Stable enough CO2?

[Florin Ilia]

Hi, I need some help optimizing my CO2/light schedule.

My current light schedule is
9-14
17-22
and my current CO2 schedule is
7:30-14
16-22

The reason for the midday break is just so I can see the aquarium in the morning and then late in the evening - I work long hours.

Using my CO2 meter I took measurements (almost) every 30 minutes for (almost) 2 days and the graph looks like this:

I think I can safely increase the general level a bit, but my main question is, is the current variation too much? As you can see, during the photoperiod, the level varies between 25 and 41 ppm. Isn't that too much fluctuation?

If it is too much, I can think of one of these solutions:
- don't stop CO2, just leave it on always. (I don't care about the waste, CO2 is cheap, I just care about the fishes' comfort)
- don't stop CO2 during the midday break; there will still be variation but less
- use a pH controller - not to control the actual pH value, but for the stabilizing effect; I imagine the pH controller has an effect like this:

I will be grateful for any advice.

 

[Tom Barr]

Seems the rate of increase is taking a long time, meaning that the CO2 diffusion is too low/slow.
Simply adding more gas is often not the solution, rather, mixing and distribution into the tank evenly is the main issue/factor.

pH controllers make those smaller dips and tries very hard to ramp up the CO2 asap.
Still, it looks like this take 1-2 hours, whereas most of the systems I try and target 30-45 minutes.

So you could likely handle more flow through the CO2 system/reactor, and/or have more current etc.

This would be better than a pH controller.
Likewise as you show in the graph, you could turn on the CO2 about 1 hour before hand and then have the lights come on.

 

[Tom Barr]

Also, do you think the CO2 is really 15-20ppm at night?

Try this: do not add CO2 during the day for one day, does the CO2 drop below 15ppm?
How low? If not, then you have something wrong in the methods.

 

[Florin Ilia]

Thanks for the pointers, Tom. You're right, I will try to validate the method first, otherwise the discussion is pointless. I'll be back.

 

[Tom Barr]

Thanks for the pointers, Tom. You're right, I will try to validate the method first, otherwise the discussion is pointless. I'll be back.

Well, it is interesting either way, and if you can figure out the CO2 is actually 15ppm lower than you think it is(often the case more than pot!), that will GREATLY enhance your horticulture!!
If not, you are still okay.........so this can only benefit you.

Little things like this can really help folks master CO2.

 

[Tom Barr]

BTW, the mid day siesta for viewing gave me an interesting idea and hypothesis for better light an dCO2 management perhaps, it's a PITA to set up the timing, but is interesting nonetheless.

.

 

[Florin Ilia]

the mid day siesta for viewing gave me an interesting idea

Tom, I'm glad to be of service, albeit unintentionally

I started to validate my measurement method. There are several things I want to do and some things that I already did:

STEP 1. Starting from your idea to measure water with no CO2 addition, I used my OxyGuard in some DI water. The reading was 1 ppm CO2.

STEP 2. I surveyed again CO2 in the tank over almost an entire day, taking measurements every 30 minutes, but this time I also measured pH (with a brand new, calibrated meter).
I also measured my KH (4.5). The KH measurement is the weakness of this step as I measured it with a hobby-grade kit. Anyway, the pH graph looks like this:



Then, I used the formula that I found here for calculating CO2 from pH and KH
CO[SUB]2[/SUB] = 3 * KH * 10[SUP](7-pH)[/SUP]
and I plotted both my actual measurement (blue) and the calculated values (red):



They're not identical but they wouldn't be, there's more things influencing pH than CO2. I was surprised to see that my measured values are higher than those calculated, but I'm not going to read too much into these values since (a) the KH was measured with a crappy method and (b) the Oxyguard was not freshly calibrated.

What I still want to do:
STEP 3. Measure my KH more precisely.

STEP 4. Calibrate the Oxyguard. For the most precise calibration I need a multimeter and I have already ordered one.

STEP 5. Leave the CO2 off for a day and see what the measurements show, as per your suggestion.

STEP 6. Cross-check Oxyguard against a titration CO2 method (I don't expect equality due to interferences, I just expect "same ballpark").

After we get these steps out of the way, is it OK to assume that I'll have a reasonably reliable way to measure CO2 in my tank?
Then we can get to the important things.

Thanks,

Florin

 

[scottward]

Hi Florin,

This is a very interesting thread. I look forward to reading more.

Tom has helped me out a lot with CO2 problems in the past.

Certainly CO2 fluctuations were causing issues in my tank, whereby my stem plants were autofragmenting (breaking apart) etc, leaves were drooping etc.

Sorry if I missed this above - but how are your plants and how long has your tanks been set up? i.e., if the plants look great and the system has been set up the way it is for, say, serveral months, then it would seem any fluctuations in your CO2 are minor enough so as not to cause any issues.

