# CO2 sensor



## mistergreen (Dec 9, 2006)

I thought I'd give the CO2 sensor another try. This time, build the casing as water tight as I can and as cheaply as I can. Thanks goodness the CO2 sensor wasn't permanently damaged. I just had to calibrate it using CO2meter's software.









2" pvc plug with cord grip









Inside I had to thin the top off with a router.









Trap adapter for the silicone membrane.









The adapter has a washer inside which I used with the membrane and silicone for safety measure.









The hardware: Arduino, CO2 senseAir K-30 10%, 5v fan, DHT-22 humidity & temperature sensor.
The fan is for circulation, and the humidity sensor is to alert me if it's too high or if a possible leak. The membrane is porous enough for H2O molecule to pass through. If the humidity is too high, I can remove the plug.


















The fit was too snug and the board broke in half so I kept it. 









Final product. I'm out of the CO2 game but I'll set up a yeast reactor in a bucket to test. I'll send the data to a mysql database online. I'll post the result soon.


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## Audionut (Apr 24, 2015)

Interested to see where this goes.


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## mistergreen (Dec 9, 2006)

Audionut said:


> Interested to see where this goes.


I've made an earlier version to control my CO2 solenoid. It worked really well until it leaked. This should be bullet proof. It might not be as responsive because it's bigger in volume. The fan might help in faster gas exchange. I've plugged the fan into the pwm pin so I can control the speed to test responsive rates.

Now I'm more keen on using it to measure CO2 in my dirted tanks.


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## mistergreen (Dec 9, 2006)

So here's the result of the logs










I added a 1 liter yeast diffused it a piece of chopstick and circulated with a powerhead in a 5 gallon bucket. I stopped at 60ppmw. It was recording every 7 seconds so I changed the code to record every 10 minutes instead.

CO2 is still climbing. It's at 100ppmw at last check. I pulled out the CO2 to see it tapered out. Who knew DIY CO2 can go that high. I guess it'll keep climbing if nothing consumes it or diffuse it out of the water. I guess shutting off CO2 at night is a good idea.


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## Audionut (Apr 24, 2015)

I think it would be interesting if you could take a sample from water that is at equilibrium with the atmosphere, and then another sample at a 1pH drop.


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## mistergreen (Dec 9, 2006)

Audionut said:


> I think it would be interesting if you could take a sample from water that is at equilibrium with the atmosphere, and then another sample at a 1pH drop.


Ah, good idea. I was looking for an excuse to buy a pH pen.

* I just got a pH pen on amazon for 'free'  . Amazon takes Amex points now.


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## mistergreen (Dec 9, 2006)

This is the log overnight. The response rate is surprisingly slow with this sensor. The rate is faster out of water. I guess the volume of air inside the encasing is too much for a fast response.


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## mistergreen (Dec 9, 2006)

So the response rate is about .005ppmw per second. It takes 7 or so hours to go from 1ppmw to 70ppmw.
I'm going to try to reduce/ seal off the area around the membrane and feed the gas in that chamber directly to the CO2 sensor via an air tube.


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## mistergreen (Dec 9, 2006)

I tried this new configuration where the cap in front seals off a section and the tubes run directly into the CO2 sensor. The fan and humidity sensor is in front.


The response rate is .003ppmw/sec. Slower that the original design.


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## ichy (Apr 6, 2015)

Don't know if this applies, but I have a buddy that builds CH2 monitors. I believe to get them to work right he has to physically pump an air sample into something like a sensor tube?? I could ask for you if you think its something that might help?

Just trying to be helpful! LOL..


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## mistergreen (Dec 9, 2006)

ichy said:


> Don't know if this applies, but I have a buddy that builds CH2 monitors. I believe to get them to work right he has to physically pump an air sample into something like a sensor tube?? I could ask for you if you think its something that might help?
> 
> Just trying to be helpful! LOL..


Thanks. Yeah, I'm using the fan to move air around but a vacuum pump would better. I was looking around for a small cheap one.


Sent from my iPad using Tapatalk HD


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## mistergreen (Dec 9, 2006)

Ok, I found a used vac pump on fleabay for $2.50. It'll arrive in a few weeks from HK.


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## mistergreen (Dec 9, 2006)

I talked to the tech guy of the CO2 sensor. My current setup with the tubes in the air ports doesn't work because the entire face of the sensor has a membrane to read CO2. So I'll have to buy their adaptor.

I knew something was off from looking at the numbers. The response rate should have been faster, not slower.

Also in such high humidity environment, I should use H2O absorbing filters and materials but they're too expensive. To extend the life of the filter, it shouldn't be in water for a long period of time.


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## jeffkrol (Jun 5, 2013)

Just throwing some stuff at you.. 

