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DIY Drop Checker

273202 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 am in a phase where I am neglecting my aquarium now - too busy with other things for now. So, I have noticed that the indicator solution gets dimmer in color over time, but still reaches the same color as it originally reached for the same bubble rate. My best guess now is that two weeks is about as long as I would let it go if I wanted accuracy. As far as the KH solutions goes, sodium bicarbonate works very well, is very cheap, and is widely available, so I don't see any advantage to trying anything else. The only things that would work would be carbonates or bicarbonates.
The color on mine stays correct, but seems to fade a bit. It is so cheap to recharge it I just do it about every two weeks.
A DIY HOB Drop Checker and pH Controller from Sept. '92:

The Krib

Don't be misled by that article. You can't hang this on the back and have part of it outside of the tank and part inside. If you try that you will get a difference in temperature between the tank water and the drop checker water, which will cause slow distillation of the warmer water to transfer water to the cooler water, changing the KH of the reference water. Also, it doesn't work with tank water in place of KH reference water, and it reacts so slowly to changing tank water conditions that it is useless as a pH controller or a CO2 controller.

If you make up a bottle of KH reference water, and seal it tightly, it should be good for several months at least. Since the indicator solution is a dye it can't be completely stable, so if you add the indicator solution to a bottle of reference KH solution, at least do it in a small bottle, and store it out of direct light. Don't forget that some plastic bottles allow water vapor to leave the bottle very slowly, which would change the KH of the water. (Store a plastic bottle of water for a couple of years and it starts to collapse.)
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This has been a very good thread, long however.

Anywya, I consider myslef pretty new to the CO2 injection and am at a point where I have to decide where to go with CO2.

Some questions I have about this CO2 dropper is:
1. The red sea model looks nice and appears to be pretty cheap, is it recommended for those that aren't creative enought or have the time to DIY?

2. The KH and the Distilled water, how do you test it? I have a test strip kit but not a liquid test kit for KH. If I mix a mason jar worth, would the water last?

3. how often do you need to refill the dropper?

4. If I go with the red sea, once the reagent runs out, can I use normal PH reaent from my AP test kit? do you use the high range or low range ph indicator?
The Red Sea model will work as well as any of them. The principle this works on is so simple even crude DIY units work fine. In my opinion the only advantage of the glass ones is that they look nice in the tank.

You can mix the KH solution without a test kit, but then you need a digital balance that will measure accurate to +/- .01 grams, and a volumetric flask that will measure at least a liter of water accurately. If you mix 4.99 grams of bicarbonate of soda that you have baked at low heat for several minutes to dry it out, with 5 liters of distilled or DI water, you will have 5 liters of 40 dKH water. Then mix 10 ml of that with 90 ml of distilled or DI water, and you will have 100 ml of 4 DKH water. This should be more accurate than depending on a test kit, and you can store both the 40 dKH and the 4 dKH solutions in airtight bottles for an indefinite time.

You can use the reagent from any pH test kit, if the kit gives yellow at about pH=6 and blue at about pH=7.2.

The solution in the drop checker will last at least two weeks, with no problems. But, eventually you will need to clean off the "biofilm" on the drop checker, and add fresh solution to it.
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NEW: Membrane style DIY Drop Checker

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 (clear acrylic tubing) 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!!

This is a result of a lot of experimenting I did and reported on the Barr Report, see: Gas Permeable Membrane Drop Checker - Barr Report

