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Old 01-11-2015, 10:46 PM   #21
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Originally Posted by jeffaquarius View Post
Testing CO2 in salt water tank is not recommended. Your readings will be meaningless since pH level is in the higher level than planted fresh water tank.
Thats why I decided to go with a drop checker. It completely removes the tank water from the equation.
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Old 01-11-2015, 11:04 PM   #22
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Testing CO2 in salt water tank is not recommended. Your readings will be meaningless since pH level is in the higher level than planted fresh water tank.
Out of interest what is the issue there? Is it past the chart range or the chart is more inaccurate at higher ph / kh or something?
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Old 01-11-2015, 11:30 PM   #23
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Out of interest what is the issue there? Is it past the chart range or the chart is more inaccurate at higher ph / kh or something?
The CO2 test kits are calibrated for fresh water with lower pH than salt water.
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Old 01-12-2015, 03:11 AM   #24
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Dela I could be wrong but I think the chart is accurate because the chart is assuming that the only dictator of ph level is co2. So at X kh with X ph where co2 is the only dictator of ph than co2 will be X with an accurate mathematical equation. The discrepancy comes as soon as you use our tank water. Perhaps that link was saying that our tanks have a tolerance as high as +- 20%

Mebbid is right. You have to use a drop checker to eliminate the discrepancies and the water used is RODI calibrated with a known kh value typically 4dkh? It's in a little chamber which allows co2 gas through a membrane. That way if you know the ph and kh of that water and the only effecting factor is co2 in theory the colour that chamber turns should match that of the chart. Actually measuring a true value is difficult but one has to assume it is what the chart says.


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Old 01-12-2015, 05:17 AM   #25
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The CO2 test kits are calibrated for fresh water with lower pH than salt water.

All co2 kits? I'm wondering if the manufacturer of the kit I linked do one for saltwater too.


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Old 01-12-2015, 05:45 AM   #26
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CO2 chart

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Dela I could be wrong but I think the chart is accurate because the chart is assuming that the only dictator of ph level is co2. So at X kh with X ph where co2 is the only dictator of ph than co2 will be X with an accurate mathematical equation. The discrepancy comes as soon as you use our tank water. Perhaps that link was saying that our tanks have a tolerance as high as +- 20%

Mebbid is right. You have to use a drop checker to eliminate the discrepancies and the water used is RODI calibrated with a known kh value typically 4dkh? It's in a little chamber which allows co2 gas through a membrane. That way if you know the ph and kh of that water and the only effecting factor is co2 in theory the colour that chamber turns should match that of the chart. Actually measuring a true value is difficult but one has to assume it is what the chart says.


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I found a later thread but it does contain the original thread I was thinking of. At the time I had no co2 injection so any error / test kit bias didn't make much difference. Low co2 was still low so to speak.

Once I flush the buffer out of the tank, I'll see if the chart comes in closer to the drop checker reading.

Edit - and now with link

http://www.plantedtank.net/forums/sh...5/topics/10041
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Old 01-12-2015, 05:54 AM   #27
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The CO2 test kits are calibrated for fresh water with lower pH than salt water.

Would the ph/kh/co2 chart still work though as it is basically a mathematical formula?

If it doesn't work, would you know what causes it to be wrong in SW?

This is just for interest to see if the chart is any more reliable in SW. For example I'm assuming driftwood is not commonly added in SW tanks.
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Old 01-12-2015, 06:00 AM   #28
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Good article on it that seems to agree with you. http://www.reefkeeping.com/issues/2007-05/rhf/

Also, youre correct driftwood is avoided in reef tanks.
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Old 01-12-2015, 07:10 AM   #29
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Would the ph/kh/co2 chart still work though as it is basically a mathematical formula?

If it doesn't work, would you know what causes it to be wrong in SW?

This is just for interest to see if the chart is any more reliable in SW. For example I'm assuming driftwood is not commonly added in SW tanks.

The chart does not care what type if water is involved. It is using a specific value of alkalinity. Whether the alkalinity is affected by calcium carbonate or sodium bicarbonate is irrelevant. The equation states that at that particular kh level with this particular ph level (only altered by carbonic acid) the carbon dioxide level will be X. The chart uses that formula right across the board.

When we take the chart across to use in saltwater or freshwater the values become meaning less because the conditions required to get the scientifically and mathematically correct answer for co2 do not exist.

The chart therefore cannot be used reliably in any setup. It is the ph value that really matters though. The chart doesn't care what kind of buffers are present because they all contribute to the measured alkalinity value. What is important is the things that are present that directly alter the ph level. For example, like you say the addition of driftwood. The chart is not taking tannic acid in to consideration during the calculation and it is impossible to subtract the ph difference caused by tannic acid. Tannic acid is just one example, there my be others.

In turn the ph value becomes unreliable for the equation because there are other factors affecting it.

The problem we face is determining kh. How accurate is the kit used for this? Already we have discrepancies and tolerance.

The chart can only be used as a very rough guide as far as I understand.


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Old 01-12-2015, 07:23 AM   #30
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The higher end test kits are quite accurate for measuring dKH. The red sea kit I use for example is accurate within 0.14dKH which for all intents and purposes would allow for a margin of error that's not going to harm anything.
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