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Correlating pH meter readings to Reference cell KH/pH measurements vs known reference

Discussion in 'CO2 Enrichment' started by Tom Barr, Aug 28, 2012.

  1. Tom Barr

    Tom Barr Founder
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    I have tried to see how well the various methods work for measuring CO2.

    In a nut shell, even if your KH is off, weird, not accurate etc........the pH meter should get you pretty close(about as well as a 2500$ meter).
    Now this is not to say that the aquarium will be the same in all cases and that the degassed aquarium will be the same, they will be if you have a wet/dry and good surface skimming.

    I used a flat tip pH probe and rubber grommet and used maybe 300-400ul of KH reference solution 4KH(71.44ppm) and managed to get the air bubbles out. PITA.
    I think a simple manufacturer made slip on cap would do well/better and be cheap. I spoke to American Marine about this a couple of years ago. Not much has come of it.
    This compared very well vs the Oxygard CO2 meter. I needed to compare these two items against a standard American Marine pH meter (one of the oldest brands and still pretty good, easy to use etc) with the flat tip KH reference cell.
    I measured the KH in my tank water carefully using 2 different methods and they matched to the ppm and the reference measure at 71.4 matched within 1ppm.

    So my tank's all have the same KH, in this case 37ppm.
    Typically from about Dec-June it's 20ppm, the water companies struggle to keep it up due to the new water from snow and rain.
    My non CO2 tank measures : 7.58 to 7.60 pH
    The 120 gal measures about 6.1 late in the day
    The 70 Gal measures 6.01 midday
    The 180 measures 5.90

    There is likely little influence on KH from non bicarbonate alkalinity.
    So the pH/KH chart works in MY CASE right now.

    I do large water changes, I also have mature older wood and ADA Aqua soil.

    The other question is while the meters all read very close to the same and the KH etc.........
    I want to be sure, so I made some CO2 solutions with some flask and rubber stoppers.
    I used Distilled water with a conductivity of 0.9uS(very pure) and added enough KH to get 17.8ppm and made 1 liter.
    I chipped off 53 ppm of CO2 dry ice and weighed it right before I added(51 mg to 1 liter of water) it to the flask and sealed it. I was careful, not to have any air bubbles after sealing(after 6 attempts).

    I placed this on the magnetic mixer and the rubber top has a 1/2" pH probe gasket. I wait about 15 min and then take the reading.
    6.10-6.09. Pretty darn close. pH meter and probe are new and they were calibrated right before this was all done.
    This is also about the same range the Flat tip with the KH reference cell came up with using this same method(4 attempts, see? I was getting better).

    So that CO2 meter worked well and had relative fast response times in under 5min.

    Next, I want to see if I can detect low CO2 in the plant beds in the low flow regions vs the high flow where the CO2 comes in etc using just the pH meter.
    I predict I can.


    Some discussion comments:
    Verification of the method and using a reference sample is a challenge.
    Many plant hobbyists have long gotten away from using a pH meter to adjust and watch their CO2.
    KH uncertainty can varied and can be large depending on tap water sources
    Wet/dry/surface skimming methods will give the best degassing after the CO2 is stopped being added at night.
    Canister filters will retain the CO2 and surface scum also appears to contribute to this issue, the latter perhaps the majority.
    In general, do not assume anything, verify and double check, particularly if some thing is going REALLY well, and if something just seems off.
    The filtration issue alone/degassing rates etc, make the pH meter a problem for some, but easy as pie for others.
    The tap water KH with non alkalinity makes life tougher for some people, but , you should be able to use a KH relative measure based on a degassed sample(or if you have a wet/dry and surface skimming, then add CO2 during he day time only.....) and add CO2 to get the appropriate scaling needed to target say 40ppm or 20ppm or 30 ppm etc.

