Gas-exchange experiments

[PeerUnk]

All electronics parts for my own sensor build are delivered except the O2 sensor. Software is up and running, hopefully I can actually do some tests next week. Fingers crossed 🤞

 

[hax47]

Hi, nice discovery and good to be aware of. How did you add the CO2? With a soda bottle I guess looking at the sudden spike? What ratio (Volume addition and tank) did you add? The amount of oxygen (or absence?) in the soda bottle might affect your oxygen reading as well…? The sensor actually reads o2 percentage right?

Yes, with soda. Typically soda water contains about 5-8 g/l CO2, so 100x dilution gets you to 50-80 mg/l range. For me, 900 ml SodaStream bottle brings the CO2 in the 120 l tank (~100 l water) to about 45 ppm.

Your finding in my own words: a lot of co2 is added to the water. This will equilibriate with the trapped and measured air. Because co2 will increase in the air and it’s co2 reading. The percentage o2 is affected and will read lower, even the amount of oxygen in the trapped air and water is the same.

Something like that, the amount of oxygen in the water is the same. In the air, the total amount of gas molecules remain, but the relative fractions will be the same as in the water. But since the air pressure remains, the total oxygen in the air will be less.

 

[PeerUnk]

Well, it took a while, the CO2 sensors were faulty, so had to order replacements. And as a average human, I was too optimistic about the build and test phase.

But, I've built it! Now I can read CO2 and oxygen levels 🙂

The setup
I've used the same sensors like Hax47, but a different setup for the diffusion part though, as I used a small fan in the gas chamber, which is directly in the water. And I added a water proof temperature sensor. I take in account the temperature when calculating the CO2 and oxygen levels in water based on the gas measurements. (I've generated formulas to calculate estimated Henry's Law Constants for CO2 and O2 in fresh water based on temperature, with a maximum of 2% difference)
You can see in the picture the gas chamber with it's sensors on top. The gas chamber and it's fan is directly in the flow of a Tunze Turbelle flow maker for maximum responsiveness in gas exchange:

The sensors were installed with the display tank in the living room The aquarium is net 540 liters and aerated with two (silent!) Eheim 400 air pumps, so 800 L/h or air runs through regular air stones. It has good flow, without CO2 injection. (bit cloudy picture, my unfiltered tank had maintenance like uprooting yesterday)


Results
So I monitored the tank for 7 days now. I've checked the CO2 readings quite often with our home air monitor.

• My well aerated tank follows the ambient room CO2 levels pretty accurate. You can clearly see when me and my family were in the living room, as major CO2 producers. The red square points out I came home late that night around 00:30, and sat 1,5 hours in the living room before going to bed.
• So CO2 levels are dropping to 0.9 mg/L at night, and fluctuating from 1 mg/L up to 2 mg/L during the day, depending on human presence.
• Oxygen levels are pretty stable, fluctuating around 20.8 and 20.9% (which is 100% oxygen saturation)

What's next?
I've ordered a second CO2 sensor, I will install near the air pumps. So an in- or decrease in CO2 of the ambient air thought the pumps will show some time delay at the gas sensors. And maybe show some result in CO2 delta during the day and night. I've skipped a second oxygen sensor, too expensive as ambient oxygen levels in a room will stay near 20.9%.

And I will run some tests like Hax47, adding CO2 from a soda bottle into the tank. This might give indications how fast degassing CO2 goes with several levels of aeration.

Will get back on that. I think this is a great learning journey for me so far. Thanks a lot for the great ideas in this topic!

Any questions, suggestions and/or comments?

PS. Somewhere during this winter I will permanently turn off the aeration and install bio CO2 reactors and target for 5-10 mg/L of CO2. And hopefully I can really see and measure the effects switching from low energy to mid energy tank in plant growth, but CO2 and oxygen levels too. But that's not the topic of this thread.

 

[hax47]

Well, it took a while, the CO2 sensors were faulty, so had to order replacements. And as a average human, I was too optimistic about the build and test phase.

