Here is a test "drop checker" using Goretex "vents", which are gas permeable. It is a 1/4" long piece of 1/2" acrylic tube, with vents glued to both faces, and a hole for filling it, which is plugged with a acrylic rod, acting as a handle.

The two latest membrane type drop checkers are in the tank now.


The Cole Parmer one is a double layer of membrane with 5 tiny drops of 5dKH water and indicator solution in between. This was a bear to assemble. I ended up putting the solution on a piece of membrane, then freezing the drop in the freezer, placing it on top of the other membrane already installed, and adding the O-ring to hold it. Then I put the assembly in a glass of water to thaw it, and keep the air out. Tricky!

EDIT: Results:
Good! The Cole Parmer membranes with a tiny amount of 5dKH water and indicator reached near equilibrium in less than 30 minutes. Reading the color is a major problem - as the color changes from blue it also gets much fainter and harder to read.

The Goretex "vents" device reached near equilibrium in 45 minutes! This is astounding considering that the quantity of fluid in it was as much as the single membrane Cole Parmer device I tested yesterday that took about 3 hours to reach equilibrium. This one has twice the membrane area, so it should have taken an hour and a half if the membranes were equivalent.

My conclusion, for now, is that for a DIY device the Goretex vents are a very promising possibility. They are relatively easy to work with, pretty rugged, react rapidly and have adhesive already on them. Their disadvantage is that they are opaque, rather than transparent, making it more difficult to design a device that is easy to view the color on. If they were white it would help, because a single membrane could be used as a backdrop for viewing the color accurately thru a clear window on the opposite side. But, working with what I have, the next obvious step is to reduce the thickness of the slug of colored water in it, to speed up the reaction still more. (The response time should be proportional to the membrane area and inversely proportional to the volume of solution in it, making response time inversely proportional to thickness of the cylindrical slug of fluid.) I will next try a section of half inch diameter acrylic tubing about 1/16 inch thick, which might give a response time of about 10 minutes, a huge improvement over the regular glass drop checkers with their one hour or more response time. And, this would be a cheap device too, assuming a reasonable cost for the Goretex vents.

Another day of testing. Today I made a single membrane Tyvek device, a two membrane 1/16" thick Goretex device, and tried to make a two membrane Cole Parmer device.

The Cole Parmer membrane is extremely hard to work with. It is extremely thin, but not delicate, just thin. Just seeing it is difficult. Unless the light reflects just right off of it, it just becomes invisible. Then, I have yet to find a way to cement it. Acrylic cement doesn't work. Super glue (methyl methacrylate) doesn't work. So, its DIY potential is pretty slim, even though you can secure it with an O-ring acting as a rubber band around the circumference.

Gortex has me puzzled. The 1/16" thick device was about as quick to react to the high CO2 concentration in the tank as I expected. It took about 10 minutes to go from blue to green. But, two anomalies: first, removing the device from the aquarium and placing it in a glass of degassed tap water didn't work nearly as fast going back to blue. It took 1 hour to do so. Then it became apparent that water could pass thru the membrane - just a slight touch with a paper towel drained the solution from between the membranes. Also, the glue holdling the membrane to the acrylic cylinder was barely adequate. It took very little to pull it off. These problems combined with its dark gray color make me wonder if it is worth pursuing it any further for a DIY probe.

