Recently on another forum there has been much talk of the inaccuracies of pH/KH charts.

I've only recently heard of the pH difference method i.e.

test tank water pH
leave sample for 24 hr.
re-test
if the difference is a pH of 1 then that's 30ppm CO2 in the tank

Is it really that simple? No need for KH testing.

Assuming this method works, if we have accurate pH testing then all sounds good.

Are there any flaws to this method?

PS I'm loving VaughnH's dropper method too. I just don't like equipment in the tank.

Well, it's not that accurate, better to try the drop checker KH ref solution method.

You can use the pH drop method if you eyeball carefully a good pH relative the degassed CO2 (48 hrs for smapled tank water).
It might be 1.4 , 1.2 or 1.0 pH reduction from ambient.

Regards,
Tom Barr

The pH of 1.0 reduction method, using the pH of "degassed" water vs. the water in the tank, only works if you know the ppm of CO2 in the "degassed" water. A 1.0 reduction of pH means there is 10 times as much CO2 in the tank water as in the degassed water. But, a 1.3 reduction means 20 times as much CO2. Big difference! And, the ppm of CO2 in degassed water can be between .5 and 3.0, depending on how long you wait and other factors unknown to me. So, this method is a comforting method, but far from an accurate one.

Thanks for that.

Do you mind if I copy/paste this to another forum (TFF) where this "new" CO2 testing technique is being hyped? Obviously I'll credit you.

No, I don't mind, but a reference to the drop checker method would help a lot of people too. My enthusiasm for that method hasn't waned a bit yet.

Cheers. I'll devote a whole thread to it and the dropper method.

George,

The dropper method with a ref KH solution is really useful.
It's easy, it's cheap and most can quickly look at any point of the day and know it's in good shape or not.

It does have some resolution issue since it's a colormetric test method.
So the best you can do there is .2-.4 pH units for color, not bad....but not that great either.

The pH probe method cost 60-100$ more.
It uses plain DI water + baking soda for the ref KH.
The holder is a pair of tubes, one is 2" ID and 1" tall, the other is a 1" ID tube about 3" long and is glued on the inside of the larger one.

The surface area increase is about 10-20X that of a drop checker.
The pH probe increase accuracy is 10-20X as well.

The 1.5" cylinder is glued to a 1.5" disc, 1/4" thick. I drill a 3/4" hole in the center of the disc's top. I next add a grommet to make the seal between the pH probe the chamber below. I then add a 1" disc to the lower part of the 1 ID x 3"L tube. This seals the lower half.

I glue two small pieces of plastic string, zip ties, whatever to suspend the 1" tube inside the air gap in the larger 3" tube.

See word doc drawing.
Note, I've applied for a patent pending.
Any non commericial use is permitted.

Regards,
Tom Barr

very cool!

If I am correct, the bottom of the "air gap" is open? This is where the CO2 dissolves into the airgap, right?

The only thing I do not like is that it is yet another internal device. So we have the internal CO2 venturi device, powerheads connected to CO2 lines and now another internal device. Get's very hard to hide all these devices.
Could it be possible to make an external design? p.e. with a very big air gap (tube) that enables you to put the pH-electrode outside the aquarium? Think it will increase the reaction time and possibly less accurate.

well, I think it is very very wise and happening! a major breakthough in the hobby!

greets,

yme

The smaller 1" tube coming down is closed on the bottom, open on the top.
The larger 3" tube is closed on the top, open on the bottom.

You may increase or decrease the size of the unit, but you also increase/decrease the contact with the air/water interface.

Note: the device need not be round, it may be thinner reactangle but such shapes can influence the air/water interface and diffusion rates.

I think a simple square with a hook or a floating or suction cup version of the round pH probe ref unit is extremely useful, but it does not require you to keep inside the tank all the time, just when you want to measure the CO2.

Note:
Such units are easy to remove and add back to the tank again!! They also may be moved from tank to tank easily!!

You keep them in a small glass/cup with DI water in it till you need to place it in the tank. Give it about 1-2 hours before a reading is to be taken. More time is better.

The air gap will equilibrate and you do not have to add fresh KH ref solution each time you move it because it's in the inside tube.

Some folks leave their pH probe inside the tank 24/7, but you do not have to, same deal here.

So unlike a CO2 reactor, this need not be an eye sore.

Regards,
Tom Barr

This is brilliant stuff. Thanks Tom.