[hoppycalif]

This is an idea that has to work - there is no technical reason for it to not work. The problems are the mechanical problems. How to make a "cup" of the membrane and attach it to the probe so it never leaks, and can be done without driving you up the wall with spillage, ripped membranes, etc.

I haven't tried it, even though I keep thinking about doing it. I still haven't worked out a good way to make the "cup" and attach it.

You can use membranes made for oxygen probes, http://www.coleparmer.com/catalog/product_view.asp?sku=0552200&pfx=EW. I tried these and they do work, but they are very, very hard to work with because they are very, very thin and fragile.

Any membrane used like this has to be cleaned periodically because biofilm builds up on it, sealing the pores. I don't know how often the cleaning needs to be done though.

 

[hoppycalif]

That cap wouldn't fit my pH probe, but it would fit many more expensive probes. It might be worth the money to see how it goes together, but not to me.

If you are thinking of using something like this to control the CO2 there is a problem. These things will take a long time to reach an equilibrium concentration of CO2 inside the membrane, unless you keep the volume of 4dKH water inside very small relative to the surface area of the membrane. That lag in reacting to changes in CO2 concentration would make a controller constantly overshoot and undershoot. By the time the controller would shut off the CO2, the concentration in the tank would be way above the limit you wanted to set, and before it would again turn on the CO2, the concentration in the tank would be way below the lower limit you wanted to maintain. It will react fast only if the 4 dKH fluid is held in a very shallow "cup", something like a mm thick or less, but a cm or more in diameter. Trying to achieve that shape is what makes it so difficult to design a membrane based drop checker.

 

[hoppycalif]

Something like that would likely work, but it would still not work to make a pH controller act as a CO2 controller, because of the very slow reaction time of the drop checker. Remember it takes an hour or more for the drop checker to stabilize at the same concentration as the tank. When the concentration in the tank reached 30 ppm, it would take the drop checker at least a half hour to reach that concentration, but by then the tank could be at 50 ppm. And, the same problem occurs when the CO2 is shut off and the drop checker is slow to turn it back on. To work with a pH controller you need a reaction time around 5 minutes or less. More than that and you get a widely varying concentration of CO2 in the tank water, which leads to algae.