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Affordable CO2 sensor?

41K views 104 replies 15 participants last post by  mistergreen 
#1 ·
So I stumbled upon this CO2 sensor manufacturer by googling.
I wrote to them explaining our hobby and we could use a sensitive sensor to turn on and off our CO2. I got a reply from them. See if this could work out. My main concern is the error range.

It uses infrared to read CO2 in the atmosphere unlike the less reliable chemical reaction sensors we're used too.

Thanks for the interest and inquiry.
We measure the concentration of CO2 in water for example by measure the head space gas.
This make use of Henrys gas law regarding concentration in air and liquid.
The measurement are made via a space cup or a gas permeable membrane.
You could do this using a simple k30 sensor;
http://www.co2meter.com/collections/co2-sensors/products/k-30-co2-sensor-module
I’m not familiar with the operation of aquarium but 30 PPM of CO2 in water sounds extremely low for water standing a state of equilibrium.

Regards,
 
#4 ·
Given this diagram, I would imagine the CO2 levels would be the same as in the water.

For example, in the case of CO2 injection, the levels of CO2 in the water are higher. CO2 will off gas from the water into the atmosphere and into the bell shaped jar. When the levels of CO2 in the jar reaches the same level as the water, then it will not increase anymore.

Of course, CO2 will be diffusing from the water into the atmosphere as well as into the jar.
 
#5 ·
Oh, there is a difference between ppm in air and in water. Read the text below the calculator.
http://www.lenntech.com/calculators/ppm/converter-parts-per-million.htm

I'll have to study it to get the math. That was my confusion when I wrote to the manufacturer.

So I can hook this sensor to an arduino (in side a drop checker or breather bag) and have the arduino do the math for water (by weight) and control the solenoid to get the proper co2 concentration.
 
#9 ·
This is an interesting gizmo that can be used for lots of things but for me it is not going to be an affordable option. It is not a stand alone item to plug in. It needs several other things before we can stick it in to measure the CO2 in tanks. Those other items will run the price way beyond the level most will pay. With mounts, power supply and connections, I see at least $200. With equipment to read concentrations in water rather than air as this is designed it will be out of my range for sure.
 
#16 ·
As always, the biggest problem with trying to measure the concentration of CO2 in a planted tank is that it varies a lot from spot to spot in the tank. Near fast growing plants it will be low, compared to near the input of CO2 enriched water. Near the surface it will be less than in the middle of the tank. I once saw this demonstrated, when Plantbrain first played around with his $2000+ CO2 probe.

If this device can measure the concentration of CO2 in the air in a .03 thick sheet of air on top of the water, it should not take more than seconds to reach an equilibrium with that in the water, just below the surface. But, if that volume of air is a cube 6 inches on a side, it will take a lot longer. Or, if it is a large volume with a small interface with the water it will take even longer.

I still like the idea - and Murphy's Law, like Einstein's Relativity Theory, has yet to be found incorrect :icon_roll (Henry and I aren't acquainted.)
 
#20 ·
I think you'd need close to 2000 ppm or so in the head space to get 25-30ppm in solution. I did the calculation back in 1998 for a class for CO2 rise and the impacts in freshwater. This assumes that the 2 systems are in equilibrium, but that is rarely the case.

The issue is that the sensor will be slow, just like those Drop checkers.
A pH meter and relative change in degassed vs enriched would work much better and be more responsive.

In other words, a 90-100 pH meter would likely fit the bill much better.

Worth a shot though.

Since lag time is a factor, a pH/CO2 standard could be done using this at different times through the day for checking the pH meter vs the pH/KH/CO2 chart. But the lag time and stability of the system need accounted for.
 
#22 ·
pH meter is, we have successfully used them quite well in the hobby for nearly 25 years.

For the $, I think you'd be better off with a good pH meter. You calibrate the probe maybe 1x a month or if you want to be very certain. Good general care for the tip etc.
 
#23 ·
If the sensor life expectancy lasts even half of the predicted >15 years quoted on the supplier site that could be an interesting pro for the new CO2 sensor vs pH probe systems as well.