To explain this some more, the top O-Ring is to seal the drop checker fluid in the chamber (the blue green color on the sketch), and the bottom O-ring is just like a rubber band to hold the membrane on the device. I know from testing that if the thickness of the fluid layer is only around 1/16 inch this will react in less than an hour, probably less than 30 minutes. I haven't tried to make the checker solution strongly colored enough to go below 1/16 inch thickness, but it would be worth trying it thinner, to see if a 5 minute response time is achievable. I expect to be able to make one of these in less than 30 minutes with hand tools.
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Cole Parmer YSI 5775 "Oxygen Probe Service Kit" is what I used, and I just noticed that this is made by YSI Incorporated of Yellow Springs, Ohio.
Ordindary Tyvek, a building wrap material is the other usable material I have tried. Both work for me, and both have their own advantages and problems. The Cole Parmer material is essentially invisible. It is a very thin, completely transparent membrane that you can see if you get light reflecting off of it just right, but otherwise it isn't visible. So, it is hard to work with easily. And, for me it didn't work any better than Tyvek. But, Tyvek is white, so you can't see through it, and it is a coarse material that likely has varying properties depending on where on the sheet you cut out a piece. Tyvek is sold in big rolls of 8 foot wide material - more than you could possibly want if you aren't building a house.
I tried Goretex "vents", which are a dark gray, adhesive backed membrane, made for venting electronic boxes. All I have of those are 1/2" diameter ones, which have a 1/4" diameter area in the middle with no adhesive on them. They work too, but the adhesive didn't stick well to acrylic for me, and I suspect they allow some water to go through the membrane. That material in a larger size would probably work, except the color and opaqueness make it hard to see the solution color changes accurately.

Once I make a couple of the devices I sketched above I plan to do some more side by side testing, including leaving one in the tank for several days to see how long the membrane will be effective before biofilm plugs it up.
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I wonder if some used clean-room type garment scraps would work, they're made of Tyvek but there's enough of a texture to them that sealing could be an issue...
Regular building Tyvek has a texture too, but holding it tight to an O-ring makes it seal well. If you have some of those scraps to spare, PM me. I would love to try them out. I am inclined to think that a white opaque material like Tyvek will end up being the best DIY device material we can get.

Another question: if garments are made of that stuff, it must be possible to buy the garment grade material itself. Do you know where to do that?
Just to update my progress on membrane style drop checkers:
Big problems! Once you reduce the quantity of fluid in the device down to where I have it now - a disk about 1/2" dia by 1/32 inch thick - it becomes almost impossible to load the fluid into it. Surface tension effects, wicking, and just unsteady hands make it very hard to seal up the fluid without losing it all. I finally managed to get one set up with the Cole Parmer membrane by freezing the fluid in the device before trying to put the cover membrane over it. That worked. And, the reaction time for the device was just about 5 minutes or so. But, the problem with loading it makes me think this is a dead end.

Next idea: Place a disk of acid free, non-buffered white blotting paper in the device, and soak that with the fluid. That will immobilize the fluid so the covering membrane can be added. If I can find some acid free, non-buffered paper I will try that today.
I have experimented with adding double and triple doses of indicator reagent to a pH test tube, to see if the color is any different. All that happened was that the color become more intense. Eventually the color gets murky and hard to "read" - that would be too much indicator reagent. In drop checkers I have been using from 2X to 4X doses of indicator. Intuition tells me that it is possible to use too much indicator reagent, but I suspect that would be more than 10% of the total volume of solution.
Just to update my progress on membrane style drop checkers:
Big problems! Once you reduce the quantity of fluid in the device down to where I have it now - a disk about 1/2" dia by 1/32 inch thick - it becomes almost impossible to load the fluid into it. Surface tension effects, wicking, and just unsteady hands make it very hard to seal up the fluid without losing it all. I finally managed to get one set up with the Cole Parmer membrane by freezing the fluid in the device before trying to put the cover membrane over it. That worked. And, the reaction time for the device was just about 5 minutes or so. But, the problem with loading it makes me think this is a dead end.

Next idea: Place a disk of acid free, non-buffered white blotting paper in the device, and soak that with the fluid. That will immobilize the fluid so the covering membrane can be added. If I can find some acid free, non-buffered paper I will try that today.
Update: The paper didn't work at all. Paper just doesn't absorb enough fluid to become colored enough to work. But, I tried a sponge and that was a different story. I bought a pack of those little sponge wedges that women use to apply or remove make up. Then sliced a thin piece off of one, about 1/16" thick or less. Cut it into a round disk. Used a 1/2" acrylic tube, about a half inch long as a holder. Put the sponge on a piece of Tyvek, soaked it liberally with 5 dKH water and added two drops of indicator reagent. Then I laid a piece of Cole Parmer membrane (transparent) on top and attached this sandwich of membrane sponge membrane to the acrylic tube using an O-ring as a rubber band to hold it on and seal it off. Immediately put it into the tank. And, in about 5-10 minutes it was green like my glass drop checker. Easy to make, easy to load with fluid, and seems to work like a charm so far. I will play with it some more tomorrow and post a photo if it still looks good.