Degassing membrane PVDF for aqueous solutions, 10 x 10 cm, pore size 1000 nm, 4 pieces | Novamem Ltd
Dissolved CO2 Sensor for Biotech - Overview - METTLER TOLEDO
not immersible..
http://www.vernier.com/products/sensors/co2-bta/

immersible:
http://www.fischundwasser.de/lang1/co2_sensor.html

w/pump:
http://www.fischundwasser.de/lang1/co2-inline-monitoring.html


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## mistergreen (Dec 9, 2006)

jeffkrol said:


> Just throwing some stuff at you..
> 
> Degassing membrane PVDF for aqueous solutions, 10 x 10 cm, pore size 1000 nm, 4 pieces | Novamem Ltd
> Dissolved CO2 Sensor for Biotech - Overview - METTLER TOLEDO
> ...


yes, I'm trying to do it on the cheap compared to these.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> yes, I'm trying to do it on the cheap compared to these.


Well, not sure it is cheaper though, the non-immerse is $250 so about the same .wrote to the German company to see if I can get pricing)

doing it "cheaply" would seem to start w/ 2 IR diodes.. 

Sorry, certainly not criticizing, but I am seeing cost of development exceeding cost of "pre-built"...

Not that that isn't what is SOP for most things though..

Thought your sensor board was south of $200...

gotta love the Chinese..
121790244440

you know what to do.. 

http://www.intl-lighttech.com/applications/light-sources/ndir-gas-sensor-lamps


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## mistergreen (Dec 9, 2006)

In addition to the sensor being the main cost. The membrane is usually second in cost, very expensive. A Silicone membrane is pretty cheap.

There are 2 price ranges for the sensors I'm using. 
If you're measuring up to 44ppmw(10,000ppmv), the sensor is only $65.
For up to 132ppmw, the sensor is $265.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> In addition to the sensor being the main cost. The membrane is usually second in cost, very expensive. A Silicone membrane is pretty cheap.
> 
> There are 2 price ranges for the sensors I'm using.
> If you're measuring up to 44ppmw(10,000ppmv), the sensor is only $65.
> For up to 132ppmw, the sensor is $265.


Understood. I'm still shooting for a $50 Li-Cor accurate PAR meter.. CO2 is way over that budget.. 
CO2 Meter - CO2, CO, Oxygen, CH4 and VOC Sensors products


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## mistergreen (Dec 9, 2006)

Yup, I'm using the K-30 from Co2meter.com. I don't know, a $150 underwater CO2 meter isn't bad compared to $500-$5000.


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## ichy (Apr 6, 2015)

I've got the interface for the Vernier one, but would be a pain to setup without being able to immerse it.


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## mistergreen (Dec 9, 2006)

Thanks to ichy,
I had this casing 3d printed. The CO2 sensor will be screwed in there. 
The screw receptacle was rather weak and broke off but I can still use it. I think my local library has a 3d printer I can test the redesign for future use.

I planned to spray it with krylon to make it gas tight but couldn't find it at the local hardware store and wasn't sure what to get instead so I got spray on polyurethane.


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## ichy (Apr 6, 2015)

If you have room to get fillets in there it really adds strength.


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## mistergreen (Dec 9, 2006)

Done. I feel like an industrial designer.


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## mistergreen (Dec 9, 2006)

Here's the new CO2 sensor box


























I might need to calibrate my sensor a bit but I always wanted to test some notions we have in the hobby.

1. There's dissolved CO2 in tap water that causes algae or extra pearling.
AIR 1.02 ppm
TAP water: 0.99ppm


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## ichy (Apr 6, 2015)

How did you end up sealing it? Looks great!


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## mistergreen (Dec 9, 2006)

I sprayed box part with polyurethane. I haven't played with acetone yet.

Next test is how responsive this setup with and how much CO2 in 1 degree of pH drop.
I've setup the yeast CO2 in a bucket and will test those.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> I sprayed box part with polyurethane. I haven't played with acetone yet.
> 
> Next test is how responsive this setup with and how much CO2 in 1 degree of pH drop.
> I've setup the yeast CO2 in a bucket and will test those.


in case you didn't see this one..


> *Calibration Using Nitrogen*
> 
> The most accurate method of CO2 sensor calibration is to expose it to a known gas (typically 100% nitrogen) in order to duplicate the conditions under which the sensor was originally calibrated at the factory. ISO certification is available on our high-end sensors to verify this process for medical or scientific purposes. Nitrogen calibration is also required if CO2 levels between 0-400 ppm will be measured.
> 
> ...


CO2 Meter - CO2 Sensor Calibration: What You Need to Know


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## pdeuchler (Jul 20, 2015)

I may be missing something here, but wouldn't it be far easier to do some simple calculations with a pH meter given a certain KH? You could even track pH fluctuations and calculate the co2 concentration ∆ given the magnitude of the fluctuations. 