But, I've built it! Now I can read CO2 and oxygen levels 🙂

The setup
I've used the same sensors like Hax47, but a different setup for the diffusion part though, as I used a small fan in the gas chamber, which is directly in the water. And I added a water proof temperature sensor. I take in account the temperature when calculating the CO2 and oxygen levels in water based on the gas measurements. (I've generated formulas to calculate estimated Henry's Law Constants for CO2 and O2 in fresh water based on temperature, with a maximum of 2% difference)
You can see in the picture the gas chamber with it's sensors on top. The gas chamber and it's fan is directly in the flow of a Tunze Turbelle flow maker for maximum responsiveness in gas exchange:

Nice work, and thanks for sharing the data! I am happy to see that there is someone else too with CO2 readings! Also, it is interesting to see the very low levels of CO2 in a well aerated tank, which is expected, but not what the common expectation would be (around 3 ppm). And quite the opposite dynamics of CO2 compared to my aquariums, which are located in the bedroom.

Do I understand it right, that you don't circulate the water in the chamber, but only the air? That was my first setup as well, but I realized that the diffusion in the air is not a limiting factor - the diffusion in water is. The diffusion rates are 10 000x faster in air, so if you manage to exchange the water surface exposed to chamber air faster (and increase the surface), it has much more effect on the rate of reaching the equilibrium than the ventilator.

This is an aquarium without lid, right? I am curious how long will the soda effect last.

 

[Andy Pierce]

This is an aquarium without lid, right? I am curious how long will the soda effect last.

I'm interested in this as well. For me in a small open-topped aquarium with active aeration and a lot of water circulation the half-life of added CO2 is about 70 minutes, or 60 minutes if the lights are on. Presumably the faster half-life with lights on represents CO2 consumption by plants. These measurements were done by adding HCl to high dKH water to convert bicarbonate to carbonic acid and then monitoring the pH over time. The HCl is fully converted to CO2 essentially in the time it takes the HCl to mix through the water column and the pH effects result from the equilibrium between carbonic acid and dissolved CO2 gas so I would expect CO2 monitoring following soda water addition measurements to be pretty comparable.

 

[PeerUnk]

Thanks Hax47,

Also, it is interesting to see the very low levels of CO2 in a well aerated tank, which is expected, but not what the common expectation would be (around 3 ppm).

If I would stop ventilating the living room with 4 person in it, I might reach 3 ppm as well. I guess 3 ppm is a rough figure for a non aerated tank after a night of respiration. I will test that at some point, shutting down aeration for the night and see what CO2 and oxygen levels do. Something to put on my To Do List.

I did a 'sanity' check for the sensors, letting CO2 from a soda water bottle escape into the gas chamber at 14:50. You can clearly see almost immediately 40,000 PPM CO2 (the maximum reading of this type of sensor SCD41, stated in the datasheet) up until 15:35 and measurable diffusion, the reading is declining with a half time of 15 minutes.
Oxygen-wise I already confirmed the sensor is a lot slower in reading values than the CO2 sensor. Nevertheless it's clear to me the curve shape and readings make sense:

Do I understand it right, that you don't circulate the water in the chamber, but only the air? That was my first setup as well, but I realized that the diffusion in the air is not a limiting factor - the diffusion in water is. The diffusion rates are 10 000x faster in air, so if you manage to exchange the water surface exposed to chamber air faster (and increase the surface), it has much more effect on the rate of reaching the equilibrium than the ventilator.

I solely maximized water and air flow. I am well aware I don't use a water pump and foam to maximize surface area for optimal diffusion, but I don't want to rinse foam, cleaning the pump intake, that type of stuff.
As for the results of diffusion rate, I'm good for now.

This is an aquarium without lid, right? I am curious how long will the soda effect last.

Well, yes and no... I do have a hood, but no lid to block evaporation. Two light weight polycarbonate plates (green in pic) are on top of my tank. The lights warm up air, which can escape in front of the hood. Fresh cooler air is drawn from the back and below the aquarium. So aerating the hood is good, without the need of a fan. No issues with condensed water and mold. (The front of the hood which the blocks light is detachable when I have to do maintenance.)