The Tyvek I used was a piece of a Priority Mail envelope, which is a thinner Tyvek than building wrap and a lot cheaper. I'm not sure if the CO2 passed thru both membranes or just one, so I don't know for sure that this Tyvek is as good at passing gas as building wrap is.
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Quick thought. I might not explain this right, but here goes:

Why not cut a large sirynge in half, and put some of the solution in the bottom half (the half with the plunger). Then attach the membrane to the opening and push the plunger in until the fluid is against the membrane. If the membrane is gas permeable, it will let the air out of the chamber , leaving just the solution.

Did I say that right?
I understand what you are proposing. And, I think it would work, but it might take a few minutes to push the air out, plus you would have to be careful not to push hard enough to push the membrane off the end of the syringe. It would look clunky, but should work ok. If you try this, try to keep the amount of fluid small enough that the thickness of the fluid in the syringe is only a 1/16" or less. The thinner the layer of fluid, the faster it can respond. Another problem you would need to solve is how to see the fluid color. If you use tyvek you can't see thru it, so the only way you could see the color is thru the side of the syringe.
I am using a drop checker from Hong Kong. I made 4 dKH water from RO/DI, put it in the drop checker along with six drops of AP pH test.

It was dark blue, and within a few hours was light green bordering on yellow. Great!

Well, the weird thing is that the drop checker water stays that color, even after a night of no CO2 and surface aeration with an air stone.

Seems pretty weird to me.. I would expect it to return to a blue color..
That does seem weird. I am finding that the CO2 level in the tank drops very slowly, but by morning my drop checkers all show blue or near blue. And, I don't use an air stone at night, or increase the surface agitation. How are you turning off the CO2 at night? Obviously, I am thinking that you aren't really shutting it off.
Oh well, I did come up with another suggestion. You could assemble it while submerged in the solution. Meaning put the container in a larger container that is filled with the indicator and then attach the membrane.


Now that is a simple, embarassingly obvious, solution to my problems with trying to get a very small amount of solution in my membrane drop checkers. I fell into the trap of mixing only as much solution as I needed, rather than waste precious water. But, there is no good reason for not mixing a cupful at a time, and dumping what isn't needed - other than the waste of indicator reagent.

Are those paint containers art paints, from an art supply store? They are most likely polyethylene, which is virtually impossible to cement anything to. But, a hole in the cap, with a membrane liner, with the cap doing the sealing and retaining of the membrane might work well. Don't forget, to get fast response you need to minimize the thickness of the "disk" of solution. The time it takes to reach equilibrium is proportional to the volume of solution and inversely proportional to the area of membrane. So, maybe cutting off the bottom of one of those containers, keeping just the lid and a thin section of the body, with the membrane covering the missing bottom, retained by the lid?
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Schaadrak's idea for how to load one of these things inspired me to try a new, even simpler design, but one which relies on an O-ring to seal the membrane where it is smooth, instead of trying to seal it by pressing the folds of the membrane to the plastic body of the device using a rubber band. This is what I came up with:

Once all of the acrylic cement, and the white nail polish dries well I will try loadiing it with Schaadrak's method, and sticking it in the aquarium.

But, I will have to make some more 5 dKH distilled water first - put on my Chemist's lab coat, with the pocket protector, etc.
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You know, I am wondering about something here.. the idea as I understand it is that the liquid inside the drop checker contains no acids and then after it is put in the tank, exchanges CO2 in the air pocket with the water from the tank until they are at equilibrium.

Fine, that makes sense.. but why won't other acids in addition to carbonic acid also enter the water in the drop checker?

I think the point of the drop checker was to isolate carbonic acid from other possible 'contaminating' acids in the tank water that render the usual KH/PH/Co2 calculation erroneous.

It seems like if there was say, tannic acid from driftwood that it too would enter the water in the drop checker..

