Tank pH effect with large indoor CO2 concentration

[taricha]

My tank is in a classroom, and I realized this means that the CO2 in the room may go quite high and this might make for an interesting demonstration of how the CO2 affects the tank pH. So here we go.
This is what the pH swing looks like when the skimmer intake line is run out the window.

The Light Blue is the measured indoor CO2 of the room. It starts around 400ppm after the building has been empty for a weekend and climbs throughout the day (jumps, dips and stairsteps are due to movement of student groups of different sizes and A/C cutting on and off.) The indoor CO2 level peaks around 2500ppm. The Red data points are the pH measurements that showed a dip throughout the daytime from 8.07 down to 7.76 before slowly recovering. The Dark Blue is a calculated concentration of how much CO2 is in the water based on pH and Alk measurements. The Alk was held constant at 7.0 to 7.1 for this data.

A few more comments:
The calculated water CO2 starts the day in equilibrium with the inside/outside CO2 level, which is a nice check on the calculation method.
The downward effect on pH from this much CO2 is sizable, especially at this low alkalinity - even with the skimmer pulling in air from outside. It is not until the room CO2 is well down from its highs that the pH can start to recover. Pulling outside air seems to have helped quite a bit, as the tank water CO2 levels went from 400 to a max of only 900ppm. If the skimmer were pulling the indoor CO2 rich air at 2500ppm, the pH shift should be even more dramatic. Hopefully I'll get data on that for comparison soon.

 

[Ron Reefman]

Nice demo and clear understanding of the issue. Thanks for showing this.

I have to admit I'm a bit surprised that even with the skimmer pulling outside air the dip in pH was so dramatic. I think this shows that the skimmer isn't as effective at oxygenating the water as many people seem to think it is. I've always felt that water motion and surface agitation makes for better transfer of gases.

 

[MabuyaQ]

Nice demo and clear understanding of the issue. Thanks for showing this.

I have to admit I'm a bit surprised that even with the skimmer pulling outside air the dip in pH was so dramatic. I think this shows that the skimmer isn't as effective at oxygenating the water as many people seem to think it is. I've always felt that water motion and surface agitation makes for better transfer of gases.

Gas exchange is water to air surface based most of the surface in a skimmer is bubble to bubble so there is no exchange happening anymore as equilibrium has already been reached in the lowest part of the skimmer where the bubbles are created/mixed.

 

[taricha]

I have to admit I'm a bit surprised that even with the skimmer pulling outside air the dip in pH was so dramatic. I think this shows that the skimmer isn't as effective at oxygenating the water as many people seem to think it is.

The gas exchange is the part that interests me the most also.
I don't know whether this should be considered a disappointing performance by the skimmer intake pulling outside air or not.

One way to look at it is the room air stayed above 2000ppm CO2 for 3 hours, which means it was roughly 1600+ppm above outdoor baseline. The tank CO2 went up from 400 to 900 for an increase of 500ppm.
So the tank moved +500ppm in a room that was +1600ppm meaning the tank went ~30% of the way towards the room.

Maybe after looking at what pulling straight room air does, we can get a better sense of whether that's impressive or not.

 

[Dan_P]

My tank is in a classroom, and I realized this means that the CO2 in the room may go quite high and this might make for an interesting demonstration of how the CO2 affects the tank pH. So here we go.
This is what the pH swing looks like when the skimmer intake line is run out the window.

The Light Blue is the measured indoor CO2 of the room. It starts around 400ppm after the building has been empty for a weekend and climbs throughout the day (jumps, dips and stairsteps are due to movement of student groups of different sizes and A/C cutting on and off.) The indoor CO2 level peaks around 2500ppm. The Red data points are the pH measurements that showed a dip throughout the daytime from 8.07 down to 7.76 before slowly recovering. The Dark Blue is a calculated concentration of how much CO2 is in the water based on pH and Alk measurements. The Alk was held constant at 7.0 to 7.1 for this data.

A few more comments:
The calculated water CO2 starts the day in equilibrium with the inside/outside CO2 level, which is a nice check on the calculation method.
The downward effect on pH from this much CO2 is sizable, especially at this low alkalinity - even with the skimmer pulling in air from outside. It is not until the room CO2 is well down from its highs that the pH can start to recover. Pulling outside air seems to have helped quite a bit, as the tank water CO2 levels went from 400 to a max of only 900ppm. If the skimmer were pulling the indoor CO2 rich air at 2500ppm, the pH shift should be even more dramatic. Hopefully I'll get data on that for comparison soon.

