determining alkalinity via pH and co2*

[karimwassef]

A very slow correcting function...

[karimwassef]

Word of warning - I have crashed my tank when I took my air injector offline for maintenance for too long.

It serves a key function in eliminating the bias due to CO2 fluctuations in a closed space. Like a limiting diode, it keeps the system stable by avoiding excursions that aren't real.

All it took was one day where my normally 400-500ppm CO2 to get up to 1000 for a couple of days - and my pH got pushed too low, driving my loop to inject so much Alk that it jumped from 8.5 to 11... predictable results after that...

Everything is back in good shape now

[dartier]

Originally Posted by karimwassef

Word of warning - I have crashed my tank when I took my air injector offline for maintenance for too long.

It serves a key function in eliminating the bias due to CO2 fluctuations in a closed space. Like a limiting diode, it keeps the system stable by avoiding excursions that aren't real.

All it took was one day where my normally 400-500ppm CO2 to get up to 1000 for a couple of days - and my pH got pushed too low, driving my loop to inject so much Alk that it jumped from 8.5 to 11... predictable results after that...

Everything is back in good shape now

Hang on, this almost sounds like the type of control I had originally assumed. Does this mean that when the injectors were off line, you (closing the loop) upped the kalk dosing too high based on faulty readings?

Are those photos from before or after the crash? If after, that is pretty impressive, as the crash was not that long ago, and that is some serious growth.

Dennis

[karimwassef]

Yes. That's what happened because pH alone (without the fresh air injector) does not align to Alk. To force the pH-Alk relationship and use the feedback loop, CO2 needs to be kept stable through the fresh air injection.

The pictures are ~ 3 months before the crash. We're selling the house so I've been selling the corals and fish ... so no new pictures but I can take some of the corals left in the tank.

Once I reestablished the air injection, the tank bounced back quickly.

[orcafood]

I still don't see why we don't try and get the actual CO2 level in the water or from a head space floating on the water surface. Plus then it could be applied to others tanks who can't assume steady state equilibrium.

I think I will set it up on my pc similarly to how you did with myself as feedback. Definitely the way to do it.

[karimwassef]

I do monitor the CO2 of the intake air and I've confirmed that it's the same in the air around the submerged injector.

If I use internal house air, I'd have to constantly adjust my loop manually.
If I use external fresh air, there's little to no variability and I can just use pH.

The injector removes one major variable (CO2) leaving pH as the controlling variable to Alk.

[orcafood]

Yup but I don't think many people have as much aeration as you. The CO2 levels outside do change over day vs night and seasonally so that variability should still be there.

I think I am going to attempt to use an IR CO2 monitor to float on my sump and give a real time read out of the CO2 levels dissolved in a small headspace. Then a .001 pH monitor to get a nice resolution on the pH. My salinity and temperature are rock solid and can be removed from the equation. Should work nicely!

All I want is for the system to text me when my approximated alkalinity is low. Then I will go in and manually raise the dosing if it is actually necessary. Controlling pH seems like a risky risky business to me.

[karimwassef]

I've been doing it for four years. The only variable is CO2 fluctuation. With two pH probes, I remove the effect of any one individual probe going off. I have built in triggers to shut it off in case the two diverge.

One friend locally, Danny, has started doing the same and he's seeing similar success.

Actually, fresh air CO2 doesn't vary significantly day-night or seasonally based on my observations. 99% of the time, it's between 400-450ppm.

My extremes have been 384 and 478 over a year's worth of observation.

My data could be off, but that's what I see.

One idea that came from Danny was to actively control CO2 by injecting it (or not) to actively maintain its level. I'm wary of two loops controlling two main variables to drive a third unmeasured, but calculated final variable. Opportunity for error and instability seems very high.

[dartier]

Originally Posted by orcafood

I still don't see why we don't try and get the actual CO2 level in the water or from a head space floating on the water surface. Plus then it could be applied to others tanks who can't assume steady state equilibrium.

I think I will set it up on my pc similarly to how you did with myself as feedback. Definitely the way to do it.

I would be very curious how this works out (CO2 sensor in a head space). The sensor you linked is pretty low cost, and has some easy to leverage interfaces (like I2C).

Dennis

[dartier]

This looks interesting http://www.pro-oceanus.com/mini-co2.php

Dennis

[karimwassef]

depends on the price

[orcafood]

Karim that is exactly the kind of changing CO2 levels I am talking about and that is just outside your house, not to mention the flux inside your house. I am pretty convinced that the amount of co2 in my reef tank has a massive control over coral growth and I am very curious to start watching the levels.

Dartier good find on that. I wouldn't be surprised if they aren't using a thin silicone membrane to get very accurate head space results.

Damn stuff costing money

ATR-IR with a small slit built for constant flow circulation from the tank would be ideal but prohibitively costly. I am pretty sure dissolved CO2 meters are just normal IR meters with a decent water resistant membrane.


http://www.plantedtank.net/forums/20...o2-sensor.html

Look at this, planted tank people are on it.

In a way, growing coral is a lot like growing plants. Gotta keep the zooxanthellae happy.


I have been thinking that injecting O2 from a solenoid might be good for solving the CO2 controll issue. If you pump more oxygen into the water, will the CO2 not be "forced out". Pumping O2 into a tank should raise the pH?

I think pumping CO2 into a tank is only a good thing if you are going to melt corals with it (Ca Rx). I am currently battling too high a CO2 level from too many people in the house. At least that is what I think the problem is.

If pH is controlled but not CO2, alkalinity will vary with the CO2. I am thinking similarly to your friend. Whenever CO2 gets too high, blast oxygen into the aquarium with a ceramic diffuser to push out the excess CO2. If CO2 and pH are controlled ....

[dartier]

Originally Posted by karimwassef

depends on the price

Ya, about that. $6,400 to $9,450 US. Doh.

Originally Posted by orcafood

Karim that is exactly the kind of changing CO2 levels I am talking about and that is just outside your house, not to mention the flux inside your house. I am pretty convinced that the amount of co2 in my reef tank has a massive control over coral growth and I am very curious to start watching the levels.

Dartier good find on that. I wouldn't be surprised if they aren't using a thin silicone membrane to get very accurate head space results. Damn stuff costing money

ATR-IR with a small slit built for constant flow circulation from the tank would be ideal but prohibitively costly. I am pretty sure dissolved CO2 meters are just normal IR meters with a decent water resistant membrane.

You are correct. It is a semi-permeable membrane over a gas head space. If we could come up with a resistant, safe, semi permeable membrane of our own, the IR unit you posted earlier would be applicable for the same thing.

Dennis

[bertoni]

Ouch, that's a pricey probe. That's one of the issues with measuring the parameters of our tanks, though. The price can be high.

[orcafood]

I'd be surprised if we can't find an appreciable membrane. That said too, why use a membrane. Why not just make a perfectly big enough channel in a floating styrofoam boat with not membrane and just a vertical headspace to get some accurate results?

[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]

Originally Posted by 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.

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/product...ags#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.