I will do the 'soda test' somewhere this week. When lights are off, I will dose 1 liter of soda water in 500+ liters, so I guess it will be 5-10 mg/L of CO2? I won't inject 30mg/L, no intention risking to harm anything.

Cheers!

 

[PeerUnk]

For me in a small open-topped aquarium with active aeration and a lot of water circulation the half-life of added CO2 is about 70 minutes, or 60 minutes if the lights are on. Presumably the faster half-life with lights on represents CO2 consumption by plants.

I would assume that as well.

These measurements were done by adding HCl to high dKH water to convert bicarbonate to carbonic acid and then monitoring the pH over time. The HCl is fully converted to CO2 essentially in the time it takes the HCl to mix through the water column and the pH effects result from the equilibrium between carbonic acid and dissolved CO2 gas so I would expect CO2 monitoring following soda water addition measurements to be pretty comparable.

Smart way of increasing CO2 levels. I've read that somewhere before as method replacing CO2 injection. I agree it won't matter where CO2 is coming from: injection, HCl or a soda bottle, it's just CO2. The chemical reaction of HCl creating carbonic acid (increasing the pH) and CO2 is pretty fast. Maybe I will do such a test as well. What amount of HCl solution do you use in your tank and what increase in CO2 do you get?

 

[Andy Pierce]

What amount of HCl solution do you use in your tank and what increase in CO2 do you get?

5 ml of a 1.0N HCl solution added to 12500 ml tank water gave a transient pH drop of just under 1.0 pH units and dissolved CO2 at 18 ppm, which is the expected 10x increase over baseline levels of CO2 at 1.9 ppm in an indoor reasonably well ventilated (poorly insulated) living room. This room has atmospheric CO2 at 450 ppm when unoccupied and around 750 ppm when occupied. I would do the acid addition 2x per day but couldn't really say it made a noticeable difference to either plants or neocaridina shrimp. After the CO2 has equilibrated with atmosphere the net effect in the tank has been to permanently decrease alkalinity by 1.1 dKH resulting in a permanent pH decrease of 0.05 pH units. Details in the link in the previous message.

 

[dw1305]

Hi all,

I've read that somewhere before as method replacing CO2 injection. I agree it won't matter where CO2 is coming from: injection, HCl or a soda bottle, it's just CO2

I think you used to be able to buy a CO2 generator that worked by dripping HCl onto a calcium carbonate (CaCO3) block. I'm guessing that this has fallen victim to <"health and safety"> as a commercial product - <"Generating enough pressure for DIY CO2">.

We have threads on DIY versions, using weaker acids, <"DIY Citric acid CO2 complete system for £35 or less"> - <"CO2 generator kits - any good?"> and a commercial one <"https://www.amazon.co.uk/ZRDR-Generator-Pressure-Automatic-Plants，Stable/dp/B083NH7BKQ">, "sera Flore Carbon-dioxide Aktiv Reactor" or the "LYEAA 1L/2L CO2 Generator System Carbon" etc.

5 ml of a 1.0N HCl solution added to 12500 ml tank water gave a transient pH drop of just under 1.0 pH units and dissolved CO2 at 18 ppm

I'm not a CO2 user, but I guess "transient" is the issue, but have a look at <"What to do when running out of CO2?">. Amano started by <"adding bottled mineral water to his tanks">. I'd guess that people use bottled CO2 just because, in the long run, it the easiest and most efficient option.

Personally I don't want the <"plants to grow quickly"> in normal conditions, but I want a plant, <"that isn't T(D)IC limited">, with the <"capability to grow rapidly"> when nutrients become suddenly available.

cheers Darrel

 

[Andy Pierce]

I'm not a CO2 user, but I guess "transient" is the issue, but have a look at <"What to do when running out of CO2?">. Amano started by <"adding bottled mineral water to his tanks">. I'd guess that people use bottled CO2 just because, in the long run, it the easiest and most efficient option.

