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DIY Drop Checker

273184 Views 357 Replies 80 Participants Last post by  HeyPK
It is quite easy to make your own "Drop Checker" or CO2 Indicator from acrylic sheet. The object is to have a small device that holds a few ml of water/indicator solution under the water line of the tank, so there is an air gap between the tank water and the indicator solution water. The commercial versions of this range from the elegant ADA glass unit, which is currently out of stock, to a much cheaper imitation ADA glass unit from Hong Kong, to a Red Sea plastic unit which is still cheaper. All do the same job. Two of the commercial versions are the imiitation ADA unit:

and the Red Sea unit:

Both are available on ebay at: eBay: Type2 Co2 Drop Checker-monitoring proper dosage of CO2 (item 250038130859 end time Oct-18-06 09:05:18 PDT)
and eBay: CO2 Indicator Red Sea Real Time CO2 Monitor (item 300036151186 end time Oct-14-06 09:48:10 PDT)

The easiest way to DIY this is to use all straight lines and rectangles, and make it from acrylic plastic. I made one a couple of years ago, but hadn't figured out how to effectively use it so I tossed it. Here is what it looks like:

All of the pieces of acrylic have to have squared edges, and the pieces that establish the thickness of the device should be cut from a constant width strip. When glueing these together, remember, the assembly has to be air and water tight, and any smearing of the cement makes it hard to see the color of the indicator solution. For an indicator solution you can use the solution from an Aquarium Pharmaceuticals pH test kit, or any other test kit that gives yellow at pH=6 and blue at pH=7.2 - use at least double the number of drops of indicator solution as the kit says to use, to get a more intense color. (Using even 4X the recommended number of drops doesn't change the test reading, only the intensity of the color.) To use this see:
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I thought of a situation where these drop checkers could potentially indicate a higher concentration of CO2 than is in the tank, when using a CO2 mist application.

Suppose the mist bubbles accumulate a little pocket of CO2 in the air chamber of the drop checker. If these bubbles are accumulating faster than they can dissolve, there will be an artificially higher concentration of CO2 in that little air pocket than in the rest of the tank. Since the little chamber of water in the drop checker is very small, the CO2 levels will become in equilibrium with the air pocket a lot faster than with the tank.

A way to test this hypothesis would be to surround the drop checker with a shield that prevents the mist bubbles from entering the chamber and seeing if the indicated CO2 level changes.
That is a potential problem, but I don't see it as a serious one. These things reach equilibrium pretty slowly, over a couple of hours, or so. So, unless there is a steady stream of CO2 mist bubbles gathering under the "horn" of the device, then dumping their remaining CO2 into the air gap, all that should happen is that the indicator will "overshoot" the true CO2 value, but settle down to an accurate reading eventually. If the ppm of CO2 in the bulb of the device is above that in the water, the CO2 will migrate back to the tank water.

I think the problem can best be avoided by choosing a good location to mount the device - where there is good water circulation, and at the opposite end of the tank from any mist generation. Those CO2 micro bubbles can't exist for very long in the water, before the CO2 diffuses into the water, leaving a microbubble of other gases. So, I suspect that most of the bubbles are "inert" by the time they are swirling around the opposite end of the tank.
That certainly looks effective if not beautiful. And, if you only want to verify that your bubble rate is giving you the amount of CO2 you desire, this should work perfectly. Hey, maybe we could get Home Depot to buy a few of the ADA ones for their tool rental department?

For those who don't read many other aquatic plant forums, Tom Barr did some experimenting with this device and came up with the idea that you could get rid of the air gap entirely, and speed up the response rate considerably, just by using a gas permeable membrane to hold the reference KH solution. Then, if that membrane is in the form of a stocking over the business end of a pH probe, you have a CO2 probe - with fast reaction and great accuracy. He has done a prototype, of sorts, and it works. And, he is working towards getting this available for all of us.
Tyvek (the stuff they wrap houses with before they add the siding) is a gas permeable membrane that is pretty much available anywhere. I've only seen it sold in the big rolls, but if there's any new houses being built around, your sure to find scraps of it.