Excellent work though, how effective are those latches for the seal?


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## Audionut (Apr 24, 2015)

The pH/KH/CO2 relationship works (or is accurate) when the only effect on pH is KH or CO2. There are a significant number of other substances that affect pH, which reduces the accuracy of the relationship.


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## mistergreen (Dec 9, 2006)

I've been using fresh air to calibrate to 400ppm. Fresh air around here is around 420. Close enough. When converting ppm by weight, that's a small margin of error.

The clips work great. It's not leaking 

Audionut it is correct and I want to see if that chart pans out.

If you have any co2 facts or myths you want to challenge, let me know. I just dispelled the tap water myth.

Sent from my iPad using Tapatalk HD


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## kevmo911 (Sep 24, 2010)

I'd like to see exactly how long after lights on that it takes for a tank of plants to really start photosynthesizing. Obviously, no tank is exactly the same as the next, nor is any plant species, and, also obviously, you were going to do that anyway. And it'd be nice to know if high vs low light changes that timeframe. I'd also like to see what effect a sunrise (gradual) simulation has on CO2 uptake. Also, what's the time difference in degassing between zero surface agitation and lots.

I'll check back for your results in 15 minutes, at which time I'll update my wish list


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## Audionut (Apr 24, 2015)

mistergreen said:


> If you have any co2 facts or myths you want to challenge, let me know.


Myth #1
With all other parameters remaining equal, the increase of KH increases CO2 concentration in the solution.


If you perform this test, can you please also log pH to show how the increase of KH only had an effect on pH, and not CO2 concentration. thanks.


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## m_t_r_86 (Jan 10, 2016)

I'm curious to see where this goes. Keep the updates coming
:nerd:


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## Hoppy (Dec 24, 2005)

How much does the CO2 concentration vary from one area of the tank to another? Is it lower at the water surface or at the bottom? How does surface ripple change this?


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## kevmo911 (Sep 24, 2010)

Also a comparison between levels and speed of CO2 increase using a reactor versus an Atomizer-style diffuser.


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## pdeuchler (Jul 20, 2015)

Audionut said:


> The pH/KH/CO2 relationship works (or is accurate) when the only effect on pH is KH or CO2. There are a significant number of other substances that affect pH, which reduces the accuracy of the relationship.


Right, but wouldn't measuring fluctuations allow you to calibrate for that? You could also account for other ph fluctuations by degassing a water sample and using that as a baseline. I'm still fairly certain you could use something like Henry's Law since it's a gas but I could be wrong.


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## mistergreen (Dec 9, 2006)

Some good ideas. I only have yeast CO2 with a chopstick diffuser at this point. I gave up on my CO2 tank.

I originally wanted to test how & and how much a dirt tank can generate CO2 on its own.


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## jeffkrol (Jun 5, 2013)

pdeuchler said:


> Right, but wouldn't measuring fluctuations allow you to calibrate for that? You could also account for other ph fluctuations by degassing a water sample and using that as a baseline. I'm still fairly certain you could use something like Henry's Law since it's a gas but I could be wrong.





> The pH/"KH"/CO2 charts have some failings, however. The charts commonly used seem to overestimate actual CO2 levels, by as much as 20%. They don't take into account various biogenic acids, aside from carbon dioxide. Or any humic acids, tannins and the like. And all bets are off if part of your buffer is based on phosphates. Details of CO2 and hardness have been discussed at length in extracts from the Aquatic-Plants Digest archived The Krib, where you can find out "All you ever wanted to know about CO2 but were afraid to ask".


CO2: dissolved CO2 and the pH scale | The Skeptical Aquarist

To me one of the whole points of the CO2 sensor is because of the possible "failings" of ph/CO2 assumptions...

http://aquaticconcepts.thekrib.com/Co2/co2_faq.htm#T06


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## mistergreen (Dec 9, 2006)

I can use my pH meter to measure the degree of CO2 that turns into carbonic acid. It would be fun either way.

I guess the water's pH originally would affect the rate of pH change.


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## Audionut (Apr 24, 2015)

pdeuchler said:


> Right, but wouldn't measuring fluctuations allow you to calibrate for that?


It would only show the already known properties. We already know that if pH drops by 1.0 pH, and that pH drop is solely from CO2, then CO2 concentration has increased 10 fold.

The problem remains, that at a hobby level, there is no accurate way to determine what substances are in the aquarium that are affecting pH.

So if you have some aquarium, and it's pH is whatever, we don't know why that pH is what it is. How much is due to KH/CO2, and how much is due to other things. If we adjust the pH of this aquarium by 1.0 pH with CO2, we know that the CO2 concentration has increased 10 fold, but we still don't know what the starting CO2 concentration (before we injected CO2) was, or even the end CO2 concentration (after we injected CO2) is. We know that the concentration increased 10 fold, but that could be from 1 ppm to 10 ppm, or 130 ppm to 1300 ppm.


