[JarredAwesome]

Hey Guys,

 Have been working on this project for a while. I am getting closer. This is just a hobby, it’s not for money or school or anything.

I am trying to calculate the Co2 in an aquarium.

I have a cup upside down in the water, and a sensor that calculates how much co2 is in the air trapped in the cup.

I am using Henry’s law to then calculate the dissolved Co2 in the aquarium.

Here are my issues.

The air in the cup is never really dropping below a certain level of Co2.

When I add Co2 to the water, the level will rise really fast in the cup. but when the Co2 drops, levels in the cup seem to take very long to catch up.


Is there something I can be doing? Would adding a fan in the cup help?

[Orcio_87]

@JarredAwesome

When I add Co2 to the water, the level will rise really fast in the cup. but when the Co2 drops, levels in the cup seem to take very long to catch up.

It looks like you have two sensors, not one. One sensor in the cup and the second in the aquarium water.

Henry's law states that concentration of dissolved gas is proportional to the pressure above the solvent (but still limited by its solubility).

Sensor in the cup will measure concentration of CO₂ in the air.

But - solubility of CO₂ depends on temperature and pH of solution.

Does the water sensor measure concentration of CO₂ or HCO₃^- ?

And - do both sensors lie in the same distance from source of CO₂ ?

pH of water should be slightly basic (7 -8) because fishes excrete ammonia.

I think that maybe cup sensor reacts faster towards adding (bubbling) CO₂ than water sensor, because water sensor measure concentration of HCO₃^- (?) and reaction between CO₂ and OH^- can take a while in near neutral pH, while air in cup sensor is rapidly saturated with CO₂.

In reverse - when CO₂ (HCO₃^-) in water drops, CO₂ in the cup drops slower because absorption from air into water is also slow.

[JarredAwesome]

I am
Only using one sensor in the cup. If an affordable sensor exists do read Co2 in water let me know. That would save me so much effort! I’m using Henry’s law, because it’s the only way I can find to measure Co2 in water using easily obtainable sensors.

The sensor I have only measures CO2, not HCO3. Is that a problem? I didn’t think HC03 was in the air… as I said, I’m a hobbyist, so I’m still learning.

I do account for temperature of the water. I have a thermometer, and I apply the Vann’t Hoff to the Co2 constant. Should I be doing more?

Would a fan in the cup work? I’m assuming agitating the water surface would, but that might be really hard 


A purely chemical version exists, using a 4dkh liquid and a ph regent

[Orcio_87]

I didn’t think HC03 was in the air

True, there are no HCO₃^- flying in the air

[Borek]

When I add Co2 to the water, the level will rise really fast in the cup. but when the Co2 drops, levels in the cup seem to take very long to catch up.

How do you know that the CO₂ (watch the capitalization) level dropped of? How do you know there is discrepancy between the levels of CO₂ in water and the cup? Sounds like you have TWO information, but at the same time you say you have only ONE sensor, so there is something wrong here.

[JarredAwesome]

I use a drop checker to compare the CO2 levels.

They aren’t super accurate, but they are good enough to let me know of my set up is working.

[Borek]

So the problem is: whatever you sensor measures, doesn't agree with what the drop checker shows?

What kind of sensor do you use?

I can be wrong, but the drop checker looks like a simple pH indicator with solution changing its pH in response to the partial pressure of CO₂.

[JarredAwesome]

So the problem is: whatever you sensor measures, doesn't agree with what the drop checker shows?

What kind of sensor do you use?

I can be wrong, but the drop checker looks like a simple pH indicator with solution changing its pH in response to the partial pressure of CO₂.

Yes to all of it.

When I add CO2 to the water, the sensor shows the increase fairly quickly. When the CO2 is off, it takes hours for the numbers to start registering a drop.

The sensor never drops below a concentration of 39ppm of CO2. Where as the drop checkers shows the levels to be at least 15ppm (that’s below the threshold of what wa drop checker can show)

The reason I think for this, is the CO2 turns back on before the levels are shown on the sensor.


The liquid in a drop checker is distilled water mixed with backing soda until it has a Kh of 4DKh. It then has a ph regent added to it.

