Thanks to underwurlde, I have a small supply of Goretex "vents", which are half inch diameter gas permeable membranes, with an adhesive backing. These should be ideal for making a DIY drop checker. So, yesterday I made one!

This was a 1/4" long piece of acrylic tube, 1/2" in diameter, glued to a long flat strip of acrylic, to act as a handle, as well as to convert the tube to a cup. Then, I loaded it with 5dKH distilled water having indicator reagent mixed in. Finally, using tweezers, I stuck one of the "vents" on the open end. Then, being a man, I read the directions! It seems that the adhesive on the vents needs to cure for 24 hours before subjecting it to use. So, I laid the assembly on top of my aquarium overnight.

Guess what? It did exactly what it is designed to do. The water inside passed thru the vent as water vapor, since the concentration of water vapor was higher inside the cup than outside. And, since the concentration of O2, N, etc. in the air was higher outside than inside, it passed into the cup. So, this morning the cup had a drop of very blue water in it, but was mostly just air. Lesson #1 learned: Do not ever allow such a device to stay out of water more than a very few minutes. Lesson #2 will never be learned: always read the directions first.

So, today I am on to my next attempt - further reports to follow.

Here is where I am now:




That is a piece of 1/2" dia x 1/16" wall acrylic tube, with a small hole on the side as a fill port. The hole will be plugged with the small diameter acrylic rod, which will also serve as a handle. The second photo is the tube with two Goretex vents stuck on either side. Now, I have to wait a day for the adhesive to cure.

While waiting for the adhesive on the Goretex vent to cure, I set up my first one with a Cole Parmer O2 membrane, using the O-ring included with the membrane kit to hold the membrane on the cup:


This time I am keeping it in a glass of water to avoid losing all of the water in the cup. As soon as my tank gets up to full CO2 concentration I will try this one out to see what time it takes to reach equilibrium.

Here is the Cole Parmer version in the tank, after 5 minutes:


It obviously requires more than 5 minutes to reach equilibrium.

wow I wish my drop checker was as pretty as that glass one!

cool project Vaughn

Well, no hope of that membrane working with a pH meter, if it takes more than five minutes to balance... Still a very cool idea for a different kind of drop checker, if you can get it work reliably.

Love the photos

5 minutes would have been nice. But, it took about 3 hours to reach equilibrium!! Tomorrow I will do some more testing with different diameters and fluid volumes to try to establish the parameters that would give us a short response time. Plus, the Goretex checker will be ready to test, in case it is the Cole Parmer membrane that is slowing down the reaction. I'm having fun with this, in case no one noticed!

I'd suggest the DO membranes vs the Goretex.
But it does not hurt either to try.

Vaughn, if you are smart about this:

A couple of things:
Make a test reference tank, maybe 1 liter.

Make a DI+baking soda ref solution, basically pure water + baking soda and add CO2 to that. Add the CO2 and wait for things to stabilize.

Next, measure the CO2 via the pH meter and KH ref.
Record that reading.
Add the various pH/Drop checker designs you have and reference them against the known pH/KH CO2 solution and record the time it takes to achieve the desired correct ranges.

This is a simple thing to set up, (easier than a tank) and simple method to verify your results further.

What you want to do is produce good results that show it works well and within a precise time frame.

This way you can predict what it does much better.
You should use a pH meter to check and have a back up method to measure the CO2.

This addresses that and is something you can do relatively easily.
Set the 1 liter test solution up and drop the pH probe and drop checkers in there and wait and record.

If you are even smarter, you'll use several 1 liter bottles and several pH drop checkers per 1 liter. Baking soda, 1 liter containers, DI water: all very cheap/simple.

Regards,
Tom Barr

Tom,
I agree with the method you outlined, but my situation doesn't allow me to do it just that way. I have a single aquarium, with a single CO2 system set up for it. It is in the dining area of my condo, with little room to set up any tables, etc. that I could leave set up for the several days this stuff will take. So, I am kind of forced to do this piecemeal.

Also, I am an engineer, not a scientist. I look for solutions and designs, and rarely data. The accuracy I use when I do things is rarely ever greater than I need. And, I change directions as soon as my testing says I am on a wrong track. Right now I am trying to figure out the following:
How well would a membrane work in this application?
Would it give a significantly better response time than the simple drop checker?
What factors determine the response time?
What simple design would allow any of us to DIY this, if it is practical?

So, I don't really care to know the response times for any configuration with any accuracy beyond whether it is hours or minutes. And, for simplicity I am using the same 5 dKH water I already have, since that's what is in my existing drop checker - the "standard". I already know it's response time is of the order of 2 hours, so I am trying to get something that responds in 10 minutes or so.

Last night I spent some time with a pencil - great invention! - and figured out the obvious - a cylindrical container of KH standard water responds only as a function of the length of the cylinder, not the diameter. That shot down my plan of attack for today, because I can't reduce that length much below the 1/4" it is now, certainly not enough to reduce the response time by an order of magnitude.

So, my next idea is a "lens" of KH solution captured between two membranes, and only 1/32 of an inch or so thick. The obvious problem with that will be getting the solution color intense enough to even see when it is that thin.

I'm also going to try the Goretex membrane just to see how it compares to the Cole Parmer one. It already has two strikes against it because it is neither transparent nor white.

You do not need an extra aquarium, just a small 1 liter container per test.
Fill with DI water/baking soda, just like the ref KH solution except 1 liter size.
Add CO2(DIY can work) and add the drop checker to that.
Don't tell me you cannot afford or have space for a 1 liter bottle:-)


See comments below.
Very well with the right membrane
Yes
Surface area, diffusion coefficient(See Fick's 1st law), concentration gradient, distance etc.
Yup, see below.


See what SeaChem did with their Ammonia/pH alert, they used the entire housing as a membrane. I've asked Greg Morin to consider and look into it as well as selling a KH ref solution to folks which we know will be highly accurate.

You maybe use a flat box model or tube etc, and look through the other short Depth side(say 5mm) while the L x W dimensions are very large (say 4 cmx4cm).
It's a narrow sliver, but you have enough room to view.
Placing the drop checker in high current will also help.

I think this has a lot to do with why I have little issue with the pH probe method, I'm not bound by viewing color. The probe can be right on top of the membrane at very small distances and measures pH faster.

You will have a trade off with resolution and color with the sizing of the drop checker. I think the best bet for a faster response time lies with Seachem (or someone else etc) etc making a pH sensitive skin that also diffuses gas.

That would change the color and allow for a super thin "bag" for CO2/pH measure.
I have my doubts as to speeding it up that much, but removing one or 2 gas liquid phase barriers should help.

At least Fick's 1st law would certainly predict it.


Regards,
Tom Barr