substrates or Peat or have tannins from
driftwood etc which make determination using the chart more

Given that CO2 stability and accuracy are critical to its use in
planted aquarium relative to other factors such as nutrients and
lighting, this is one of most overlooked parameters and the most
poorly tested. As a result, I've become a "CO2 nag":)

Unlike NO3, PO4 etc, CO2 ppm values may rise and fall much
faster in relative terms. They can vary greatly hour to hour and
day to day. No so with the NO3/K+/Ca++ etc values.
To resolve this issue, I used two ideas, one from Vaughn
concerning the KH reference solution and one of my own borrowed
from other sources.

There is no reason such a semi permeable gas membrane + ref KH
solution could not be used in our applications for getting a
very accurate CO2 determination. Thanks to Vaughn namely.
(Aqua Medic sells a CO2 diffuer with a membrane as well).

This would be very responsive and easy to maintain.
Essentially removing the air gap issue with the "pH probe+ KH
solution + air gap" and the "drop checker method+ Kh ref
solution" as describe by Vaughn prior.

Condensing the size and interface down as well as the ref
solution volume will increase the flux of CO2 and thus increase
accuracy/response time.

This is neither here nor there at the moment.
We need the membrane first and way to put on a pH probe and add
the Kh Ref solution, we can make the ref solution easy enough.

Take a membrane mesh bulb that snaps on like sock would work
well, then add a rubber band so as to seal the CO2 membrane on

This would be easier than making the CO2 testing gap device.

It would certainly be smaller and thus more responsive due to
less distance, no air gap required. The probe could be placed in
good current ansd that would give you a high degree of accuracy.

There are two main type of membranes that are highly effective
diffusion membranes, namely from the medical field in making
artifical lungs that diffuse O2 in and CO2 out.

Background reading:

Factors governing gas exchange efficiency in membrane


and an example:


I placed an order for some various materials, and sealing
methods for the membrane(not dupont's).

I'll make/cut the membrane to size and then make a sock like cup
on the probe, then, use a rubber band for the seal like the DO
probes use and place the KH ref solution inside the membrane
with the probe tip before sealing it up.

This should not cost much and little membrane will go a long
long way.

It's also small, easy to adapt with present materials.

Why the heck didn't some one think of this before?
It's all your fault Vaughn!!
You are to blame for it, haha. 

Well, that solves me from doing a bunch of funky chambers with
acrylic. I think if you wanted to make some $$ on the side
Vaughn, you really should offer reference pH probe kits.

Sell some membranes and rubber bands and the a small bottle of
KH ref solution for a pH probe. Say 25$ for the kit.
Folks would buy it.

So now we took that simple idea and beat it up to the point of
making a simple cheap way to resolve all the CO2 ppms issues:

1.It's still small
2. It can be adapted to the accuracy of the pH probe.
3. It has a very high surface to area to volume of the ref
4. It'll have higher response times than any other available gas
exchange method 
5. It is easy for the user to add to the pH probe.
6. It'll come with a ref KH solution
7. You can DIY it.
8. The minimal KH ref solution means faster response times than
other designs.
9. It's cheaper than other methods/designs.
10. It's still Vaughn's fault.

I have several membrane types to choose from still. I can get a
little of each type and see how they work.
Small rubber bands that will tightly seal the membrane red
solution from the outside water are not difficult to obtain. KH
ref solution, I can make easily enough, pH probes, I have
several, just need the membrane is all really.

So what are the draw backs and weak points here?

Note: the membranes do not allow the water solutions to mix(tank
water and KH ref solution), just the gases.

There more discussion on my site.

It's funny, for all the testing folks squawk about, they seldom
go after CO2 carefully.

Without isolating CO2 in our tanks as a variable(make sure the
other parameters are in good shape with high degree of
confidence), we are not able to deduce what causes the
algae/growth problems are due to.

I think this simple membrane method should solve many issues for
folks that are unsure about their CO2 ppm's, have a pH
probe/monitor and yield much more accurate CO2 readings the
world over.

EI resolved many things for folks, this does as well and
cheaply. I do have one new thing yet up my sleeve I am working
on. Maybe two. 
Tom Barr


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