I'm sure this has been covered.. so help me understand why we are so positive that CO2 is the only thing that can enter the water in the drop checker.
The membrane is supposed to be gas permeable, but not liquid permeable. So, liquids can't pass thru it. And, as long as any gas on both sides is in equilibrium across the membrane, no net exchange of gases will occur either. So, if there is tannic acid on one side, but not the other side, the acid will be a liquid, which cannot pass thru the membrane. But dissolved gases are not liquid, so they do pass thru. I'm not yet clear on whether O2 and N in solution in the tank will pass thru the membrane but they should.

I found that if you just leave the membrane drop checker out in the air water vapor will pass thru it from the water inside to the low humidity outside, until the water is all gone. But, the indicator solution and the sodium bicarbonate seem to remain in the drop checker, leaving a very blue liquid there.

If you were to put a hot drop checker in cold water there would be water vapor passing from the drop checker to the tank, until the temperatures equalized. That would be from the higher vapor pressure inside the device than the tank water.
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blyxa, the basic idea for a membrane type drop checker is to use the membrane to allow CO2 in the water to equalize with the CO2 in the drop checker, and detect the amount of CO2 by watching the change of color of a pH indicator solution mixed with the drop checker water. The only advantage I see for doing it this way is to get a drop checker that reacts much quicker to changing CO2 concentration than does the regular air gap drop checker.

The reaction time for a membrane type drop checker is proportional to the volume of fluid in it that has to dissolve CO2, and inversely proportional to the area of membrane that allows the CO2 to enter the drop checker fluid. To minimize the reaction time requires maximizing the membrane area, while minimizing the volume of fluid. If the fluid is in the form of a round disc, like a coin, for example, the volume is the round cross section area times the thickness. If there is a membrane on one side of that disc, the area is pi times the diameter of the disc squared. So, the reaction time is proportional to the volume of fluid divided by the membrane area, or pi times the diameter squared times the thickness divided by pi times the diameter squared. That reduces to just the thickness. So, minimum reaction time requires minimum thickness of the fluid disc.

In order to see the color of the fluid disc the disc has to be thick enough to see the color - obviously a one mil thickness will never show a color, and a one inch thickness will show the color brilliantly. So, achieving a minimum reaction time conflicts with being able to see the color. One way to get around that is to view the color through the diameter of the disc instead of the thickness, but the mechanics of assembling the device, holding the device in the water, sealing the membrane, etc. make it very hard to keep a thin disc while still allowing for viewing the fluid thru the diameter.

A compromise is to make one side of the container very opaque white, so the color shows up better against the white background. This can be best done by using a Tyvek membrane for one side, and a transparent membrane for the other side - the viewing side. But, doing this only doubles the membrane area, cutting the reaction time in half. Since a big slug of fluid, like in the air gap drop checkers, takes a couple of hours to reach the final color, a reduction by half isn't very signiificant. We need a reduction by more than a tenth.

Another problem with using membranes, at least it's a problem for me, is sealing the membrane to keep the fluid inside.

All of the above led me to the simplest design - a 1/16 inch thick O-ring glued to a plastic plate, to make a recess for the fluid, with a membrane draped over that, so the O-ring seals against the membrane to hold the fluid in. The mechanical problem of attaching the membrane can be solved by making the plastic plate be a solid plastic cylinder, the same diameter as the outside diameter of the O-ring. Then the membrane can be held in place with a rubber band around it where it drapes over the plastic cylinder, and another O-ring makes a great rubber band. In order to hold this device in the aquarium I chose to glue an air line holder suction cup to the side of the plastic cylinder, which meant the cylinder had to be about 1/2 to 1 inch long.

To make the back face of the fluid disc white while still using clear acrylic for the plastic cylinder, I used white nail polish on the end of the cylinder. It took about 4 coats to get an opaque white coating, and I used the last coat as glue to attach the O-ring to the plastic cylinder. (Nail polish is acrylic glue with coloring in it.) Now, since I didn't have any solid plastic rod, other than a 3/8 inch diameter, for which I didn't have an O-ring to fit, I used a piece of 5/8 inch acrylic tube, with a 1/2 inch acrylic disc glued in the end, in place of the solid plastic rod. (I used the parts I had on hand.)

How do you get Cole Parmer membrane? Go to:YSI Meter Accessories And Replacement Membrane Kits - Cole-Parmer Catalog
The membrane kit costs about $30 with shipping included, and you get about 20-30 membranes, good for at least that many drop checkers. Each membrane is about 1.5 inch by 4 inch.

