DIY Drop Checker

Naja002 said:

A DIY HOB Drop Checker and pH Controller from Sept. '92:

The Krib

HTH

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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.)

u2_crazy said:

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?

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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.

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.

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.

hoppycalif said:

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.

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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.

schaadrak said:

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.

Tschuss,

Kent

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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?

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.

banderbe said:

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.

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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.

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.

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.)