I remember I worked for a place that sold raincoats made out of Tyvek. The company gave us a display that was a clear cylinder with a sheet of it in the middle dividing it in two. Water was poured into the top chamber and an air pump was connected to the bottom chamber which forced air through the Tyvek and up through the water. Even while the pump was off there was never any water in the bottom chamber.
Now that is a creative burst of inspiration!! I should be able to find some scraps of Tyvek, and I had forgotten completely that it is a semipermeable membrane, and a very rugged one at that. If I can find some scraps at Home Depot, for example, I will try to make a KH reference "condom" for my pH meter and see how it works. THANK YOU!! If you also post at Tom Barr's forum I suggest you post this there. If I don't see it in a day or two I will post the idea myself.
Hoppy do you have any more of the Drop checkers you built that you were selling??? I'd be really interested in buying one from you.
No, I only built two, just to demonstrate that you can easily make one and it works well. I was also trying to see which configuration would work best. I may make a few more sometime, but it isn't likely, and I don't want to do this for profit. I do have a big hunk of 1" acrylic tube and 5/8" acrylic tube left over, if someone wants some. I could easily cut it into 8 inch pieces and ship that by priority mail, but you would still need to buy from somewhere a tube of medium viscosity acrylic cement and a small scrap of thin acrylic plastic.
As best I can determine, bromothymol blue is the only pH indicator dye that gives yellow to blue readings, that is used in such test kits. There are a few others that give different colors, and for differenct pH ranges, but not for yellow to blue.
I experimented with the AP test kit, adding varying amounts of the pH reagent to tubes with the same water - tank water - in them. It made no difference to the results whether I used the three drops the instructions call for, or 2 or 6 or even 9 drops. With larger numbers of drops the result becomes a murky opaque liquid, where reading the color is harder, not easier. But, the color stayed the same for all. So, I just add whatever amount makes it easiest to judge the color.
The reason for keeping the entire device under water is to avoid any temperature difference between the device and the tank water. If there is a difference some distillation will occur - water will migrate from the warmer to the cooler area, which changes the KH of the tester water.

Are you sure there is no leakage between the pvc tube and the bottle neck? If so, that has to be the easiest one yet to make. (But not the most unobtrusive or most elegant or coolest!)
Just a thought, but if the membrane idea pans out, these could be made even easier by gluing a piece of membrane to the top of any small clear container filled with indicator solution.

Like this:

It wouldn't matter which way was up since water can't go through the membrane and you could use any small clear container you have lying around with virtually no modifications. Heck, you could just glue membrane to either side of a piece of air line tubing. Then if it starts going bad, toss it out and throw a new one in.
As I recall, the membranes are made of teflon or polypropylene plastic, and neither of those is at all easy to glue. Both are, I think, considered "release agents". If you can use glue, I hope someone with the knowledge will tell us which glue to use. If I had some membrane, other than Tyvek, which is very thick and stiff for something like this, I would just wrap it around a piece of tubing and rubber band it in place. I do have a pretty nice piece of transparent vinyl 1/2 inch tubing I could easily use this way. But, the first steps are to find a source of membrane, and to find out if there is a glue that will work with it.

EDIT: I did some googling and found the obvious source of membrane material - Kordon breather bags! Who has some and is willing to supply one or two to me - of course I will pay shipping and cost of the bag(s)?

EDIT AGAIN: It only took a few minutes to get two offers of these bags, and both are relatively local to me! If I can make this work out I will make two or three extras and offer them to whomever wants them. But, I hope someone else tries this idea too.
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The reason the Kordon bags "let oxygen in and CO2 out" is the same reason it will work for the drop checker. The gas will always reach equilibrium conditions on each side of the membrane. So, if the fish or plants are using O2, the O2 concentration will drop on that side of the membrane and O2 will have a net inflow to that side. But, if the fish or plants release CO2, the CO2 concentration will increase on that side and there will be a net outflow of CO2. The membrane is little more than a net with extremely small openings, so it must be able to let gases go either direction.

If I were designing a membrane drop checker for sale, I would have a sheet of membrane with 4-5dKH water and pH reagent spread on it, lay a second sheet on top, then heat seal a grid of 1" squares, so it separates into a bunch of 1" square pillows with standard KH water/reagent sealed inside. One of these could just float in the aquarium - it would have near neutral buoyancy.

So, who has a machine that does this - seals two heat sealable plastic sheets together in 1" squares? Surely one of us has one hidden away in the garage??
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Yes, and I completely missed that. I was obsessed with how to make a container that I could easily replace the membrane on at that time, not the membrane itself. It is still a great idea, and a really cheap source for membrane materials.
what do you mean with a shot glass??

it's just a small cup..
The photo looked like the bottom of the cup was thick like a shot glass is made, so I just thought it would be a good joke! I keep being amazed at the variety of ways there are to make this little device. When I first thought seriously about it I assumed that the ADA type glass device was the only good idea - now we are down to a piece of tubing, a piece of breather bag and a rubber band!
If you are going to use pH reagent to find the pH of the water you need to adjust the KH of the water to around 4 or 5 dKH, assuming you will be measuring around 30-40ppm of CO2. The color is just too hard to judge accurately unless you are reading about 6.6 pH. And, plain distilled water won't work because it has no measurable KH, so it gives an ambiguous reading of CO2. But, the device you describe will work fine.
I'm not so sure the breather bag idea works. I've had a couple differnt models in my tank for over an hour and there is no color change.
Dennis, this idea has to work doesn't it? Assuming the breather bags really do allow CO2 and other gases to pass thru freely? The time it takes for them to reach equilibrium might be long, but I don't see how they can fail to work eventually. Tom Barr believes a device like this will have a short response time, but I haven't seen any data to say, one way or another. Of course a breather bag, used for its intended purpose, doesn't need to react very quickly. So, it is possible that the density of the pores in the bag material might be too small for the material to work rapidly for this.
I think Tom is getting that notion from the fact that he plans to use DO probe membranes. The DO probe has a fast response time and works basically exactly like he has described in his posts. As usual though, I don't follow his notion of cheap since I can't find any that are less than ~$25 per membrane. I obviously don't know where he is looking as he never tells....