My objection to the bottled water approach is that it isn't very environmentally friendly (or economical) to bottle and ship water. Same deal as to why dry salts are the much more efficient approach than liquid fertiliser. Maybe there is an aquarium product business model for fertilised bottled carbonated water. 😉

 

[dw1305]

Hi all,

My objection to the bottled water approach is that it isn't very environmentally friendly (or economical) to bottle and ship water. Same deal as to why dry salts are the much more efficient approach than liquid fertiliser.

Point taken, I'm really not recommending it as a viable option. Assuming that you are happy to use CO2, compressed gas has the advantage of putting a large volume of compressed CO2 in a small (and potentially refillable) container.

Maybe there is an aquarium product business model for fertilised bottled carbonated water.

Unfortunately <"there almost certainly is">. It would appeal to those companies that have <"no moral compass"> and <"like a huge mark-up"> and a <"huge profit margin">.

cheers Darrel

 

[PeerUnk]

Hi all,

I did a small soda bottle test: Shutting down aeration and adding some soda at 21:45. At 22:45 the increased in measured CO2 in the gas chamber is maxed out, so I enabled aeration again.

Some observations I made:

• Measuring the full increase in CO2 gas sensor takes 1 hour window. @hax47 Looking at your post #78 it seems your full increase window is 30 minutes? I guess I can improve with increased surface area like Hax47 did or some other type of wet and dry drip system.
• I have the similar situation where adding CO2 via soda drops in oxygen percentage a little like Hax47 had as well in post #79. I guess I could apply the same correction formula.

Based on the 60 minute window for actually measuring the added soda, I won't drawn any conclusions (or assumptions) on the degassing effect of the aeration. I already ordered a pH probe and controller board so I will incorporate this in the sensor setup (monitoring platform). This way I can do a couple of things:
1. determine the amount of time it takes for the CO2 to diffuse into the gas chamber the gas sensor.
2. together with my second CO2 sensor which I will install at my air pumps, I can actually distinguish between the speed of diffusion of the aeration itself and the the bullet above.
3. hopefully calculate and keep track of the KH based on the 24h rolling average pH and CO2. This in a non labor intensive way... Which to me seems a welcomed benefit having a heavy aerated tank relying on carbonates as a carbon source, on a KH around 1 dH. I even might get a sense how the uptake of nutrients AND carbonates affect conductivity as I measure this on weekly basis, but that's an entire different topic.

I will do a new test when the pH probe and second CO2 sensor is installed.

Cheers!

PS. In the tank hood I am installing a light sensor (amongst others) in the same monitoring platform, so we might see the direct link between light and oxygen levels in the same graph. The aquarium light intensity fluctuates during the day.

 

[PeerUnk]

I think you used to be able to buy a CO2 generator that worked by dripping HCl onto a calcium carbonate (CaCO3) block. I'm guessing that this has fallen victim to <"health and safety"> as a commercial product - <"Generating enough pressure for DIY CO2">.

We have threads on DIY versions, using weaker acids, <"DIY Citric acid CO2 complete system for £35 or less"> - <"CO2 generator kits - any good?"> and a commercial one <"https://www.amazon.co.uk/ZRDR-Generator-Pressure-Automatic-Plants，Stable/dp/B083NH7BKQ">, "sera Flore Carbon-dioxide Aktiv Reactor" or the "LYEAA 1L/2L CO2 Generator System Carbon" etc.

nice links you provided Darrel!

Personally I don't want the <"plants to grow quickly"> in normal conditions, but I want a plant, <"that isn't T(D)IC limited">, with the <"capability to grow rapidly"> when nutrients become suddenly available.

Same here, I'm no CO2 injection user as well. Done it, been there, but I don't like systems which potentially could go terribly wrong: explode or capable of releasing enormous amounts of CO2 in seconds or minutes.

Same thing with leaking canister pumps, inline CO2 diffusers with cracked hose beneath the tank... My wife called me a decade ago, we had a flooded wooden floor with 300 liters of water drained out of the tank... So from that point on, water and pumps stay in the tank and no return valves at strategic positions.