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## mistergreen (Dec 9, 2006)

Oh, I found something interesting. This CO2 setup is more sensitive underwater than floating on the surface. I think water pressure helps push CO2 through the membrane.
I've made the reading public for you if you want to check it out. This is live.

http://2noodles.com/arduino/publictable.php

It updates every minute so you can refresh your browser to get the data.
At 2016-02-05 09:03:39, is where I dunked it underwater. 132 ppmw is the max reading for this sensor I think.


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## Zapins (Jan 7, 2006)

I've looked into CO2 sensors several times and the main issue I keep running into is that it even with the most expensive and accurate sensors they don't seem to have the accuracy to detect a difference of a few ppm change in CO2 or even measure the range of CO2 that we commonly use.

How have you gotten around this problem?


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## ichy (Apr 6, 2015)

mistergreen said:


> Oh, I found something interesting. This CO2 setup is more sensitive underwater than floating on the surface. I think water pressure helps push CO2 through the membrane.
> I've made the reading public for you if you want to check it out. This is live.
> 
> http://2noodles.com/arduino/publictable.php
> ...


Not sure how you are pushing your data to the web but,
If you look at Internet of Things websites, such as Zapier you can push data to a web page, have it graph it and Zapier will send email alerts at given set points. 
Just a thought.
Carry on.


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## mistergreen (Dec 9, 2006)

Zapins said:


> I've looked into CO2 sensors several times and the main issue I keep running into is that it even with the most expensive and accurate sensors they don't seem to have the accuracy to detect a difference of a few ppm change in CO2 or even measure the range of CO2 that we commonly use.
> 
> How have you gotten around this problem?


Are you looking at the right sensor?
It's pretty sensitive and have onboard calibration.
The $85 sensor will give you a max of 40ppmw
The $225 one will give you a max of 135ppmw.]

Note that ppm by weight and by volume are different.

Bump:


ichy said:


> Not sure how you are pushing your data to the web but,
> If you look at Internet of Things websites, such as Zapier you can push data to a web page, have it graph it and Zapier will send email alerts at given set points.
> Just a thought.
> Carry on.


The sensor is connected to an arduino with WIFI. It pushes the data to my online database. Ok. Off to work. I'll take the CO2 out of the bucket and you can see it drop.


BBAH, it sprung a leak.
will have to let it dry out.


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## Hoppy (Dec 24, 2005)

mistergreen said:


> Some good ideas. I only have yeast CO2 with a chopstick diffuser at this point. I gave up on my CO2 tank.
> 
> I originally wanted to test how & and how much a dirt tank can generate CO2 on its own.


That would be a very good test to do, including how long it takes to generate that CO2.


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## ichy (Apr 6, 2015)

mistergreen said:


> Are you looking at the right sensor?
> It's pretty sensitive and have onboard calibration.
> The $85 sensor will give you a max of 40ppmw
> The $225 one will give you a max of 135ppmw.]
> ...


oh no! did it seep through the plastic layers?


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## Zapins (Jan 7, 2006)

mistergreen said:


> Are you looking at the right sensor?
> It's pretty sensitive and have onboard calibration.
> The $85 sensor will give you a max of 40ppmw
> The $225 one will give you a max of 135ppmw.]
> ...


For example this is one that I was interested in a while back and it seems pretty typical of the sensors I looked into.

CO2 Meter - COZIR Ambient CO2 Sensor - CO2 Sensor by GSS


```
[B]Measurement Range: [/B]0-2,000ppm, 0-5,000ppm, 0-10,000ppm
[B]Measurement Rate[/B]: 0.5 sec / measurement
[B]Accuracy[/B]: ±50 ppm ± 3% of reading
```
An accuracy of +/- 50 ppm means that if it reads 50 ppm the real concentration could be 0 ppm or 100 ppm or anything in-between. This basically means that for our purposes it isn't useful at all. 

Even the several thousand dollar lab CO2 meters don't seem to be that sensitive. 
https://www.licor.com/env/products/gas_analysis/LI-7000/

Any chance you can post the sensor you are talking about? I'd love to look into it & possibly buy it.


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## mistergreen (Dec 9, 2006)

I think I need more silicone to seal any leaks.


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## 691175002 (Apr 28, 2009)

Zapins said:


> An accuracy of +/- 50 ppm means that if it reads 50 ppm the real concentration could be 0 ppm or 100 ppm or anything in-between. This basically means that for our purposes it isn't useful at all.


I'm not a chemist but I think what is going on here is that the sensor is measuring air concentration instead of water concentration.

