determining alkalinity via pH and co2*

[dartier]

I was thinking about the idea of a head space open to the water and would think that condensation may be an issue. Well that and salt air corroding the device. The manufacturer of the inexpensive one that was posted, has an option for a cap for remote sensing along with a small pump and filter to prevent moisture from fouling the sensor. You may be able to use that with a confined headspace and get around potential moisture problems.

Dennis

 

[karimwassef]

That's why I use fresh air. When you run the numbers, the fresh air variation impact is small and slow. I would not do it with indoor air.

Also, photosynthesis by day consumes massive CO2 and injecting external air actually helps offset this - basically providing just what is needed to be stable.

 

[karimwassef]

I would not run two loops.
I would not inject CO2.

Fresh air is substantially stable and my tank's growth rate shows it.

 

[orcafood]

I would never run any loop

I also want to know the amount of CO2 in my water, very interesting. As you have seen it is heavily related to Fresh Air Exchange (FAE) (anyone grown mushrooms before hehe). Which is probably related to the amount of CO2 in the air. I think we are right on board with exactly the same ideals, glad your tank is doing so well, my tanks not worthy!

Wow Dennis nice find again

 

[dartier]

That's why I use fresh air. When you run the numbers, the fresh air variation impact is small and slow. I would not do it with indoor air.

Also, photosynthesis by day consumes massive CO2 and injecting external air actually helps offset this - basically providing just what is needed to be stable.

I am not sure I understand what you mean by indoor air. I think what orcafood is proposing is using a trapped air pocket, like a submerged open ended pipe (protected from bubbles), with one of the infrared CO2 sensors sampling that air column. This would work out the same as the probe I posted earlier, as that is the premise of that unit, though the gas permeable membrane makes it impervious to water.

As for the air injection, I think it is a great idea and would support doing that in addition to trying to measure the CO2 content directly from the water. You could even run a second CO2 sensor sampling the incoming air to again detect a sensor going bad, or an unexpected tank event taking place (like your dual PH probes do for your implementation).

Dennis

 

[dartier]

Ok, now we are getting somewhere.

Take a look at this: http://www.kordon.com/kordon/products/aquarium-pond-accessories-2/breathing-bags#suppliers!

What if we used a piece of one of these bags to seal our sensor from water? That should remove the risk of water incursion. Though I would suggest putting the sensor above the waterline with the pipe extending below by a few inches.

Dennis

 

[dartier]

I am reading a thread on a planted tank forum about using this sensor in the exact same fashion as orcafood is proposing. That is where I stumbled upon the breather bags. I have not got to the end of the thread yet to see if it end up working as hoped, or was a bust.

Dennis

 

[orcafood]

Breather bags is a great idea

lol I read that same thread but never read further to the guys forum, tell me if you find anything more!

 

[bertoni]

The bags could be useful. I don't know what the velocity of the gas exchange would be, but someone might be able to measure that.

 

[karimwassef]

I meant that I wouldn't trust pH without injecting fresh air to stabilize.
Allowing the tank's pH to be driven by indoor air CO2 volatility would be a disaster imo.

Measure CO2 or not, I would use fresh air to reduce volatility.

 

[karimwassef]

I like the idea of measuring in-water CO2 as well as the air being injected.

It would help optimize my air injection. I'm currently running on maximum pressure ~30ft through two penductors with a massive vent fan to pressurize the external air through an 8" duct... overkill! But I know that I'm getting gallons of fresh air heavily aerating.

 

[dartier]

I read the thread on the planted forum, both the one I found and the one that orcafood posted. They were by the same person. In both cases he was able to measure the amount of CO2 in his FW tank using the inexpensive IAQ CO2 sensor.

The 2 challenges he had was keeping the sensor dry, he was mounting it inside the housing with a silicone membrane and putting it below the water surface.

My suggestion would be to use the remote sensing cap that they offer and use an aqualifter pump to setup a closed loop through the sensing head. The pump offered from the vendor is quite pricey. For the sensing head, a 1/2" PVC tee, with the inline legs sealed with a 1/2" to 1/4" NPT adapter so that a John Guest fitting can be screwed in for each side of the loop.

For the 3rd leg of the tee, the membrane needs to seal this portion. I am still thinking about how to do this and make it maintainable. I am thinking a union that can be tightened on with the membrane trapped between the O-ring and the union face may be all that is needed. The breather bags were slow to pass the CO2 and reach equilibrium, but for our needs, that may be fine.

Lastly, they found the sensor was much quicker to update the lower below the water surface it was positioned. So for safety it would be nice to be able to detect a leak for water incursion and have the pump not pump it through the sensor. Still thinking about how to do this without impacting the internal air volume in a significant way.

Dennis

 

[orcafood]

I like the idea of measuring in-water CO2 as well as the air being injected.

It would help optimize my air injection. I'm currently running on maximum pressure ~30ft through two penductors with a massive vent fan to pressurize the external air through an 8" duct... overkill! But I know that I'm getting gallons of fresh air heavily aerating.

It would be very interesting to see how stable the levels of dissolved CO2 are in your setup

 

[karimwassef]

I would compare the air CO2 to the water CO2 at different injection pressures over time...

The high pressure injection reduces the time for the air CO2 to impact the water CO2.

The water CO2 is not benign. It has its own forcing function as the tank actively consumes CO2... so the injection both adds and removes CO2 to keep the air and water tight.

 

[bertoni]

Is there any documentation on the type of silcone that he used? That's another interesting idea.

 

[dartier]

Is there any documentation on the type of silcone that he used? That's another interesting idea.

I am not sure if this is the product he used, or he DIY'd one, but he did link this site as a possible supplier: http://www.sspinc.com/products/Thin-Silicone-Membranes_4_category.htm

At one point he was talking about making his own between 2 pieces of saran wrap with a rolling pin, lol.

He mentions that the site above has 12" x 12" sheets that can be purchased.

Dennis

 

[karimwassef]

The silicone diffusion rate would need to be factored in.

I'm interested in time ... so this is a big deal.

 

[bertoni]

That looks like an interesting approach. I agree that we'd need to know the velocity of carbon dioxide across the membrane. It'd be an interesting experiment if the cost is tolerable.

 

[karimwassef]

I'm trying to gauge the rate of diffusion of air CO2 into aqueous CO2 and need to account for the rate of diffusion of CO2 through the membrane!

Don't like measuring time with a time error... but maybe it can be corrected for enough.

 

[orcafood]

I think a pipe sealed from the top in an overflow (low flow area) extended all the way to the bottom having no cap and some sort of inline fan like an aqualifter to keep the air moving through the top of the pipe to the sensor and back near the bottom so it is always full of air? Keep the sensor far from the water and bring air, which is under decent pressure from the top-capped bottom-open tube submerged. Then pack a bunch of desiccant into an inline filter before the sensor.