Please tell me if I understain:
I put kh ref solution in the cap
make a hole in the cap
put a membrane over the cap (sanwich bag?)

and what?
i do not understain "how this work".

This works just like a "drop checker" works. The 4-5dKH water captured around the works of the pH probe absorbs CO2 from the tank water until it is in equilibrium with the tank water, meaning until it has the same ppm of CO2 as the tank water. That changes the pH of the 4-5dKH water according to the equation used to generate the table of ppm of CO2 vs pH and KH. So, the pH probe reading is a reading of the CO2 in the tank water. The cap for that probe would work as a compartment for the 4-5dKH water since it is already the right size and seals pretty well.

I haven't tried this probe modification yet, and may not try it at all, since I have to cut a hole in the probe cap to do it. I don't yet have a gas permeable membrane to use, but an ordinary sandwitch bag isn't likely to work well, because even if it is gas permeable it would be too slow. A Kordon breather bag will work, but extremely slowly, which isn't usable for a hand held probe. You can also buy a package of membranes for an O2 probe from Cole-Parmer for a reasonable price and they might work fine. I just haven't gotten around to doing it yet. Tyvek is highly unlikely to work at all well, for the same reason - slow response.

great! i understain and want to try it. I have to assume it, I'me a gear freak, but I also have to assume that I am not ritch... yet

so lets try that DIY

so the first thing to get will be that membran. what do you think about Gore-Tex? would it work? I have an old jacket here so maybe I could use that.
Other than this, that would be bether to have something more than just a membran hold with an elastic. What I have is a Hanna ph checker. do you think that milwakee cap showed previously on this post would fit on it? is the membran good on this?

Also, i understain that the kh ref will absorbe the Co2 from the water. does it mean i will have to use new kh ref for each test?

thx!

OK, Just a very quick post to say thanks to VaughnH and Tom for persuing the drop checker and PH probe methods .....
I've recently been trying VaughnH's drop checker method Here and Tom suggested a more accurate way of mixing up the ref solution and am happy with the results.

There are at least 4 other people over on that forum who are now giving the drop checker method a try. I'm posting off some ref solution this morning for one of them.
I think many in the hobby will benefit from your efforts !

I'm following the progress of the probe method and if I find anywhere in the UK where I can purchase some suitable membrane (without breaking the bank), I'll give it a try.

Thanks again gents !

YSI Meter Accessories And Replacement Membrane Kits - Cole-Parmer Catalog is a possibility for membranes, at a reasonable price. I just ordered a package of these last night, so I haven't tried them yet. The Cole-Parmer website is an international site, with "stores" for other countries, so these may be available in Europe or nearby also.

To give credit where it is due: Dennis Dietz found this product and posted the link on APC.

thx for the link

and about that kh ref solution, what do you use?

Cooked/baked bakign soda: 400F for 45 min.
Use about 2x what you think you need.

10 grams may yield 6 grams after dehydrating.
Sometimes 8 if you live in drier areas.

I add 4.99 grams to 5 liters of DI water.
This makes 40KH.

Add 10mls of this to 90 mls of DI water to make a 4KH solution.

Regards,
Tom Barr

Just to say, I've tried this method of Tom's and it works very well and is easier than using a smaller volume of water and trying to adjust it with tiny amounts of bicarb. After diluting to 4dKH I double checked it with a KH test kit .... comes out at 4 !

I can confirm Toms advice to use more baking soda than you think you'll need.
I cooked off 7g and was left with less than 4.5g, so had to repeat the process.
Bicarb is cheap .... so cook it up !

Al

it's cooking! now I just need that membrane.
thx lot for the advises

Anyone ever figure out a good DIY cap/membrane for pH probes?

I bought a drop checker and cooked up the kH solution for that. Works good...

but onto the next project. I would like to build a cap for my pH Meter (Milwaukee Cheapo SMS122)... Wondering if anyone has found the "magic membrane". I would love to see someone spell this out clearly.