For example in equilibrium, the ~400ppm of CO2 in the atmosphere is equivalent to roughly 0.5ppm of CO2 into the water. In other words, any sensor you use becomes roughly two orders of magnitude more precise when measuring water.

One problem is that the ratio of atmospheric to dissolved CO2 is dependent on temperature, but assuming you know temperature and atmospheric CO2 its just math.

https://en.wikipedia.org/wiki/Henry's_law


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## mistergreen (Dec 9, 2006)

Zapins said:


> For example this is one that I was interested in a while back and it seems pretty typical of the sensors I looked into.
> 
> CO2 Meter - COZIR Ambient CO2 Sensor - CO2 Sensor by GSS
> 
> ...


this guy CO2 Meter - CO2 Sensor for OEM Applications. Quantity Pricing. - CO2 Sensor by Senseair 
for $85

ppm by volume (air) is not ppm by weight (water)
accuracy +-30ppm (air)
is
accuracy +-0.132ppm (water)


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## Zapins (Jan 7, 2006)

Thanks for the explanation. I'm still not entirely sure how that makes the sensor more accurate in water?

Are you exposing the sensor to water directly or creating an air pocket that the device measures? If the second setup is true then how is that different from sampling the atmospheric air? They are both gasses so shouldn't the accuracy be the same? 

Many of the sensors also are not designed to work in high humidity environments, the sensor in the link didn't mention humidity so I am not sure how that affects the sensor you have.

Also, I seem to remember seeing specially designed water CO2 probes somewhere that also didn't have terribly good accuracy in water. Not sure if I can still find the link, but I'll have a look for it if anyone is interested in it.


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## 691175002 (Apr 28, 2009)

The sensor is measuring an air pocket that is in equilibrium with the water using a semi-permeable membrane.

It is no different from sampling atmospheric air, the accuracy is still +-30 or whatever. The difference is that each 1ppm increase of water CO2 ends up being a huge change in the air CO2 concentration. This ratio is governed by Henry's law.

I'm not sure you can do a straight volume to weight conversion since the equilibrium ratio depends on temperature. Maybe temperature only has a small effect, I know a chemist who could probably tell me.


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## mistergreen (Dec 9, 2006)

691175002 said:


> The difference is that each 1ppm increase of water CO2 ends up being a huge change in the air CO2 concentration. This ratio is governed by Henry's law.


Not really. It's just by weight takes a lot of by volume. It's just different measuring stick.




691175002 said:


> I'm not sure you can do a straight volume to weight conversion since the equilibrium ratio depends on temperature. Maybe temperature only has a small effect, I know a chemist who could probably tell me.


yes, it's temperature & pressure dependent. I'm taking 70F calculations, sea level.


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## Zapins (Jan 7, 2006)

I found the water CO2 sensor I mentioned above:
Mini-Pro CO2 Sensor: Dissolved CO2 Probe by Pro-Oceanus

Designed specially for measuring CO2 in water.


```
[B]Specifications[/B]

[B]Performance[/B]

[B]Standard measurement ranges:[/B]

pC02 concentration
0-2000 ppm CO2,
by volume
0-5000 ppm
0-1%
0-100%

[B]Resolution[/B]

1 ppm
10 ppm
0.01%
0.1%

*Other ranges available

TDGP: 0-3 bar

[B]Accuracy[/B]:
[U]pCO2: + 2% of maximum range
TDGP: + 1%[/U]
```
It says 2% of max range which is 40 ppm +/- I think?

Though this one seems quite bit better:
http://www.pro-oceanus.com/co2-pro-atmosphere.php

Data Sheet:
http://www.pro-oceanus.com/images/co2-pro-atmosphere.pdf

```
[U][B]Accuracy[/B]: +/- 2 ppm[/U]
[U][B]Resolution[/B]: 0.01 ppm[/U]
[B]Range[/B]: 0-600 ppm
[B]Equilibration time[/B]: 2.5 minute
[B]Sample rate:[/B] 1.6 seconds (user selectable with datalogger / controller)
```


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## mistergreen (Dec 9, 2006)

Notice, this sensor is by VOLUME. People don't bother converting to ppmw (weight) like we do in the plant hobby. All of the literature I've read (education research) , they kept their CO2 reading by volume.

5000ppmv will give you 22ppmw. Maybe a change for the hobby is needed and we should start using by volume readings because we won't need to bother with temperature & pressure as factors.

For instance, we should say, "For optimal growth, you should get your CO2 to 9,088ppm."






Zapins said:


> I found the water CO2 sensor I mentioned above:
> Mini-Pro CO2 Sensor: Dissolved CO2 Probe by Pro-Oceanus
> 
> Designed specially for measuring CO2 in water.
> ...