Every week doing required water changes and lots of trimming is not my thing in the long run. I'm not lazy in general, but I like a tank which can't derail easy in terms of healthiness and doesn't need much attention to do well. But I am willing to put much effort in accomplish this and understand and test how and why things work in a tank.

Cheers!

 

[dw1305]

Hi all,

nice links you provided Darrel!

I started looking for a commercial HCl ~ CaCO3 version, but I couldn't find one, which led me to citric acid (C6H8O7) and sodium bicarbonate (NaHCO3) based generators. I can see that two liquid components (the salt product, sodium citrate, is soluble) is a sensible way forward in terms of ease of cleaning etc. <"DIY CO2 kit for acid citric and baking soda">

or capable of releasing enormous amounts of CO2 in seconds or minutes.

Agreed it is not my sort of thing (even in this version), but some of the stainless steel flasks look really nice and shiny <"https://www.amazon.co.uk/LYEAA-Generator-Dioxide-Aquarium-Cylinder/dp/B0B7QXC6DC">.

cheers Darrel

 

[PeerUnk]

5 ml of a 1.0N HCl solution added to 12500 ml tank water gave a transient pH drop of just under 1.0 pH units and dissolved CO2 at 18 ppm, which is the expected 10x increase over baseline levels of CO2 at 1.9 ppm in an indoor reasonably well ventilated (poorly insulated) living room. This room has atmospheric CO2 at 450 ppm when unoccupied and around 750 ppm when occupied. I would do the acid addition 2x per day but couldn't really say it made a noticeable difference to either plants or neocaridina shrimp. After the CO2 has equilibrated with atmosphere the net effect in the tank has been to permanently decrease alkalinity by 1.1 dKH resulting in a permanent pH decrease of 0.05 pH units. Details in the link in the previous message.

Thank you for the detailed reply!

Hmmm... Currently my tank has just 1 dKH.
That's way too less headroom in terms buffer capacity to do such a test.
I'll keep the method in mind if I go to full tap water of 10 KH.

I'm not a chemist or expert in any way, but I tried to do some validations and calculations, which is off topic:

(I'm not familiar with the unit 1.0N, but chatGPT's final answer: "The mass percentage of HCl in a 1.0 N HCl solution is approximately 3.61%.")

So 5 ml of it gives you an increase of 18-1.9=16 mg/L in a tank of 12.5 liters? So total increased diluted CO2 is 16 x 12.5 = 200 mg.
And 5 ml is roughly 5000 mg x 3.61% = 180 mg HCl.
Seems to make sense... but I don't know the exact chemical reaction nor the molar weights...
I've asked Chatgpt this question: "How does 12.5 liters of aquarium water containing CaCO3 reacts with 5 ml HCl solution of 3.61% in terms of KH?"
It gave a very precise answer I can't validate but its conclusion was:
"The reaction between 5 mL of 3.61% HCl solution and 12.5 L of aquarium water containing CaCO₃ will lower the KH by approximately 1.11 °dKH.
This reduction occurs because CaCO₃ reacts with HCl, removing carbonate ions from the water and thus lowering the water’s buffering capacity."

Your result of decrease of 1.1 dKH and the calculations are extremely close!

(When you would do the calculations reacting HCl with Ca(HCO₃)₂, the drop would be doubled... not knowing if this could be a valid situation though)

I can buy 10% HCl in my local hardware store.
If I would test via your method I would want half your increase in CO2: 8 mg/L
In 540 liters this would need 4320 mg.
So I need 4320 * 200 / 180 = 3888 mg of HCl
So that would be 3888 / 10% solution = 38.880 mg or 39 ml...
And it would drop the KH 0.55 dH if it reacts with CaCO3.

 

[PeerUnk]

Agreed it is not my sort of thing (even in this version), but some of the stainless steel flasks look really nice and shiny <"https://www.amazon.co.uk/LYEAA-Generator-Dioxide-Aquarium-Cylinder/dp/B0B7QXC6DC">.

Price is very good and sure looks shiny, credits for that. But I laughed looking at their promo pic's:

There are no plants in those tanks😵‍💫! And do they actually mean the fish as CO2 generators 😉 ?
If the product is as good as the text (full of typos)...