Natural latex is a gas permeable membrane, as I recall. So, a natural latex glove finger could be used to make a membrane type drop checker. All that is required is the engineering to figure out how to hold it together, seal the fluid inside, be able to see the fluid inside, with a white background, and hold it in the aquarium. The design possibilities for membrane drop checkers are almost endless.
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In other news, I removed the drop checker from my tank and stuck it on my mirror in my bathroom. Within a day the color went back to blue.

I had also taken a glass full of water from the aquarium and left it out over night to degass.

I submersed the drop checker (now blue) into the glass of water, and it is staying blue, confirming that indeed the CO2 level in the glass of water went way down.

So I guess I still have no idea why on Earth the drop checker's color stays green even after a night of surface aeration and no CO2.

Weird. But, I am comfortable in believing that green means good CO2 IF it is true that ONLY CO2 can enter the drop checker's water.

I still wonder why other acids in the tank water can't also enter a gas phase and enter the drop checker's water.

Hoppy, can you confirm for me the intent of the drop checker?

In a tank with a giant piece of drift wood, e.g. tons of tannic acid, your drop checker (not the membrane idea) should still give an accurate reading of CO2. Is that correct?

So there is the unstated assumption here that acids other than CO2 cannot pass through the gas pocket inside the drop checker.

I still want to know how we know that...
We know it because there will be no distillation occurring unless the drop checker water and the tank water are at different temperatures. Acids are just water with ions in it, with more H+ ions than OH- ions. If the acidic water becomes a gas it has to be H2O molecules, not a mix of ions and molecules. Even if there were distillation, the distillate would be pure water with no ionic solutes. That's why distillation gives us water without any salts in it.

So, only gas can go across the air gap from one glob of water to the other. Oxygen, nitrogen, CO2 and any trace gases will also go across. Volatile organics such as alcohol will also go across the gap. But acids and bases won't, since by definition, they are ions, not molecules.

That is how I see it anyway. I'm a bit troubled by knowing I can smell muriatic acid, which certainly appears to mean the vapors I smell are gases and acid. Perhaps someone with a lot more chemical knowledge than I have would like to discuss this?
We need to keep in mind that gas permeable membranes need to be cleaned periodically to eliminate the biofilm buildup. I understand that a bleach solution would do it. But, I'm not sure if any of the bleach would make its way into the indicator solution and spoil it. Also, if a fast reaction time is a goal, the thickness of the fluid behind the membrane has to be as thin as possible. A penny shaped slug of fluid is ideal, a "square" cylinder shaped slug isn't ideal. The perfect drop checker would have a slug of fluid only a few molecules thick sandwitched between two membranes, and would detect the pH electronically. That would be an almost instantaneously reacting device.
Another simple way to make membrane type drop checker

I made another, almost foolproof membrane drop checker. It has a 1/16 inch thick layer of KH reference fluid, for rapid response, an O-ring to seal the membrane, and is easy loading.

The blue fluid photo was when I first put both drop checkers in the tank. The green fluid photo is 1 1/2 hours later. So far I haven't checked on how fast the membrane one reacts. I had no trouble at all putting the fluid in it, with a couple of drops of pH reagent.

My next effort is one that I think will be easier to see the color on from outside the tank, and which can be used with either a tyvek or clear membrane. But, I haven't tried making it yet. (This is fun!!)

(Originally posted on the Barr Report.)
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That one is pretty easy to see. You should pull the ADA style one & yours above the water line, let both turn blue and see which turns green faster when returned to the tank.
You can't leave either one of those out of water very long or water vapor escapes, raising the KH. I pulled the membrane one out when it was very green and put it in a glass of tap water. It took about 30 minutes to go to a blue green and about an hour to become virtually blue. Like any device of this type the color approachs the equilibrium color asymptotically, so to characterize the response time you need to time it to an intermediate color. Once it is thoroughly blue I will put it back in the tank and time it the other way.

Then I plan to put a tiny sponge in the fluid chamber to reduce the fluid volume and see how much that reduces the reaction time. I did this before on another one and it makes a significant differenct.
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