If your CO2 level is fluctuating from 25-41 (during the photoperiod of course) I would expect plants to be doing very poorly??

That is a long CO2 ramp up period. It's not possible to see where the CO2 level plateas, as the CO2 turns off and it starts to fall. It would be interesting to see where CO2 level plateus if you left it on longer. If it did eventually plateau, I suppose this would (solely) suggest an inefficient diffusion method. On the other hand, if it never reached a plateau, but continued going up and down, this would probably suggest a mixing issue?

Scott.

 

[Florin Ilia]

Hi Scott, thanks for your thoughtful reply.

The original issue is that I have a stubborn infestation with Staghorn that didn't go away even when I reset my tank (different substrate, light, hardscape, plants). I am thinking there must be something wrong with one or more of the major factors - light, CO2, fishload/maintenance. I started with the investigation of CO2 because I read that stability is important and I felt that my 2-hump routine may not be stable enough, but I plan to also optimize the other factors.

I posted some background about my tank in a separate thread in order not to obscure the CO2 investigation (here it is: http://www.barrreport.com/showthread.php/9107-Stubborn-Staghorn ).

As soon as I get my meter calibrated I will measure different scenarios - including trying to find the CO2 plateau and seeing if I can improve diffusion - all are good suggestions, thanks!

Florin

 

[yme]

great thread!

just my two cents from another owner of an oxyguard co2 meter:

-In the past, I also measured 15-20 ppm CO2 in the morning. The addition of a MP20 vortech helped to get the co2 level down to 7 ppm. way more surface agitation and thus more degassing.
Also, if I remember correctly, the graph tom showed is derived from a tank with a wet/dry filter or a biological filter. In any case it is a lot easier to degass in such a tank compared to, e.g. my tank, that only uses eheim pot filters.

-if you have the money: go for the pH-controler. you will have a much more stable system.

-could you elaborate a bit more on how you plan to calibrate the co2 meter using a multimeter. I would like to know since I am also not 100% sure about the readings that I get.

-you indeed should work on the system in get a faster increase in co2 levels. it takes too much time. something is not optimal.

greets,

yme

 

[scottward]

-if you have the money: go for the pH-controler. you will have a much more stable system.

I don't own a pH controller - but I think they are more trouble than they are worth from what I've heard?

Scott.

 

[Tom Barr]

great thread!

just my two cents from another owner of an oxyguard co2 meter:

-In the past, I also measured 15-20 ppm CO2 in the morning. The addition of a MP20 vortech helped to get the co2 level down to 7 ppm. way more surface agitation and thus more degassing.
Also, if I remember correctly, the graph tom showed is derived from a tank with a wet/dry filter or a biological filter. In any case it is a lot easier to degass in such a tank compared to, e.g. my tank, that only uses eheim pot filters.

Yes, the rate of degassing is under 45 min for my 180 tank and about the same on the 1600 gal tank.
Both have CO2 mist so the CO2 goes up very fast and is removed very fast.

The CO2 meter also read about 20ppm without a wet/dry, eg with a cansiter filter.
O2 was about 1-2ppm lower with a canister filter than with the wet/dry.

As YME states, he is NOT certain about the CO2, neither was I.


No one should be. Only calibrated confirmation is going to give us a high degree of confidence.

CO2 cartridges they sell for paintball or bike tire, air pellet guns etc have a KNOWN volume/mass of CO2.
Say a 12 gram cartridge.
These can be released with minimal head space in a sealed container of DI water with a known KH.

The concentration can then be calculated.

Then this can be diluted inside the sealed contain to whatever concentration you want.

It's a bit tricky not degassing the water, but this should get you pretty close.

the other way to test and measure the CO2 meter is to use a KNOWN KH DI mixture of water, then add CO2 gas to get 50ppm based on the equation.
Then see how close the CO2 meter is to this value.

If you only have 1 point calibration, then that's not very useful.

 

[Florin Ilia]

Thanks for your reply, yme. Just a quick answer to your question:

-could you elaborate a bit more on how you plan to calibrate the co2 meter using a multimeter.

I plan to use the output lead - it has better precision than the native display (which rounds up to integer ppm values). The lead carries an analogous signal in the 0-1V range that is proportional to 0-50 ppm so zero is zero not 0.44 etc.

Only calibrated confirmation is going to give us a high degree of confidence.

CO2 cartridges they sell for paintball or bike tire, air pellet guns etc have a KNOWN volume/mass of CO2.
Say a 12 gram cartridge.
These can be released with minimal head space in a sealed container of DI water with a known KH.

The concentration can then be calculated.

Then this can be diluted inside the sealed contain to whatever concentration you want.

It's a bit tricky not degassing the water, but this should get you pretty close.

the other way to test and measure the CO2 meter is to use a KNOWN KH DI mixture of water, then add CO2 gas to get 50ppm based on the equation.
Then see how close the CO2 meter is to this value.