    When I did this, I noticed I'd not adjusted the timer correctly for the only problem tank I have which is the 70 Gal manzagumi. So I was able to catch that and adjusted the timer to account for the lighting time.
    For all the verification, it's the simple stuff that gets me time and time again, so I know not to trust myself and feeble mind and instead, need to go through and check things one by one.
    And if it gets me, then there's certainly going to be a good sized chunk of the public that also gets nailed.

    and not just the "new" folks either...........
     
  2. yme

    yme Lifetime Charter Member
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    great post!

    and interesting to read that your co2 levels in the 120 are not very high! if the pH/KH tabel is more or less accurate, then the co2 level is around 40 ppm. not that high.

    it seems to show that great plant growth is more than just increasing co2 levels. your tank proves this!

    greets,

    yme
     
  3. Forumsnow

    Forumsnow Member

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    In your opinion what is a good decently price meter to use? I know you can get them for like $15 but I would imagine you kinda get what you pay for.
     
  4. Tom Barr

    Tom Barr Founder
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    Yes, but I also noted than when I dropped the CO2 down just a little bit(turned the valve the wrong way slightly to increase, but was actually decreasing the CO2), I ended up stunting a few species and also getting hair algae.

    So I am really right at the cusp. I adjusted it a little higher since, but I only do so very slowly.

    The 180 Gal which is very resistant to growth/algae issues has another 20ppm extra and has no issues.

    Which is right along the lines of what the CO2 meter said about 6 months ago.

    So while this is not unexpected, it's fairly well in line with the general theory behind good CO2 being critical.
    I'd add to that saying that the most critical part of the day period is that 1st 1-2 hours as well.

    So it depends on WHEN we take the CO2 measurement as well.
     
  5. Tom Barr

    Tom Barr Founder
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    Pinpoint meters are good and easy to use.
    Run about 80-100$.
     
  6. Forumsnow

    Forumsnow Member

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    Cool thanks for the suggestion. Might have to pick one up in the coming months. Pretty interesting stuff
     
  7. Tom Barr

    Tom Barr Founder
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    Well, careful, I recall one guy we measured the kh/ph and ended up 220 ppm CO2 but there was no way that was correct!
    If you can reference the method and get close or use a conversion, then you are okay.

    Also watch the CO2 when it first comes on and watch for that 1-2-3 hour time frame in the photoperiod. See how fast the CO2 knocks the pH to the targeted range. Relative pH should be able to do this. I shoot for about 1.2- 1.4 pH drop. Then tweak as needed.

    I think this might be along with close observations, about folks best bet.
     
  8. yme

    yme Lifetime Charter Member
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    which means that for your 120, a CO2 level of more than 40 ppm is sufficient. If you didn't have your co2 sensitive animals, you would be confident that when you measure 60 ppm (and water flow is all right), that the co2 levels are adequate. no tweaking, just making sure that the current is perfect. From your measurements, it seems that obtaining sufficient CO2 is fairly easy :) !

    2 cents from my tank:

    according to the pH/kH tabel, my tank should contain 93 ppm CO2. The oxyguard CO2 meter measures 89 ppm CO2. Thus, also in my case, the pH/KH tabel is fairly accurate. (tank contains flourite dark and 50% WC weekly).

    greets,

    yme
     
  9. Forumsnow

    Forumsnow Member

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    Yep got ya on that. Was thinking of using it more to check distribution within the whole tank especially in plant bed. I could also use it to se how quickly my ph is dropping in the first crucial couple hours of the photoperiod. I believe I am getting very close to the correct levels of co2 in my 20 as there is very good growth in the stellata and other plants, and I see very little stunting. Also I have yet to see a trace of the hair algae return even though it is to early to know for certain.

    But at the same time when I trimmed my very very thick stand of colorata yesterday I did notice a few stunted tips on mixed next to perfect growth. I was hoping to use the meter to help me judge a good flow into the stand.

    I guess the meter would be more of a toy than anything as it appears watching the plants and animals is the most sure fire way of getting co2. It would just be nice to know what is going on in those first couple hours when the drop checkers are to slow to respond.
     
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