Nice data. There seems to be about a two hour delay between the pH curve response to the indoor CO2 curve. I lookedat the onset of pH decline, pH minimum and pH rise. Very roughly two hours prior, the CO2 began to rise, the CO2 hit its maximum and the CO2 started its decline, respectively. All very rough.

 

[Randy Holmes-Farley]

Nice demo and clear understanding of the issue. Thanks for showing this.

I have to admit I'm a bit surprised that even with the skimmer pulling outside air the dip in pH was so dramatic. I think this shows that the skimmer isn't as effective at oxygenating the water as many people seem to think it is. I've always felt that water motion and surface agitation makes for better transfer of gases.

Just to clarify, it is not oxygenation, but CO2 equilibration which is technically more difficult and slower than oxygen equilibration because CO2 hides in forms like bicarbonate that are not in instant equilibrium with CO2.

 

[JimWelsh]

My tank is in a classroom, and I realized this means that the CO2 in the room may go quite high and this might make for an interesting demonstration of how the CO2 affects the tank pH. So here we go.
This is what the pH swing looks like when the skimmer intake line is run out the window.

The Light Blue is the measured indoor CO2 of the room. It starts around 400ppm after the building has been empty for a weekend and climbs throughout the day (jumps, dips and stairsteps are due to movement of student groups of different sizes and A/C cutting on and off.) The indoor CO2 level peaks around 2500ppm. The Red data points are the pH measurements that showed a dip throughout the daytime from 8.07 down to 7.76 before slowly recovering. The Dark Blue is a calculated concentration of how much CO2 is in the water based on pH and Alk measurements. The Alk was held constant at 7.0 to 7.1 for this data.

A few more comments:
The calculated water CO2 starts the day in equilibrium with the inside/outside CO2 level, which is a nice check on the calculation method.
The downward effect on pH from this much CO2 is sizable, especially at this low alkalinity - even with the skimmer pulling in air from outside. It is not until the room CO2 is well down from its highs that the pH can start to recover. Pulling outside air seems to have helped quite a bit, as the tank water CO2 levels went from 400 to a max of only 900ppm. If the skimmer were pulling the indoor CO2 rich air at 2500ppm, the pH shift should be even more dramatic. Hopefully I'll get data on that for comparison soon.

Which set of constants for K1 and K2 are you using, and which pH scale? It looks like you might be using the programs's defaults, which are likely not correct for your pH units (your pH probe very likely uses the NBS scale, rather than the Total scale). Is the "CO2 in the water" the pCO2 value from the calculated results, or something else? Also, what temperature and salinity is this tank?

 

[taricha]

Ok, here we go. Side by side comparison of what pulling outside air vs indoor air for the skimmer does to the tank pH and dissolved CO2.
First, same data posted above with a few more labels.

And here's the data for the skimmer with the air being pulled from the room.

Note that the room and therefore the tank didn't fully recover overnight to the weekend CO2 levels of 400ppm matching outdoors. The tank water tracked much closer to the high indoor CO2 - when the room CO2 finally started coming down below 2000ppm, the aquarium had nearly caught up to it! The tank finally topped out at 1550ppm CO2. The pH at that level (7.56) was a full 0.50 pH units lower than the pH at normal outdoor 400ppm CO2 (8.07pH) from the day before.

 

[taricha]

Which set of constants for K1 and K2 are you using, and which pH scale? It looks like you might be using the programs's defaults, which are likely not correct for your pH units (your pH probe very likely uses the NBS scale, rather than the Total scale). Is the "CO2 in the water" the pCO2 value from the calculated results, or something else? Also, what temperature and salinity is this tank?

Yep! definitely more features and options than I could make sense of, so I didn't mess with the defaults much.
"K1, K2 from Mehrbach et al., 1973 refit by Dickson and Millero, 1987"
Seawater pH scale, but yeah the hanna pHep+ meter likely isn't using that.
And yes, I mean the calculated pCO2 when I sloppily say the "CO2 in the water". Temp went from 26.8 in the morning to 27.2 afternoon for both data sets, and salinity 34.

 

[taricha]

There seems to be about a two hour delay between the pH curve response to the indoor CO2 curve.

Looks like the very rough 2 hour delay holds up pretty well.
Which is an interesting contrast with when I messed around with dissolved O2, it was much faster to respond to a change in aeration as explained below...

it is not oxygenation, but CO2 equilibration which is technically more difficult and slower than oxygen equilibration because CO2 hides in forms like bicarbonate that are not in instant equilibrium with CO2.

 

[Brian_68]

Those levels above 1500 and 2000 ppm are concerning in a classroom considering the negative affect of cognitive ability in humans.......