The idea is, the partial pressure from the CO2 will change the ph of the liquid and because the Kh is known, ph can be used to roughly measure the CO2 in the water

[Orcio_87]

When I add CO2 to the water, the sensor shows the increase fairly quickly. When the CO2 is off, it takes hours for the numbers to start registering a drop.

The sensor never drops below a concentration of 39ppm of CO2. Where as the drop checkers shows the levels to be at least 15ppm (that’s below the threshold of what wa drop checker can show)

How is this possible if solubility of CO₂ at p=0,004 atm (present-day conc.) is about 6 ppm ?

I hope that you made a correction on the pressure inside the cup in the water.

The idea is, the partial pressure from the CO2 will change the ph of the liquid and because the Kh is known, ph can be used to roughly measure the CO2 in the water

True - carbonate hardness (kH) and pH can be used to calculate concentration of CO₂ - but ammonia will false outcome as it will increase kH in aquarium water and even raise pH in drop checker.

After all - kH and pH tests are better, as they give data about all CO₂ (CO₂ and HCO₃^-) while your sensor measures only concentration of CO₂ in the air above aquarium water.

[Borek]

What kind of sensor do you use?

Yes to all of it.

?

[JarredAwesome]

How is this possible if solubility of CO₂ at p=0,004 atm (present-day conc.) is about 6 ppm ?

I hope that you made a correction on the pressure inside the cup in the water.

I’m not entirely sure I understand what you’re asking. But if you’re asking why the Concentration of Co2 is so high, it’s because I inject Co2 into the filter so it dissolves.

The idea is, the partial pressure from the CO2 will change the ph of the liquid and because the Kh is known, ph can be used to roughly measure the CO2 in the water

True - carbonate hardness (kH) and pH can be used to calculate concentration of CO₂ - but ammonia will false outcome as it will increase kH in aquarium water and even raise pH in drop checker.

After all - kH and pH tests are better, as they give data about all CO₂ (CO₂ and HCO₃^-) while your sensor measures only concentration of CO₂ in the air above aquarium water.

I’m not worried about ammonia. A cycled tank has organisms in it that remove ammonia, and convert it into nitrates.

How would I compensate for the
Pressure in the cup? I have a barometer in the cup, but I don’t think that’s why you’re referring too.

The CO2 Sensor I have in the cup is a Senseair S8. It’s supposedly one of the best you can buy without going into scientific lab quality stuff.

[Borek]

How would I compensate for the Pressure in the cup? I have a barometer in the cup, but I don’t think that’s why you’re referring too.

Don't worry about that. The cup is submerged so the pressure inside is a bit higher than the atmospheric one, which in turn means a bit higher amount of gases. The error is something like +3% per each foot the cup is below water surface.

39 ppm that you listed as measured in the air in the cup (that's what you measured, right?) is a bit strange. According to the specs the sensor is not capable of measuring the concentration below 0.04% or 400 ppm (that's more or less how much CO₂ is in the air these days).

[JarredAwesome]

Sorry, maybe I wasn’t clear.

39ppm is the concentration of CO2 I calculated in the water after applying Henry’s Law.

The cup was reading at a PP of 26150PPM

[Borek]

39ppm is the concentration of CO2 I calculated in the water after applying Henry’s Law.

This is tricky, as CO₂ not only dissolves, but also reacts with water and dissociates, and the amount dissolved depends on pH. You never mentioned that you already apply the Henry's Law, nor what value/kind of constant you use.

[JarredAwesome]

Here is the Calculation I have been using

To get the Value in moles:
C = Concentration of CO2 in water
T = Temp of water in Celsius
P = PPM of CO2 in cup

C = (P / 10^6) / (29.42^(-2400/8.314*(1/T+273.15)-1/298.15))

I then convert moles to PPM like this:

   ppm = C*44.1 * 1000

The equation is basically Henrys law, with the CO2 constant. But I used the Van't Hoff equation to adjust the constant for the temp of the water


I'm just trying to digitize the drop checker I talked about about. The 4Dkh liquid reacts to the CO2 off gassed from the aquarium. I should be able to replace the 4dkh liquid with a digital sensor, right?

Maybe there is a better way to measure this?

The other Idea I had was to agitate the water surface in the cup. I figure that would speed up Equilibrium