Converting a pH meter to a CO2 meter for more accurate CO2 control

[MiamiArt]

Hi everyone,

In my humble opinion, I think that we need to get a better handle on one of the most important aspects to plant keeping- CO2. If you're adding it to the aquarium, you need to be able to control it.

I think it was back around 1997 that I first read about the possible impact of humic acids on KH and the fact that the KH/PH/CO2 table popularized by Dupla's The Optimum Aquarium may be off in aquariums. For those of you interested, I think it was James Purchase on the APD list but I recall George Booth also being involved. Over time, the issue became better understood and the consensus now is that KH interference in aquariums makes controlling CO2 via pH control inaccurate.

As the issue is interference of KH by things we can't control, methods of stabilizing KH have been developed. One is the drop checker. The downside to the checker is that there is a rather long lead time before it shows a change in CO2 and it's lack of accuracy. Another method is simply to increase CO2 until you balance plant growth with your fish being gassed (the method I use). This is time consuming and risky. Either works for sure but is there a better way?

An alternative method was discussed quite some time ago by Erik Olson and then Tom Barr. Tom just recently discussed it in his talk with Robert Hudson on his podcast. The method calls for converting a pH meter to a CO2 meter. All that's involved is finding a way to place the pH probe in a gas permeable bag that contains a KH solution with a known KH. This gets placed in the aquarium.

By doing this, we have now have a way to measure a KH solution that only changes with CO2 concentration in the aquarium. As we have removed the air gap associated with drop checkers, the reaction time goes from several hours to just a few minutes. We can then use the KH/PH/CO2 table to 1) measure exactly the CO2 concentration in the water and 2) control it via a controller. For example, a pH controller could be set to a pH of 6.5 giving you a known CO2 concentration in the water.

As no company has come up with a pH probe that works in the above way, hobbyists are left to do it themselves if they want this type of accuracy. To me, the hardest part is coming up with a gas permeable bag for the probe. I recently ran into Parafilm sheets that seem to provide this ability. Now we need a good DIYer with time to come up with a way to use these with our probes. Any takers?

Any problem with my logic above?

[SMinNC]

How about a Breather Bag for shipping fish?

*Disclaimer:
I've never used one(breather bag). Still new to plants and only DIY CO2 so far. Never had a probe of any type.

[Jeffww]

I never knew parafilm was permeable I just thought it was wax lol. Interesting...I'm not sure if it would provide a good rate of exchange though. It might end up just taking several hours to react so it wouldn't be a good meter.

If we're really wanting to know how much CO2 is in our water we might as well just buy a CO2 probe.

You just need a laptop with a logging program or even a TI calculator with the appropriate installations...the 600 dollar probe...and you're set to go.

[MiamiArt]

The thinking is to try to come up with something anyone can do.


Regards,

Art

[Hoppy]

That film is a good find. If I had that a few years ago when I was trying to make drop checkers with a film in place of an air gap I would have been delighted.

It would be very easy to enclose a pH probe end with this film, but getting the standard KH water enclosed inside is much more difficult. I would probably try it by making an assembly tool that would hold the film shaped into a pocket, then fill the pocket with the water, stick the probe into the water filled pocket and mold the film around the end of the probe with the water trapped inside. Easy in principle, but probably very tricky to actually do it. The less of the water you trap, the shorter the reaction time for CO2 measurement.

I no longer have a CO2 system or a workshop, and don't have a pH meter or probe, or I would be tempted.

[MiamiArt]

Hoppy,

I was thinking just a piece of acrylic tubing with one end capped. The capped end would have holes drilled into it and the film would lie on it somehow. The KH solution would be in this tube with the probe placed into it.

Like you said, easy in principal but harder to actually make...

[Hoppy]

This probe is shaped about right, and the diameter is small: http://www.google.com/products/catal...d=0CIUBEPMCMAM

It has a sensor diameter of 5/16 inch, so you could drill a slightly larger hole in a piece of wood, maybe 1/4 inch deep. Take a square of the film and a piece of 5/16 wood dowel, and poke the film into the hole leaving a 1/4 inch deep pocket for the water. Fill the recess with the water, and stick the probe in on top of the water and push it down as far as possible. Form the film around the probe and put a rubber band or O-ring over it to be sure it stays there. The result might be a standard KH filled probe end, so when you stuck it into the aquarium it would be a pH sensor/drop checker. Now, I just have to find a factory to make a few thousand of these before the world beats a path to my door (And, I need to get my money before any problems show up, like the probe taking 10 hours to stabilize, or the KH water oozing out, etc.)

