Squawkbert

The membrane has replaced the PTFE/silicone rubber cap liner in the yellow cap (that has a big hole in it for instrument needles to pass through).

If your wife can bring you LC vials & some good water (high resistivity, polished, Milli-Q type) & you get some gas permeable membrane, you can make these things all day.

hoppycalif

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.

schaadrak

I have a idea for anyone who has an electric pH meter; make a long tube out of some membrane a little longer than your probe and only a smidge wider, seal up the bottom and fill the tube with some 4GH water. Then put your probe in and seal the top (somehow, I don't know if they have attachments that would work or if you would have to seal it with silicone or something). That would give you a large amount of membrane surface area and a small volume of "solution"

hoppycalif

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.)

Squawkbert

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.

hoppycalif

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.

hoppycalif

The sponge helped the response time a bit, but not by as much as I expected. It took 10 - 15 minutes for the color to reach about 2/3 of the way to the final color. So, I tried one last design, using the Tyvek Priority Mail envelope material:




This one used the same size KH reference sample, the same amount of indicator reagent, a sponge, but a Tyvek membrane. It reacted exactly the same as the one above using the Cole Parmer transparent membrane. This one is easy to make, a piece of plastic, an O-ring, and a suction cup airline holder. It took me about an hour to make. It is easy to load, easy to read, easy to make, and responds almost 10 times faster than the glass drop checker. And, everyone should be able to get a Priority Mail envelope to use as a membrane. I think I have exhausted all of the ways to make a drop checker!

caymandiver75

Wow nice job Hoppy! Should get a patent on that idea.

caymandiver75

Today I put distilled water with a KH of 5 in my Red Sea CO2 indicator but I'm having trouble deciding what my CO2 ppm is from it. Below is a photo that I just took. I can't decide if the ph in it is 6.6, 6.7, 6.8 or what.

Original

Jubs

I like this design as I have all the supplies to do this one but I can't figure out where the reading is coming from... Is it just the sandwiched Tyvek membrane spaced from the O ring that is allowing the exchange in reading ? Or am I missing something as to where membrane is exposed to the water ?