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## Zapins (Jan 7, 2006)

Over the years how happy are you with the silicone membrane you bought? Did it hold up all these years or did you have to replace it?

Also, have you tried using kordon breather bags as a membrane?


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## mistergreen (Dec 9, 2006)

Zapins said:


> Over the years how happy are you with the silicone membrane you bought? Did it hold up all these years or did you have to replace it?
> 
> Also, have you tried using kordon breather bags as a membrane?


The Kordon bags are slow in gas exchange compared to silicone.
The silicone is fine in durability. This project isn't meant for long term underwater use. I've linked to a DIY underwater sensor from some university. They use hydrophobic materials and absorbent materials to deal with the humidity in the casing. Water molecules can enter the membrane as well if you read my earlier posts. Water & electronics don't match. I don't use these materials because it's cost prohibitive.

For now this setup is fine temporarily, at least you should do weekly maintenance where you get it out of the water for a few hours to check for leaks, let the humidity disperse, and let the CO2 sensor's ABC (auto calibration) feature do its work.


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## mistergreen (Dec 9, 2006)

ok, the sensor is live again. It looks like the responsiveness is getting 1ppm / minute. It's the fastest I've gotten yet. I'll check for leaks again in 30 minutes.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> Notice, this sensor is by VOLUME. People don't bother converting to ppmw (weight) like we do in the plant hobby. All of the literature I've read (education research) , they kept their CO2 reading by volume.
> 
> 5000ppmv will give you 22ppmw. Maybe a change for the hobby is needed and we should start using by volume readings because we won't need to bother with temperature & pressure as factors.
> 
> For instance, we should say, "For optimal growth, you should get your CO2 to 9,088ppm."


Eek... anyways thanks for bringing this to my attention.. So is your "conversion factor" (disregarding the STP factors)
correct?


> Typically, surface waters contain less than 10 ppm free carbon dioxide
> The acceptable range of carbon dioxide for most finfish is <2.0 mg/L (ppm).
> To take in fresh oxygen, fish must first discharge the CO2 in their blood stream, a process which is slowed down considerably when there are high concentrations of CO2 in the water itself.


http://www.alken-murray.com/TESTS01.htm

2 ppm(v) = .0088ppm(w)

seems we have a conflict of interest.. or am I not looking at something correctly?

Actually most charts are ppm(v)


> Important notes on CO2 levels
> The optimal safe C02 level for plant growth is 15-35 mg/l.
> CO2 levels above 35 mg/liter are dangerous to fish.


http://aquadaily.com/2009/01/29/co2-ph-kh-chart/
0.154 ppm(w)

IF this large discrepancy is due to the difference between "bound" CO2 (carbonates) vs free CO2 your sensor (I assume is measuring just free CO2(g)) is better used as a negative indicator.. i.e "don't exceed" 2ppm(v)...


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## mistergreen (Dec 9, 2006)

My conversion takes into account 1 atm and 69F or 70F (I forget). Standard Temperature and Pressure (STP) is defined as 0 degrees Celsius and 1 atmosphere of pressure.

I believe the alken-murray site is talking of ppm(w); Lots of confusion. Most don't know the difference and freak out by the numbers. 2ppm(w) is basically equilibrium with CO2 in the air (400-500ppm(v)).

I checked the sensor this morning. No leaks!.. And if you saw the CO2 drop, that's because the floated to the top and only reading CO2 right at the top of the water line. It's too buoyant and airline holder wasn't strong enough.

There is a relationship to CO2 in the water and the air right above it  Henry's Law.


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## mistergreen (Dec 9, 2006)

I lifted the sensor an inch out of the water. It's dropping to ambient air CO2 (really slowly). So Henry's Law doesn't work unless the container is closed off? I just covered the bucket to see.


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## mistergreen (Dec 9, 2006)

I learned something interesting. If you inject CO2 into a tank with no current/filtration, the CO2 slowly builds up without end (seemingly).


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## ichy (Apr 6, 2015)

seemingly? LOL..


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## mistergreen (Dec 9, 2006)

ichy said:


> seemingly? LOL..


Yeah, my sensor only goes to 135ppm max. I can't measure beyond that. I'm sure that's a limit at some point. Water can only hold so much CO2 without being pressurized like a soda can.

I'm starting to see yeast CO2 is perfectly fine for planted tanks except for the mixing and preparing.

I'll test the 1ph drop an how fast yeast CO2 can fill up 3 gallons of water.


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## aja31 (May 25, 2013)

mistergreen said:


> I learned something interesting. If you inject CO2 into a tank with no current/filtration, the CO2 slowly builds up without end (seemingly).


Interesting thread, curious to see what else we learn.

If there is nothing to break the surface tension on the water then I suspect you can pump CO2 in there for days before you hit the limit. 

