Testing Carbon Dioxide Levels

[George Farmer]

There's been much emphasis on CO2 testing. Rightly so, poor CO2 causes algae issues without a doubt. A CO injected tank with mod-high lighting and regular ferts should keep CO2 at around 30ppm. This eliminates CO2 as the limiting growth factor, leaving us to concentrate on EI or other dosing and plenty of plant bio-mass to ensure algae is kept at bay.

pH/KH/CO2 charts have their uses for a ball-park CO2 figure but unless we are using accurate testing and other acids in the tank aren't affecting results then a true CO2 figure is unlikely.

Recently the "pH drop of 1" method has been hailed as our saviour.

How does this work?

Test tank pH and note result
Take sample of water and leave 24 hrs to "de-gas"
Test sample pH and note result
A difference in pH of 1 indicated 30ppm

But, in the words of Barr Report's VaughanH .....

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.

All is not lost though. For the enthusiasts out there - http/www.barrreport.com/co2-aquatic-plan....html#post10996

and http/www.aquaticplantcentral.com/forumap...html#post244534

 

[lljdma06]

Great post, George. I think it was either zig or Jimbooo who showed me the method via PM. It could have been you too, for all I know. It's great seeing it again, and if I had a moderately lit tank, it would be a Godsend.



llj

 

[George Farmer]

Did you read toward the end of the post? It's not as accurate as you may believe.

 

[lljdma06]

Did you read toward the end of the post? It's not as accurate as you may believe.

Oh, I did, and it's still a little better than the kH/pH conversion thingy, which I have used too. Will anything ever be completely and totally accurate? I seriously doubt it, unless massive amounts of money aor brainpower are forked over. Perhaps "Godsend" was not the correct term. I tend to exaggerate when it is past my bedtime.

It's still a good post, George. Just read throught the links in your post, and I'm a musician, George, not a chemist! That was way beyond my ability to comprehend on a Saturday night. When the English Idiot version for llj becomes available, I will read it.

Goodnight, George. Or is it Goodmorning?

llj

 

[George Farmer]

Good morning. 2.30am, night shift!

 

[plantbrain]

I added a drawing in a word.doc file tio show how to make a DIY pH probe KH reference testing tube.

It's about 10-20x more responsive and about 10-20x more accurate.
Maybe +/- 2ppm of CO2 if you are careful irrespective of tap/tank water interferences.

Beat that

Note, this unit will have issues if you use the CO2 mist/disc diffusers methods as the gas will travel and get caught in the air chamber and give a false positive if you assume the CO2 bubbles are different in CO2 concentration than the water solution.



Regards,
Tom Barr

 

[zig]

I added a drawing in a word.doc file tio show how to make a DIY pH probe KH reference testing tube.

Cant seem to find the word.doc file, anyone point me in the right direction? Thankyou

Edit: ok found the file but i cant seem to open it even though im logged in, anyone else able to open it?

 

[George Farmer]

Works fine for me. Very cool in fact!

 

[Bowman]

But, in the words of Barr Report's VaughanH .....
(snip)
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.

If the sample is left until it reaches equilibrium with atmospheric CO2 would this not be quite stable and predictable ?
It would vary with ambient temperature I suppose, but at STP I think I remember an equilibrium concentration of 3ppm ?? *

A pH drop of 1 has worked well for me for about a year now and has been stable and predictable without needing special reference(test) solutions.
The downside of this method is that for pH drops of greater than 1 the scale gets compressed too much and needs great care and accuracy.
As Vaughan points out, a drop of 1.3 makes quite a difference ! but for a drop of 1 or a bit less the scale opens out and should not cause problems.

* (not counting global warming and how much more CO2 we put into the atmosphere of course ! )

 

[plantbrain]

The point is not so much a method that works or not, but rather one that is acurate and reproducible for everyone in all cases.

This pH probe method is very accurate in all cases, I'm not saying the pH 1 unit drop does not work for many, it does and will. But not everyone will experience this and may need to tweak the CO2 up a bit more.

Now using the pH to do that at the upper ranges might not be wise.
Bubble rates are perhaps beter, since adding more CO2 is still addijng more CO2 and the samll ioncrease in gas additions is still small additions. If you use the pH, then it becomes much larger changes per unit.

If I add say 1 bub/sec to get a CO2 of 10 ppm, 2bub/sec for 20ppm and 3 bub sec for 30ppm, this is not the same as using the pH, the level does not jump way up as I increase the bubble rate.

The bubble rate is relative, more something you would eyeball.
That leaves some leary. So this is another way to skin a cattail.


Regards,
Tom Barr

 

[Bowman]

The point is not so much a method that works or not, but rather one that is acurate and reproducible for everyone in all cases.

Hi Tom, I guess you were responding to my post ? Sorry if I gave the impression that I was trying to score a point
My main purpose in posting was to ask, in my first line, of the assembled 'brains' on the forum, if the equilibrium CO2 level was quite as uncertain as Vaughn was suggesting, and, by implication, how significant this would be on the overall efficacy of the "1-pH drop" method.

The rest, about how it had worked ok for me for about 1y was a bit of an addendum by way of conversation and encouragemnt to others who may not yet be kitted-up with the extra gadgets yet,, i.e. they shouldnt be "leary" of it !

If I add say 1 bub/sec to get a CO2 of 10 ppm, 2bub/sec for 20ppm and 3 bub sec for 30ppm, this is not the same as using the pH, the level does not jump way up as I increase the bubble rate.

Yes, I take your point and that is a good way. In my case (I dont use bubbles) I can observe the rate at which CO2 dissolves out of my extended cylindrical inverted bell chamber.

The bubble rate is relative, more something you would eyeball.
That leaves some leary. So this is another way to skin a cattail.

I agree, totally !

PS. What a busy forum, I had to dig really deep to remember where I had been !