The breather bags should work in theory though don't know exactly how they work. There might be an issue with pressure across the gradient, or lack of with water on both sides. I really don't know much about osmosis type functions, other than the basics of things wanting to be at equilibrium.

I'll let them float around longer. It is possible that there is to large an air space above the indicator sample. Let me say what I did... I took a small specimen jar with a snap on lid, about 10ml volume. I cut away most of the lid leaving only the outer rim. Placed a small piece of Kordon bag over the opening and snapped on the lid rim. Works great. The hole in the lid is about 1/2" diameter. I added 1ml to this and placed it inverted in the aquarium so that the indicator solution is sitting on the Kordon. I also made another that has a 3/16 hole but the container is 7ml and I used 2ml of solution. This one is tall and skinny compared to the first with is shorter and wider. Neither is showing a color change after ~3hrs.
With the air space sitting above the KH standard solution it will moderate the change in ppm of CO2 in the KH liquid. The air space has to reach equilibrium with the KH liquid as the KH liquid is reaching equilibrium with the tank water. So, wouldn't that just slow down a regular drop checker even more? By forcing the tank water CO2 to go thru the membrane holes instead of through a big air/water interface? I'm still betting that eventually the color will change as expected. And, intuitively I would say the smaller the quantity of KH water divided by the area of the membrane, the faster it could react. So the ideal would be a .030 thick layer of KH water with a membrane on both sides having a square inch of area. I haven't made any effort to prove this, nor do I know how to start to do so, but if I get bored I will make the attempt, at least to do it with a mind experiment.
Dennis, do you have anything new to report on your testing with Kordon breathing bags? I just got a few from Bill Harada to play with, and the first things I notice are that the bags are covered with lots and lots of writing, part of that writing says it doesn't work under water. That surprises me, and I don't really believe it. If a gas can pass from the air to the water and from the water to the air, I can't see a physical reason it can't pass from water to water. I plan to try a simple experiment to verify that.

Meanwhile, over on Tom Barr's forum, Tom says the bags will have such a slow response time they will not work. That doesn't surprise me too much, since the original purpose for the bags doesn't require much of a response time. Fish just don't breathe a whole lot of O2 in and CO2 out - they are kinda small. But, for a DIY drop checker, a fast response time isn't all that important either. Response time is only important if you are using this for lab experiments of some kind and want to see the effect of a change fairly quickly. (In my opinion.)
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Back to the drawing board I guess. I still plan to try one of the breathing bags to see what I can find out, and I have a small piece of Tyvek to try the same way, not that I expect it to work at all.

I have been doing a lot of google and wikipedia research on gas permeable membrane and I have learned that I know nothing whatever about the subject. That is a challenge!
Dupont is selling samples of this material for $500 to $1000! Now, I don't mind spending $10 now and then just to experiment with, but that's about tops for me.

I am continuing to read up on gas permeable membranes, and I notice that silicone rubber is a good material for CO2 permeability, but I don't see where one could obtain thin sheet silicone rubber. And, I saw a hint that latex rubber also is gas permeable - which (blush) leads me to wonder about using condoms, the extra thin kind. Does that sound promising to anyone?
I found CO2 probes of this type already exist for blood gas analysis. They have a gas permeable membrane with a carbonate/distilled water solution between the membrane and the pH sensor, just as Tom is proposing. Looks like once again someone has figured it out before us, and probably patented the design.

The Cole-Parmer membranes are intriguing, and I may decide to buy a package of them just to play with. But, the season, the other jobs (making a Murphy bed, producing two newsletters, finding a new camera, etc.) are interfering right now. I'm still wondering if an ordinary condom acts as a gas permeable membrane.
I haven't found a source of thin silicone rubber tubing. The stuff we sometimes use for CO2 tubing is too thick and too opaque - mine is blue also. The secret of all of this is to find consumer products that use what we are looking for, so the price is reasonable. I'm still thinking about the Cole-Parmet membranes, and I did sign up with their web site.

Edit: I bit the bullet and purchased a pack of the membranes. It's my Christmas present to myself!
I don't think it is likely that well water will contain 40 ppm of CO2, and I know it won't if that water is allowed to sit in the open for very long. You just have to use either DI or distilled water for this method to be any better than just measuring the tank pH and KH.
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