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Old 01-28-2007, 07:40 PM   #221 (permalink)
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banderbe- The way that I understand it, gas molecules in general are do not have the cohesive properties that liquid molecules do, so they are able to pass through the small holes of the membrane. Plus I think tannin molecules are just plain old too big.

Hoppy- I'll start testing to see what kind of attachment method works best. I don't think anything could fit in that between that lid and still shut properly

Also paint container update; its been almost twenty-four hours with no sign of leaking or wicking of the red dye. Things are looking good.
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Old 01-28-2007, 09:25 PM   #222 (permalink)
 
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hoppy, I don't quite understand how you make your new design?


Thanks
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Old 01-28-2007, 09:32 PM   #223 (permalink)
 
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Default Cole Parmer

How would you get a membrane from Cole Parmer? Now, I understand the device however couldn't you make the tube smaller? What about a natural latex glove? It is like a balloon and deflates when filled with air but it doesn't leak.
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Old 01-29-2007, 09:44 AM   #224 (permalink)
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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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Old 01-29-2007, 04:33 PM   #225 (permalink)
 
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I wasn't asking about the membrane. I was asking about the basic drop checker.

What assurance is there that only CO2 will enter the colored water in the drop checker?

If CO2 can enter, then other acids might be able to as well.

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The membrane is supposed to be gas permeable, but not liquid permeable. So, liquids can't pass thru it.
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Old 01-29-2007, 04:38 PM   #226 (permalink)
 
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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...
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Old 01-29-2007, 06:28 PM   #227 (permalink)
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Quote:
Originally Posted by banderbe View Post
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?
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Old 01-30-2007, 10:04 AM   #228 (permalink)
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When you smell acids you are indeed smelling the acid that has evaporated into a gas. This means that there is a possiblity that some acids could cross the air gap and end up in the indicator solution, but that pecentage is probably too small to affect the results of what we're testing for, since the surface area in the air gap is small and the air gap is saturated with evaporated water. The membrane would not allow acids in gaseous form through since evaporation only occurs at the surface.

I think that's right.
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Old 02-01-2007, 06:03 AM   #229 (permalink)
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Regular drop checkers staying green - maybe (I'm speculating here) there isn't enough water movement around it, or it's too close to the outlet of the CO2... if CO2 actually displaces air in the checker instead of equilibrating with it, you won't get it back out unless you remove the drop checker so that regular air is mixed in again (CO2 is heavier than air and could blanket the surface of the checker, but I would think it would go yellow if this happened).

Also - it isn't faster (or slower) than the open types, but it won't leak, even if flipped... and it's super easy to assemble (2mL LC sample vial, some of Hoppy's probe membrane, 4dKH purified DI, API pH indicator solution):


The red thing is the setting knob from my Visitherm Stealth heater. It's a tight fit, but that's a christmas mini-light suction cup holding the checker to the inside wall of my 46 (also w/ new pic today - see sig. link)
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Old 02-01-2007, 09:58 AM   #230 (permalink)
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Quote:
Originally Posted by Squawkbert View Post
Regular drop checkers staying green - maybe (I'm speculating here) there isn't enough water movement around it, or it's too close to the outlet of the CO2... if CO2 actually displaces air in the checker instead of equilibrating with it, you won't get it back out unless you remove the drop checker so that regular air is mixed in again (CO2 is heavier than air and could blanket the surface of the checker, but I would think it would go yellow if this happened).

Also - it isn't faster (or slower) than the open types, but it won't leak, even if flipped... and it's super easy to assemble (2mL LC sample vial, some of Hoppy's probe membrane, 4dKH purified DI, API pH indicator solution):


The red thing is the setting knob from my Visitherm Stealth heater. It's a tight fit, but that's a christmas mini-light suction cup holding the checker to the inside wall of my 46 (also w/ new pic today - see sig. link)
Where is the membrane located on that?

*Paint Container Update*- I can see a tinge of red in the water, so it looks like there is some wicking going on. Also that sample vial made me realize that my wife could get me something similar to those since she works in a DNA lab and they have to guard against cross contamination. I'll see if she can suggest anything that might be closer to the shape that Hoppy has.
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