CO2 on the Brain and the Brain on CO2 - Advanced Science News

Opinion: Do you think we have a serious greenhouse gas climate change problem caused by anthropogenic CO2 emissions in our atmosphere, which is threatening the future health and well-being of our civilization?

www.advancedsciencenews.com

 

[JimWelsh]

Yep! definitely more features and options than I could make sense of, so I didn't mess with the defaults much.
"K1, K2 from Mehrbach et al., 1973 refit by Dickson and Millero, 1987"
Seawater pH scale, but yeah the hanna pHep+ meter likely isn't using that.
And yes, I mean the calculated pCO2 when I sloppily say the "CO2 in the water". Temp went from 26.8 in the morning to 27.2 afternoon for both data sets, and salinity 34.

Just a suggestion: For better accuracy, try the NBS sale and the GEOSEC or Peng, et. al. constants. It will make a significant difference.

 

[taricha]

Thanks a ton @JimWelsh for the help. You're right - it's a big correction. At first glance, the recalculated values are bigger than makes sense.
The new K's and NBS pH scale increase the calculated pCO2 by somewhere in the ballpark of 50%.
400 pCO2 -> 640 on the low end and 1550 -> 2360 on the high end. Hmm.....

 

[Rick Mathew]

My tank is in a classroom, and I realized this means that the CO2 in the room may go quite high and this might make for an interesting demonstration of how the CO2 affects the tank pH. So here we go.
This is what the pH swing looks like when the skimmer intake line is run out the window.

The Light Blue is the measured indoor CO2 of the room. It starts around 400ppm after the building has been empty for a weekend and climbs throughout the day (jumps, dips and stairsteps are due to movement of student groups of different sizes and A/C cutting on and off.) The indoor CO2 level peaks around 2500ppm. The Red data points are the pH measurements that showed a dip throughout the daytime from 8.07 down to 7.76 before slowly recovering. The Dark Blue is a calculated concentration of how much CO2 is in the water based on pH and Alk measurements. The Alk was held constant at 7.0 to 7.1 for this data.

A few more comments:
The calculated water CO2 starts the day in equilibrium with the inside/outside CO2 level, which is a nice check on the calculation method.
The downward effect on pH from this much CO2 is sizable, especially at this low alkalinity - even with the skimmer pulling in air from outside. It is not until the room CO2 is well down from its highs that the pH can start to recover. Pulling outside air seems to have helped quite a bit, as the tank water CO2 levels went from 400 to a max of only 900ppm. If the skimmer were pulling the indoor CO2 rich air at 2500ppm, the pH shift should be even more dramatic. Hopefully I'll get data on that for comparison soon.

Awesome work...I love it when data is added to our observations and ideas...thanks for sharing..

 

[gbru316]

Necro'ing this thread (my apologies) but I recently purchased an indoor air quality sensor (measures pm2.5, pm10, co2) that ties in with my personal weather station and placed it directly over my tank.

I run a 40 breeder with 1x Maxspect Jump 2k and 2x Tunze 6040 providing a great deal of surface agitation (there's also an mp10 and I use a Neptune COR-15 for return). I'm using a Reef Octopus Classic 110-INT skimmer with BRS Co2 scrubber, configured to (mostly) recirculate, and I inject 25 mg/ml O3 from 00:00 to 03:00.

I've also found that indoor co2 concentrations seem to have a much bigger impact on pH than anything I do with the skimmer and scrubber. Typically, the day after my wife makes a more elaborate dinner (natural gas, internally vented hood), pH is depressed by about 0.2 units. This corresponds to an indoor Co2 concentration of about 1500. While I'm not surprised overall, I do feel this has placed a renewed interest on my end for adding an ERV into the renovation plans for next year -- and we were already adding an externally vented hood to the range.

Until then, I've been airing out the house a bit after she cooks using a box fan in one window, and opening the sliding door next to the tank for 30-45 minutes. This drops indoor CO2 to slightly above outdoor for a small period of time. I've also turned the HVAC fan "on" instead of "auto" which has evened things out a bit, too. Currently, indoor air is sitting between 700-800 ppm CO2, pH stays between about 8.0 and 8.15 consistently, which I'm satisfied with. I don't have days with pHmax of 8.1 followed by a day with pHmax of 7.9. Stability promotes success, right? It's just one more thing on my list of daily tank stuff to worry about. Hopefully the ERV and vented hood help with that.

After going through all this, I'm convinced that anyone that has low/unstable pH concerns absolutely needs to start monitoring indoor Co2. Otherwise, you're kind of flying blind.