Once you break the tension it should hit a much lower equilibrium.

DIY CO2 is definitely fine for smaller tanks, the issue is usually in large 75g+ tanks where you have a ton of surface area and no surface tension, you simply can't produce CO2 fast enough to keep up with the outgassing. 

Without any current the CO2 probably also takes a lot longer to diffuse around the tank, I wonder how big of a difference height and position makes with no current as compared to normal current.


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## mistergreen (Dec 9, 2006)

Hmm, the surface area could be a factor. It takes forever for the bucket to drop CO2 back down to 3ppm.
If I need to reset the reading, I have to replace the water.


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## Zapins (Jan 7, 2006)

It makes sense that it would take a long time to return to 3 ppm since most of the CO2 in water is dissolved in carbonic acid form and would need to go back into a gas to exit the water and be detected. 

In terms of speed most of the CO2 probes I've seen seem to use a very thin tube sort of setup connected with the CO2 sensor. I am guessing this is to minimize the amount of air that needs to equalize with the membrane. The more air, the more time it takes for the CO2 in the water to equilibrate with the air CO2 concentration.

I wonder if you could try something like a perforated tube (http://www.ebay.com/itm/Aquarium-St...065043?hash=item2a5b943153:g:m3MAAOSwuTxV~-bP) with the silicone membrane wrapped around it to make it water tight, then use an aqualifer pump to circulate air through the steel perforated tube & membrane in a closed loop. This would help speed the transfer of gas between the tube system and the membrane and give a very large area for gas diffusion to happen while minimizing the total amount of air that needs to equalize.

You could even add the little pre-filter container inline and before the sensor and fill it will water absorbant material. This would help dry the incoming air and prevent it from messing with the readings.

This is the pump and filter I mentioned. 
http://www.no ebay links allowedcom...832320?hash=item1e9476cc00:g:zBIAAOSwmUdUV9xj

By the way, in retrospect do you think you could have used a thinner membrane or is 0.01" the limit without tearing it?


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## jeffkrol (Jun 5, 2013)

Just for "fun" I checked into this sensor's cost..............
Mini-Pro CO2 Sensor: Dissolved CO2 Probe by Pro-Oceanus



> Standard measurement ranges:
> 
> pC02 concentration
> 0-2000 ppm CO2,
> ...


Care to guess????


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## mistergreen (Dec 9, 2006)

jeffkrol said:


> just for "fun" i checked into this sensor's cost..............
> mini-pro co2 sensor: Dissolved co2 probe by pro-oceanus
> 
> 
> ...


$5000

Bump:


Zapins said:


> It makes sense that it would take a long time to return to 3 ppm since most of the CO2 in water is dissolved in carbonic acid form and would need to go back into a gas to exit the water and be detected.
> 
> In terms of speed most of the CO2 probes I've seen seem to use a very thin tube sort of setup connected with the CO2 sensor. I am guessing this is to minimize the amount of air that needs to equalize with the membrane. The more air, the more time it takes for the CO2 in the water to equilibrate with the air CO2 concentration.
> 
> ...


I tried the pump route for fun. Using a cheap pump, I couldn't get a good airtight pump. I would pump air out but wouldn't pump in correctly. I scrapped it.


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## Zapins (Jan 7, 2006)

I edited my post while you were replying. An aqualifer pump would make a closed loop, air tight system if you hooked up the tubes in a circular pattern with the sensor and the tube membrane.


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## jeffkrol (Jun 5, 2013)

mistergreen said:


> $5000


GOOD guess, for the analog version..


> $6750 USD for an analog output model. Shopping costs to the US would range between 175-250 USD


Maybe I was confused on what "mini" meant.. 

Carry on..


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## mistergreen (Dec 9, 2006)

jeffkrol said:


> GOOD guess, for the analog version..
> 
> 
> Maybe I was confused on what "mini" meant..
> ...


yeah, I'm an expert at costs for CO2 sensors at this point


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## mistergreen (Dec 9, 2006)

ok, some data


















There are lots of data to mine but this is it for now.


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## aja31 (May 25, 2013)

That's odd... the pH drop is fairly linear, but the increase in CO2 concentration is exponential. A linear pH drop seems reasonable, but I would expect CO2 to increase rapidly at first and then slow down as it approached saturation. I can't think of any reasonable explanation that would make CO2 hard to absorb into the water at first and then easier as more and more CO2 built up in the water...


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## mistergreen (Dec 9, 2006)

aja31 said:


> That's odd... the pH drop is fairly linear, but the increase in CO2 concentration is exponential. A linear pH drop seems reasonable, but I would expect CO2 to increase rapidly at first and then slow down as it approached saturation. I can't think of any reasonable explanation that would make CO2 hard to absorb into the water at first and then easier as more and more CO2 built up in the water...


