Affordable CO2 sensor? - Page 7 - The Planted Tank Forum
Old 12-15-2012, 01:57 AM Thread Starter
Children Boogie

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Location: Cincinnati
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Yeah, it's noticeably faster than the breather bag. I'm not sure what the relationship is but .001 would be faster in theory.

Gas flow in and out easier. The breather bag seems to let co2 in but very slow in letting it out.

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Old 12-15-2012, 03:08 AM Thread Starter
Children Boogie

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Oh, if you're interested how the relay is connected to arduino, check out here
http://aquatictechtank.net/viewtopic...8&start=10#p19

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Old 12-16-2012, 12:26 AM
Algae Grower

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I'm not a chemist and this is only my second post around here, but I'm curious about this. Why are you bothering with membranes? Couldn't you mount the sensor in a container above the water's surface but open to the water. The concentration of CO2 in the air space should reach equilibrium with the concentration in the tank water. Then you could use Henry's Law to calculate the concentration of CO2 in solution.

Caq/Cgas = 0.8317 for CO2 @ 25 C

Last edited by coonass; 12-16-2012 at 12:37 AM. Reason: added formula
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Old 12-16-2012, 03:32 PM Thread Starter
Children Boogie

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Quote:
Originally Posted by coonass View Post
I'm not a chemist and this is only my second post around here, but I'm curious about this. Why are you bothering with membranes? Couldn't you mount the sensor in a container above the water's surface but open to the water. The concentration of CO2 in the air space should reach equilibrium with the concentration in the tank water. Then you could use Henry's Law to calculate the concentration of CO2 in solution.

Caq/Cgas = 0.8317 for CO2 @ 25 C
Yup that's an option as well. If water floods the electronics, we have an issue. The membrane is a safer solution and responsive solution.

It's worth a try though.... Maybe the sensor can hang from the tank. Depending the the design, it could be affected by ambient co2 or slow in response. It's pretty amazing how much co2 increases when there are people around the sensor. You can use it to detect people.

I'll give it a try down the road.

Last edited by mistergreen; 12-16-2012 at 03:49 PM. Reason: ....
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Old 12-16-2012, 05:07 PM Thread Starter
Children Boogie

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I did a quick sketch what do you think?

If the top is closed, it doesn't follow Henry's law any more since it's an enclosed atmosphere? It's like a drop checker at that point.

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Old 12-16-2012, 08:59 PM
Algae Grower

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Quote:
Originally Posted by mistergreen View Post
I did a quick sketch what do you think?

If the top is closed, it doesn't follow Henry's law any more since it's an enclosed atmosphere? It's like a drop checker at that point.
Exactly! It is just like a drop checker but you are measuring CO2 directly rather than inferring it from a change in pH. Why do you say Henry's law wouldn't apply? With a drop checker, you have two aqueous solutions connected by an air passage, so the concentration of CO2 in the tank water equals the concentration in the test solution since they are both aqueous. If you measure the CO2 concentration in the head space, you have to calculate the concentration in the water using Henry's law. As long as bubbles of CO2 (or any other gas) aren't constantly rising directly into the test chamber and the pressure within is equal to ambient air pressure, the concentrations of gasses in the head space should reach equilibrium with the concentrations of gasses dissolved in the water at the ratios described by the Henry's law constants for the given temperature and pressure. You can't have the head space open to the atmosphere because the concentration of CO2 in the tank water is greater than what you would have at normal equilibrium. The tank is constantly outgassing CO2 into the atmosphere trying to reach that equilibrium like a big glass of carbonated water fizzing its CO2 into the air. The room will never reach equilibrium with the tank unless it is sealed because CO2 is constantly diffusing away. The head space must be sealed to reflect the concentrations of gas in the water below it.

I would not clamp the test chamber to the side of the tank but rather build a floating platform so the pressure in the head space does not change as water level rises and falls. This could be as simple as an up side down plastic cup pushed through a piece of Styrofoam. In my opinion the use of a membrane of any type will only slow the response of the sensor (which will always have some lag as is).

This is a really cool idea. I just think it can be executed more simply without the dry box and silicone membrane.
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Old 12-16-2012, 09:23 PM
Algae Grower

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Quote:
Originally Posted by Hoppy View Post
There must be a relationship between ppm in the water, and ppm in the air trapped above the water. I assume the partial pressures of the CO2 would be equal, but what would the relationship be between the two ppm's?

Hoppy had it right from the beginning. Do this!
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Old 12-16-2012, 10:17 PM Thread Starter
Children Boogie

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Ok, this is worth a try out.
So what's the math here?

Say the sensor reads, 5000 ppmv in this drop checker.

I have to do 5000 x 0.8317 = 4158.5 ppmv? Then do the math to convert to ppmv to ppmw if I want. If that's the case I might have to do this to my current set up since it's a drop checker as well.

This design solves the ugly box in the tank problem

The benefit with the membrane solution is you can place it different parts of the tank and measure the difference in 3D. I guess with this design it's the overall concentration of the liquid or in 2D (the surface of the water line).

Last edited by mistergreen; 12-16-2012 at 10:33 PM. Reason: ...
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Old 12-17-2012, 12:07 AM Thread Starter
Children Boogie

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I went ahead and modify my existing sensor. I pulled the membrane out and put in a styrofoam raft. I'll test out the responsiveness of the sensor. There is probably .75" of head space all together with space behind the sensor as well.

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Old 12-17-2012, 02:17 AM
Algae Grower

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I went back through this thread and I didn't see that you calibrated this thing. What does it output? A mV reading? Are you applying an offset or multiplier to get the numbers you posted in ppm?

I would flow a pure gas other than CO2 through the sensor to set a zero point or derive an offset. Once you know that, whatever it reads in ambient air outside away from any local CO2 sources should equal approximately 0.00034 atm CO2 (don't know what that's supposed to be in ppm). What it reads in the head space of your test chamber should be the partial pressure (P) of CO2 above your tank. The solubility in air multiplied by 0.8317 should give you the solubility in aqueous solution. I've only ever done it in mol/L. Don't know anything about converting to ppm.

Another thing to consider is that it looks like you are supposed to flow the gas through this sensor.
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Old 12-17-2012, 02:26 AM Thread Starter
Children Boogie

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The output is ppmv. It's pretty convenient. All the info is in the datasheet.

The sensor is sent out calibrated to atmospheric CO2 at 400ppmv or something like that.... They also suggest to use Nitrogen and that should give me a 0ppm... I'm not looking for absolute values so what came out of the factory to me is fine.

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Old 12-17-2012, 02:27 AM Thread Starter
Children Boogie

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Quote:
Originally Posted by coonass View Post
The solubility in air multiplied by 0.8317 should give you the solubility in aqueous solution.
Yeah, that's what I thought. I should fix my code to do that. So with the new math, the maximum ppmw from the sensor will be 36.6ppmw.

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Old 12-17-2012, 02:17 PM Thread Starter
Children Boogie

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The floating design seems to be as responsive as the membrane design.

I wonder how the humidity will affect the electronics in the long run though.

Last edited by mistergreen; 12-17-2012 at 02:38 PM. Reason: ...
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Old 12-17-2012, 02:42 PM
aka Nick

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Can always pot the electronics in a polymer if you are worried about humidity. The water should act as a large thermal mass so I doubt it would get super hot.
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Old 12-17-2012, 03:55 PM Thread Starter
Children Boogie

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If you're interested in these projects, you can check them out on my forum.

Last edited by mistergreen; 11-03-2015 at 02:38 PM.
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