Assuming 1 year life span on double junction probes. (8 -14 months has been my personal experience) @ 35 bucks a probe. This obviously adds up.

And please tell me if I should be using better probes. that give me better longevity. Ive been going with the Milwaukee's and been happy with them since I usually get a year out of them with monthly calibration and gentle cleaning.

Tom, while I understand what you are saying about lag time and the sensor not being all that responsive, i think there is still one major advantage here.

While the pH meter obviously has served us well and can tell us the maximum amount of CO2 that could be dissolved in our water for a given KH. This could finally help the hobbyist with more money than sense, that won't listen to me that the number on the chart is a maximum and continues to insist that his pH/KH chart tells him that he has 100ppm CO2 so his lackluster plant growth could not possibly be from lack of CO2 :icon_roll

But seriously Mistergreen I am excited about this. As a group I don't think aquarists accept change well. Its nice to see someone pushing the limits with new tech.
 
#24 ·
Hey MisterGreen- This sounds like another really cool project. If the science involved can be sorted out and a fairly accurate prototype built, maybe we can build a "Close enough Ph Controller" too.
 
#26 ·
I had a chance to play with it. It's reading ambient air with ~2000ppm of CO2 and as soon as I dunk it in water inside a breather bag it shot up to ~8000ppm - 10,000pmm (maxed out)... I contacted the manufacturer to see what's up. I though air was only suppose to have ~360ppm and CO2 in water is suppose to be less than air.
 
#30 ·
Does this work by shining a beam of IR light through a length of air gap, and measuring the absorption in the wave length band that CO2 absorbs? If so, it must require a flow of air through the air gap. And, if that's the case there would be a minimum volume of air around the sensor that would be needed to refresh the air in the gap so it can follow the fluctuations in CO2 content. Or, am I off on a tangent here? :icon_roll
 
#31 ·
Since reading that the sensor has to "warm up" a little while before it produces reliable readings- I'd bet your description of how the sensor works is accurate. Providing for gas flow through the sensor will be an important step to keep it working accurately. I'm wondering if the sensor and a very small fan or other air moving device, could be mounted remotely in a sampling tube. The sampling tube would connected at both ends to a chamber that has a open end submerged in tank water. The air trapped in the tube would be constantly recirculated through the sensor, while the Co2 concentration would be allowed to diffuse freely in and out of the chamber, Similar to how a ph drop checker functions.
 
#32 ·
Yes,
it uses IR (non-dispersive infrared (NDIR)) to read the CO2 spectrum. It lights up when plugged in. It has 2 air ports for air movement. I think the issue when I dunked it in water, the breather bag collapsed to plug up the air ports. I'm building a proper plastic box for it now with a breather bag membrane. I'll use rubber bands to hold the bag on. I'll have to find something heavy to glue to the box so it would sink.

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From looking at the ppm in the air, it shifts from 400ppm - 500ppm. I'm thinking it'll shift back and forth as well in water. So I'll plan to take the average reading within a certain time frame like 10 seconds or so.
 
#33 ·
I made the box for this and threw in a bunch of washers to see how much weight it takes to sink the thing. It'll need a lot. Looks like this route is not a good one. I'm thinking suction cup is the way to go.

My only concern is there is quite a bit of air in the box but the membrane surface area is quite big so the gas exchange will be ok.
 
#34 ·
Things look good for a bit!


1701ppm outside water



I didn't see any noticeable leak so I went ahead with the experiment


It took a few minutes but the ppm started dropping. It dropped 1 ppm every 2-3 seconds. This is totally responsive in an aquarium where we're looking at 9ppm - 50ppm max. The ppm is a lot more stable in the container compared to air. There was a 2-3 ppm deviation sometimes.

I wanted to see where it would wind up but the readings stopped and notice there was a little water. The experiment ended at 1648ppm. I'm drying things out now. Hopefully there's no permanent damage. I'll need more silicone and thicker band next time.
 
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