The CO2 is yeast so there might be a slow to fast buildup. The pH could be on a curve too. I might need to take more pH readings or get a pH sensor to hook up to my arduino to log as well.


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## mistergreen (Dec 9, 2006)

Dang, this thing keep leaking. The sensor is ok.
The water collects at the bottom over time. So my guess is the wire holder isn't water tight.


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## Zapins (Jan 7, 2006)

aja31 said:


> That's odd... the pH drop is fairly linear, but the increase in CO2 concentration is exponential. A linear pH drop seems reasonable, but I would expect CO2 to increase rapidly at first and then slow down as it approached saturation. I can't think of any reasonable explanation that would make CO2 hard to absorb into the water at first and then easier as more and more CO2 built up in the water...


I think that makes sense. pH is done on a log scale, and ppm CO2 is linear, said another way: it takes exponentially more CO2 to get a linear change in the pH.


mistergreen, what about using a gasket to seal the container? I've seen them on ebay in many different sizes.


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## mistergreen (Dec 9, 2006)

hmm, yeah, a gasket might help. I can make a diy gasket if silicone where the membrane is. I'll test the heyco cable grip connection.


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## Zapins (Jan 7, 2006)

For most water tight applications like underwater camera cases they use a rubber gasket and then clamp the case tight around it to create a seal. Usually some petroleum jelly is also used to lubricate the gasket and ensure no air spaces. Of course you have to be sure to clean off any dust or hairs from the gasket seal or that can create a space for water to enter.


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## mistergreen (Dec 9, 2006)

thanks, petroleum jelly is a lot easier to remove & take apart than sealing it up with silicone.


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## aja31 (May 25, 2013)

mistergreen said:


> The CO2 is yeast so there might be a slow to fast buildup. The pH could be on a curve too. I might need to take more pH readings or get a pH sensor to hook up to my arduino to log as well.


Yeah the pH change follows the CO2 curve pretty well, it should be linear if the CO2 is exponential since pH is logarithmic. 

It must have to do with the yeast CO2. Probably takes a while to build up, then really takes off. Especially if you made the yeast mixture at the same time that you started measuring. It would have to build up CO2 pressure before it even started going into the water.

Would be interesting to compare with a pressurized CO2 setup. 

If you can I would repeat the experiment with some powerhead or something in the bucket to break the surface tension to simulate a tank situation where we would expect the CO2 to reach an equilibrium below the very high concentrations you are reaching in a static bucket.


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## mistergreen (Dec 9, 2006)

If anybody wants the raw data,

https://docs.google.com/spreadsheets/d/19q_OL20Qn430j--ACvk5GPz6Kec7vPem9hZtvR8Ia9c/edit?usp=sharing


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## Zapins (Jan 7, 2006)

Any progress on this? A functional CO2 sensor would be a phenomenal achievement for the hobby. But we would need to figure out a way to improve the read speed time, I have a few ideas that I can fabricate for you and send over for you to test if you like. I live fairly close in cumberland gap TN so I might even be able to drop it off.


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## mistergreen (Dec 9, 2006)

The membrane is what influences the response rate. The only cheap option is a thin silicone membrane.

I think what I have is good enough. I want to tweak the casing a bit and reprint. Some parts are weak and break in time.

I also worked out the math to convert ppmv to ppmw finally.

Sent from my iPad using Tapatalk HD


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## Zapins (Jan 7, 2006)

Would love to see the math. 

Also how large are the silicon sheets? 

What are the other options?

It would be nice if they made silicon tubes with tiny wall thickness.


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## mistergreen (Dec 9, 2006)

The math is here

AquaticTechTank.net ? View topic - CO2 sensor!

You can buy the silicone sheets (12"x12")
Thin Silicone Membranes


The other membrane option is a special teflon tube. It's like $200+ a pop.


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## Zapins (Jan 7, 2006)

Which site sells the tubing? 

Thanks for all the other links.


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## jeffkrol (Jun 5, 2013)

Zapins said:


> Which site sells the tubing?
> 
> Thanks for all the other links.


Maybe referring to bioreactor tubes w/ PTFE membranes..

Helena Biosciences Europe :: TubeSpin Bioreactors

PTFE venting


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## mistergreen (Dec 9, 2006)

Zapins said:


> Which site sells the tubing?
> 
> Thanks for all the other links.


Look for 

Teflon® AF 2400
PTFE

I have an attachment of a DIY CO2 sensor made by University Of The District Of Columbia.
http://aquatictechtank.net/viewtopic.php?f=11&t=11&start=40#p506


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## Zapins (Jan 7, 2006)

One of these perms select membranes would be perfect. 2.1 square meters of surface area. Too bad they are pricey as hell.

https://www.permselect.com/membranes


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