hoppycalif

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.

Squawkbert

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...

hoppycalif

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?

hoppycalif

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.

junco

Quick question... do you have to use the same amount of pH indicator that the test kit suggests? For example, if I'm supposed to add 7 drops of the indicator solution to 5 ml of water... do I need to do the same for my drop checker? I only ask because I made mine very small. I don't think all that liquid will fit in it.

hoppycalif

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.

junco

well that's great. man i'm stupid. i just checked and my drop checker can hold a total of 2.5 ml of liquid. so what do I do, use a half dosage of indicator solution? that won't even work though... if i put 2.5 ml of water in it, the indicator solution would make it overflow. i guess i'll have to do 2 ml of water + 3 drops of solution or so. would that work?

redstrat

I use 2-4 drops per 1ml water

hoppycalif

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

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?