The Tyvek was a surprise. I had previously sneaked a 2 inch square of the stuff out of Home Depot, so I cut a 1 inch diameter circle out of it and used an O-ring to hold it on my one membrane tester. The ruggedness of the Tyvek made that easy. And, the white color of Tyvek makes it easy to see and a good backdrop to see the color of the fluid inside. This single membrane, 1/4" thick cylinder of fluid took 1 hour and 20 minutes to become green. Remember, using this same device, with a Cole Parmer membrane took 3 hours to reach green. I wasn't much surprised at this, because you can blow thru the Tyvek easily, meaning it has either big pores or a lot of them. The Tyvek device took about 1 hour and 45 minutes to go back to blue when placed in a glass of degassed tap water. So, its reaction time is in the neighborhood of 1.5 hours per 1/4" thickness of fluid and single membrane. I would expect a 1/16" thickness of fluid, with a single membrane, to take about 20-30 minutes to reach eqilibrium with the water. And, with two membranes, one on each end of the cylinder, it sould be a 10-15 minute device. I consider that worthy of pursuing as a DIY device. Tyvek is very cheap (even if you don't just swipe it), easily available, and very easy to handle. Whether it can be glued is a question needing an answer, but using O-rings to hold it it place is workable. My next attempt will likely be trying to make an easy to assemble and re-assemble, thin cylinder device, followed by thinking how to make such a device easily usable in the aquarium. Ideas welcomed!

Been following this thread and finding it very interesting.
Has anybody used one of these ( Aqua Medic Membrane Reactor ) and could they be modified into a CO2 tester ?

I think the membrane should be ideally suited, but was wondering if the device as a whole could be used rather than just the membrane.

Al

I have one of these and I dont think it would suit. If theres no gas pressure inside the unit, water leaks back through the membrane, its a pretty low quality product. Expensive for what it is, I regret buying mine.

OK thanks ...... never actually seen one up close - guess I'll keep it that way

its all right I guess for what its designed for, but it really struggles if you have anything more than about 15 bubbles/min going through it, and its extremely hit and miss in getting the water flow across it right, if its not bang on, you loose alot of your CO2 due to it leaking out of the membrane in large floaty bubbles. I have a mister now and its so much better, and it was 1/2 the price of it.

I have a workable, DIY, easy to make design now for a membrane version of the drop checker. Tomorrow I will make a couple to do some more testing. Here is the sketch:


This design will allow you to reduce the thickness of the KH water solution to as thin as you can use and still see color differences, thus making it react rapidly. It also will seal well, so leakage of the solution will not be a problem. And, you can use either Tyvek or Cole Parmer membrane, or just about any other membrane on it. Installing the membrane will be as easy as I can make it be. All of the materials are readily available from a Tap Plastic store and a hardware store (for O-rings), plus a 1/2" diameter probe holder/vacuum cup from the LFS. I feel good about this one!!

EDIT: Add photo of finished device. I made three of these, so I can do side by side tests of 2 or 3 membranes if I wish. The first test will be a piece of a Priority Mail envelope, which is Tyvek, but thinner than building wrap. Of course we are supposed to only use those envelopes for mailing something by Priority Mail, so I have to strongly recommend that we search out used envelopes for this purpose. Being totally law abiding I would never, ever dream of picking up one of those envelopes at the Post Office for this purpose, since they are free.

EDIT AGAIN: I ran a quick test with Priority Mail Tyvek. There was a major problem trying to load it with the solution. I ended up having to freeze a big drop of the solution in it, then add the membrane. Otherwise, the tiny amount of fluid would wick out before I could seal it. But, the good news is that this time it reached near equilibrium in 4 minutes! In 10 minutes it was as green as it would get. So, PMT (Priority Mail Tyvek) does work. And, reducing the amount of fluid in the device definitely speeds it up drastically. I wish I could think of a good DIY design that could be much more easily loaded with solution.

If a membrane type drop checker had acid free, neutral pH, blotting paper in the fluid chamber, would it work as well as pure fluid there? Who knows enough about chemistry or whatever specialty would be involved to make a good guess or knowledgeable answer to that? I can see using such blotting paper, white, soaked in 4 dKH fluid and indicator, with a clear membrane over it, as a sensor.

It should work but the blot paper often has poor resolution.
I think a white background would help with determining the liquid coloration though.

10 minutes is getting down there and much better than the 2 hours you posted prior Vaughn! Now you have a simple working unit that is a good DIY project for all.

Since many folks will never do the DIY and make the KH ref solution, I might suggest talking to Greg Morin at SeaChem and goading him into making these along with a ref solution (say 500mls of 40KH solution).

Regards,
Tom Barr