 

[dw1305]

Hi all,

Hmmm... Currently my tank has just 1 dKH.

You shouldn't need a strong acid to lower the pH (if that is your aim?). A weak acid, like citric acid, or botanicals like Alder (Alnus spp.) "cones" or Oak (Quercus spp.) leaves should do. That won't generate much CO2, you don't have enough carbonate present, nearly all the T(D)IC is already CO2.

I'm not familiar with the unit 1.0N

These are just "normality" values, rather than molarity - <"Normality-Measuring the Concentration of an Element">

..... Normality is similar in concept to molarity. Where molarity (M) represents the concentration of an ion or compound in solution, normality (N) goes one step further and represents the molar concentration only of the acid component (usually the H+ ion in an acid solution) or only the base component (usually the OH- ion in a base solution). ......

.... Here is a simple example to show the relationships of Normal acid and base solutions: a 1N solution of the acid H2SO4 will completely neutralize an equal volume of a 1N solution of the base NaOH. Even though the H2SO4 provides two (acid) H+ ions per molecule verses only one (base) OH- ion per NaOH molecule, the calculations of N take into account these differences and puts it all into an equivalent scale......

It is a useful unit for scientists (lab. technicians etc) making up solutions:

There are no plants in those tanks😵‍💫! And do they actually mean the fish as CO2 generators 😉 ?
If the product is as good as the text (full of typos)...

Agreed, that is fantastic and it doesn't exactly fill you with confidence.

cheers Darrel

 

[PeerUnk]

You shouldn't need a strong acid to lower the pH (if that is your aim?). A weak acid, like citric acid, or botanicals like Alder (Alnus spp.) "cones" or Oak (Quercus spp.) leaves should do. That won't generate much CO2, you don't have enough carbonate present, nearly all the T(D)IC is already CO2.

I was trying to reason out whether increasing the CO2 could be done by adding hydrochloric acid. Via such a test we could determine the diffusion / degassing rates. It can be done like @Andy Pierce pointed out, but I don't have sufficient KH for it. So far easier for me is to buy a bottle of soda water.

 

[Andy Pierce]

Thank you for the detailed reply!

Sure. Your follow-up calculations look reasonably correct to me.

I was trying to reason out whether increasing the CO2 could be done by adding hydrochloric acid. Via such a test we could determine the diffusion / degassing rates. It can be done like @Andy Pierce pointed out, but I don't have sufficient KH for it. So far easier for me is to buy a bottle of soda water.

These days there are reasonably priced entry-level pressurised CO2 gas setups available so I recommend just picking up one of those. If the goal is to add dissolved CO2 gas then maybe just adding CO2 gas and letting it dissolve is your best approach. If I'm honest, my all-time favourite system conceptually is the CO2 spray bar approach.

 

[hax47]

So from that point on, water and pumps stay in the tank and no return valves at strategic positions.

That is my approach as well. Filter and water stay in the aquarium, fail-safe and there is the added benefit of no tubes running in and out of the tank.

Done it, been there, but I don't like systems which potentially could go terribly wrong: explode or capable of releasing enormous amounts of CO2 in seconds or minutes.

That is one added benefit of the soda-water approach (or HCl), it is not easy for it to go wrong. I make the soda water myself, and it can't be filled accidentally with too much CO2.
An additional advantage is that no scale builds up on the surface of the aquarium, as I am constantly filling the aquarium.

@hax47 Looking at your post #78 it seems your full increase window is 30 minutes? I guess I can improve with increased surface area like Hax47 did or some other type of wet and dry drip system.

That is right, approximately 30 minutes... If the equilibration time is critical, move the water and increase its surface.

Hmmm... Currently my tank has just 1 dKH.
That's way too less headroom in terms buffer capacity to do such a test.
I'll keep the method in mind if I go to full tap water of 10 KH.

You could add HCl to tap water and pour it into the tank, it will not lose the CO2 that quickly. But then you could just make fresh soda water and use that.