[jcgd]

What about a Heyco liquid tight cord connector, threaded into a coupling with the film somehow attached/ glued over the end. This is all I have lying around, but you get the idea. You would only have to unscrew the coupling and screw on a new one to change out the film.

[jcgd]

Okay, apparently the film is self sealing. I assume it's like a sticker? That would make it easy to stick it to a coupling. I someone knows where I can get these films I could try it. I have a spare ph controller, and, well, everything else that I anyone could think of.

[Hoppy]

That particular site only sells big lots of the film. Enough for thousands of probes. Googling didn't help a lot either, $20 for one roll was the cheapest I saw, but at least there are several places to buy it. Maybe it is possible to get a free sample?

[jcgd]

Checked for it on the Bay of E.

I found a 2"x10 foot roll for about $7 shipped. I bought one. I'll update when if finally makes it to Canada and send out some samples if people like.

[Hoppy]

Originally Posted by jcgd

Checked for it on the Bay of E.

I found a 2"x10 foot roll for about $7 shipped. I bought one. I'll update when if finally makes it to Canada and send out some samples if people like.

That's great news! I never once thought of checking that place. What does your pH probe look like? Maybe there is a really easy way to use the membrane to trap water around the sensing part - like assembling it with your hands and the probe under water.

[plantbrain]

There are silicone films that have a better transfer rate of gas.

A flat tip pH probe also would be easier to add a cap to and refill with a known reference KH solution.

Now.........once all this is said and done..........how do plan to check to see the response times and the accuracy against a known standard?

Here's how:

You need a 500ml full bottle/jar, flask with a grommet, sealed 1/2" for the pH probe to added with the film membrane and cap so it does not tear.

Try and get all the air gap out with some DI water.
Measure the DI water, or better yet, bubble N2 gas to remove all the other gases like O2 and CO2.

Now buy some dry ice at the local grocery store, cut and weight a small amount of it, say 25 mg and add this very fast to the flask and cap.

25mg in 500mls = 50ppm of CO2 once it melts.

Next, you can do the same thing for 10ppm, and 100ppm etc.


If you use a larger volume of water, and dry ice, the accuracy should increase. N2 gas is likely available from a few different outlets.
Or you can measure the DI+ sodium carbonate with a pH probe and get a certain ppm etc.

This should be pretty accurate. I did this a few times recently.

I suppose the fish breather bags should work also, might be a bit fragile.
They have a a very large surface area relative the gas exchange rate for fish vs a small tiny pH probe tip. The trade off is the volume in the tip of the pH needs to be small and the KH solution layer needs to be thin.
This increases the response time considerably.

I spoke awhile ago to American Marine about making one for their pH probes, but they dropped the ball. A good idea and a way to make extra $.........down the tubes, I guess they make enough money??

Rather than DIY, it would be nice for a vendor to make and carry one.

[Hoppy]

Originally Posted by plantbrain

There are silicone films that have a better transfer rate of gas.

Have you done any testing with the Parafilm M as a membrane? It is hard to relate the CO2 diffusion rate they claim to a response time for the way we would use it.

For the accuracy part: If it would just measure the pH accurately, say to +/-0.1 pH, we should be able to make a KH solution with very good accuracy, so that would still be a much more accurate CO2 measurement than a drop checker gives us, where judging the color is so hard. For an absolute calibration vs a known ppm of CO2, that seems like a losing proposition all the way. We could certainly make a solution using dry ice, but getting it to be accurate sure looks questionable.

[Hoppy]

From the website for Parafilm M:
"Permeability Characteristics:

Carbon Dioxide (Modulated IR Method): 1200 cc/m2 d at 23°C and 0% RH"

Does this mean 1200 cc/m2 per day? If so, this isn't likely to work. I did some calculating and for a .5 inch diameter probe, with a .5 inch diameter by .5 inch volume of standard dKH water enclosing the probe, it would diffuse about 8 ppm of CO2 per hour through the film. That wouldn't work for fast response, assuming I didn't make mistakes.