If you only have 1 point calibration, then that's not very useful.

The Oxyguard comes with a 2-point calibration procedure described as follows:

The kit contains a white powder called "pH conditioner" and which according to the label contains citric acid. It also contains a clear liquid called "calibration fluid", which has to be kept sealed as much as possible, and which has the following effects:
- when added to DI water, "absorbs all the free CO2 in the water";
- when added to DI water that was treated with the white powder, releases a known amount of CO2 in the water.

In case anyone's curious about the whole procedure here's the Oxyguard manual (calibration is at page 4).

 

[Tom Barr]

Yes, but this is a range of about 0 to about 3ppm, not say 0, 5ppm, 20 ppm and 50 ppm etc.
Their method has some issues and they admit to it.

 

[Florin Ilia]

Yes, but this is a range of about 0 to about 3ppm

You mean the Oxyguard calibration points? They're 0 and 50 ppm. (Or 0 and you can choose another point that's close to your most typical values).

In any case, I've added an "independent standards check" step to my TODO list. From the 2 methods that you mention I think I can implement easily the 2nd one but I'll try the 1st one too as it seems fun . I have also ordered a CO2 standard solution but I couldn't find one in the 0-50 ppm range, so I'll have to dilute and worry about degassing from manipulation.

 

[Tom Barr]

Well, you can add 12 grams of CO2 from a paintball cartridge to 1 liter of DI/RO water for a known solution of CO2 ppm.

 

[Florin Ilia]

Yeah that's the method that sounds like fun (the fun being to design a sealed environment in which I can inject CO2 into water, mix it up thoroughly, then also dilute that water. Nice engineering problem )

 

[Tom Barr]

Yeah that's the method that sounds like fun (the fun being to design a sealed environment in which I can inject CO2 into water, mix it up thoroughly, then also dilute that water. Nice engineering problem )

It's not quite as hard as you might think.

Still, some DI/RO water plus a good calibrated pH probe and some reference KH solution usign sodium carbonate should work pretty well and be easier. But providing a known CO2 ppm solution [aq] is certainly possible since the weight of the CO2 in those cartridges are known. They might be off a little bit, maybe 1/2 a gram? I'm not sure.

 

[Florin Ilia]

That's not a problem, I have a milligram scale, I can weigh the cartridge before and after to know exactly how much CO2 I injected.

 

[Florin Ilia]

Aeration matters

I ordered some CO[SUB]2[/SUB] standard solution (without paying attention to the product description) and I was quite disappointed to realize that it isn't actually CO[SUB]2[/SUB], but Potassium hydrogen phthalate and I have no idea how to check my OxyGuard with that.

In any case, I did calibrate the OxyGuard with its own 2-point procedure and using a digital multimeter for added accuracy. It had a deviation but very small (about 3%).

And then, I went ahead and used my aquarium for a few tests. For each of the tests below I left the OxyGuard running for several days and logged the values every 10 seconds. I then averaged the data over 10-minute periods, selected one 24h period (from midnight to midnight) which looked most "representative" and voila.

Between the tests I tried to change ONLY the variables from the test title. I screwed this up twice (see red text).

Why did I go through this? I wanted to see for myself, in the context of my tank, how things change when I tweak things around.

Anyway, here goes:

Test 1. CO2 0.3 bps, no aeration, then CO2 stopped

This test started from Tom's question

Also, do you think the CO2 is really 15-20ppm at night?

I turned off surface agitation as well as I could (no airstone and my filter current doesn't break the surface). Here's how degassing went. This graph covers 56 hours:

Test 2. CO2 0.3 bps, no aeration

The degassing test above started a bit prematurely because my CO2 solenoid was accidentally turned off (you can see the second CO2 period being shorter than the light period). So I redid a full day with CO2 at 0.3 bps and no aeration:

Screwup: I didn't notice that water evaporation left the upper hole from my vertical spraybar very close under the water surface, which caused a bit of surface agitation and reduced the peak concentration from around 72 to around 64.

Test 3. CO2 0.3 bps, violent aeration

Same as above, but with strong aeration from an airstone set to maximum. I was impressed to see how big an impact it has on CO2 concentration:

Test 4. CO2 1 bps, violent aeration

I had to increase my CO2 pressure 3 times, up to 1 bps, in order to reach a reasonable peak concentration of about 35 ppm. The gassing and degassing are now much sharper (although full gassing still takes 3h30 as opposed to Tom"s target of 30-45 min):

Test 5. CO2 1 bps, violent aeration, no siesta

Last test, I removed the CO2 siesta (I just left the light siesta). Screwup: I increased filter output from around 400 l/m to around 550 l/m - with the effect that CO2 now peaks around 45 ppm instead of 35 ppm. However the overall shape of the graph is now very close to Tom's.