Affordable CO2 sensor? - Page 4 - The Planted Tank Forum
Old 11-18-2012, 05:14 AM
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Have you figured out the relationship between the ppm in the air volume vs. ppm in the adjacent water? If the room air is at 500 ppm, and conventional wisdom isn't way off, that would correspond to around 3 ppm in the water. But, without that relationship the ppm in the air volume doesn't tell us much. I'm sorry my knowledge isn't sufficient for me to figure out the relationship.

Hoppy
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Old 11-18-2012, 12:22 PM Thread Starter
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Not yet. I think I just need to convert the ppmv to ppmw <- by weight. That's what is used to measure co2 in water.

Last edited by mistergreen; 11-18-2012 at 12:45 PM. Reason: A
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Old 11-18-2012, 03:26 PM Thread Starter
Children Boogie

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Quote:
Originally Posted by Hoppy View Post
Have you figured out the relationship between the ppm in the air volume vs. ppm in the adjacent water? If the room air is at 500 ppm, and conventional wisdom isn't way off, that would correspond to around 3 ppm in the water. But, without that relationship the ppm in the air volume doesn't tell us much. I'm sorry my knowledge isn't sufficient for me to figure out the relationship.
is it as easy as this?

500ppmv = .0005 mole (500/1000000)
.0005 X 44.01 (molar weight of CO2) = .022005 / 1000000 = 2.2005 ppmw

(molar weight/1000 kg/m^3 = ppmw)
1000 kg/m^3 (density of water) = 1000000 mg/l

There are lots of assumptions of course like water density and temperature. If you want absolute accuracy, you have to take those into account.

So if this is right, the tub of tap water has 4.26ppmw of CO2

and my low tech tank this morning has 20.18 ppm of CO2

Last edited by mistergreen; 11-20-2012 at 12:29 PM. Reason: Fix math
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Old 11-19-2012, 12:48 AM
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That gives the right order of magnitude for ppm of CO2 in water. So, it can't be way off in theory, but it could be ignoring other factors that would alter the ppm. As long as we don't have unusual conditions, like very cold or very hot atmospheric air, very high or very low atmospheric pressure, etc. that should work. (I can't think of any other parameters that might affect it.)

From http://www.350.org/en/node/26 "Since the beginning of human civilization up until about 200 years ago, our atmosphere contained about 275 parts per million of carbon dioxide. Parts per million is simply a way of measuring the concentration of different gases, and means the ratio of the number of carbon dioxide molecules to all of the molecules in the atmosphere."

http://www.xylenepower.com/Carbon%20Dioxide.htm is a very thorough discussion of CO2 in the atmosphere and in water, but it will take me many days to understand much of it.

Hoppy

Last edited by Hoppy; 11-19-2012 at 01:21 AM. Reason: add some reference info
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Old 11-19-2012, 02:08 AM Thread Starter
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From looking at the formula, I know for sure the density of water affects the calculation. My simple formula accounts for pure water I believe. SO saltwater or very hard water will be off but hopefully by not too much.

You'd need a combo CO2 sensor and density sensor to figure out true concentrations in water. I wonder if the \$2000 CO2 sensor is off as well ... Then why pay \$2000!?

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Old 11-19-2012, 03:51 AM
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From Wikipedia: "The density of surface seawater ranges from about 1,020 to 1,029 kg·m−3, depending on the temperature and salinity. Deep in the ocean, under high pressure, seawater can reach a density of 1,050 kg·m−3 or higher." So, if the ppm of CO2 is directly proportional in some way to water density, even seawater would only introduce a 5% error, which would be a spectacular accuracy.

Over the range of 10C to 30C the density of water varies less than 1%.

I'm inclined to believe that we can ignore water density variations in measuring ppm of CO2 in water.

Plantbrain has long been advocating measuring CO2 with the plants. In other words the absolute value of the concentration of CO2 isn't important, as long as we have determined that we have as much as the plants need at the light intensity we have. Where a sensor like this would be valuable is for monitoring changes in concentration, and even a moderately accurate sensor would work for that as long as it is sensitive enough.

Hoppy
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Old 11-19-2012, 01:41 PM Thread Starter
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I think I figured out a way to make the sensor more reactive. I placed the air ports right next to the membrane. It'll get co2 directly from the membrane instead instead of ambient co2 in the container. The readings will probably be more erratic. We'll see.

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Old 11-19-2012, 04:01 PM Thread Starter
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dang it! sprang another leak. Will have to find another case.

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Old 11-19-2012, 07:06 PM
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Is it possible to fill much of the volume of the air chamber with something like plastic beads? This would greatly reduce the air volume, making it inherently faster responding. Or, perhaps uncooked rice? Or, fit a saran wrap shroud around the sensor, then fill most of the volume with epoxy, making it a fitted case, with reduced air volume.

Just brainstorming!

Hoppy
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Old 11-19-2012, 07:26 PM Thread Starter
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yup!, I built a simple laminated carboard base for the sensor. I can seal the base with silicone or I can build a smaller container that'll fit over the sensor. Seal that off from the bigger container.

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Old 11-19-2012, 09:44 PM Thread Starter
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So I was wondering since I've been playing with measuring CO2. There's way more moles of CO2 in water than in air.

So why do aquatic plants need more of it?

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Old 11-25-2012, 04:56 PM Thread Starter
Children Boogie

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Well, things look good.

I found a little box with clasps at Michael's.

I built a little container within it out of acrylic sheets and silicon glue. The lip has silicon on it so it'll make a good seal with the membrane.

The top is the lid with a home made gasket make out of silicone.

Everything assembled with silicone for extra measure.

It is leak free.

So then it got dunked into my high tech tank.

It rose at about 1ppmw per minute. Final reading was 44.01 ppmw in my tank. It took 30 minutes to get there from 6 ppmw.

All in all a successful experiment. I have ideas on making it even more responsive. Adding more holes in the lid might help and I made a thin sheet of silicone I will replace the breather bag.

So now the next step if to hook this up to a relay to control the solenoid.

Last edited by mistergreen; 11-25-2012 at 05:31 PM. Reason: --
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Old 11-26-2012, 02:37 AM
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This is way over my head but it's cool seeing someone experimenting. That's how you further a hobby.
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Old 11-26-2012, 02:49 AM Thread Starter
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Quote:
Originally Posted by hedge_fund View Post
This is way over my head but it's cool seeing someone experimenting. That's how you further a hobby.
It should be pretty simple once i work everything out. It reads co2 to control the solenoid to the co2 tank instead of the guessing game of bubbles per second, timer, or ph meters.

I really do hope this gizmo will wipe out killing your fish and the dreaded BBa. No more co2 swings. That was the primary motivation for working on this.

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Old 11-26-2012, 04:40 AM
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Here are some silicone membranes: http://www.sspinc.com/ssp_store/Thin..._to_040_21.htm
Tyvek, which is gas permeable - http://www.amazon.com/E-I-DuPont-Tyv.../dp/B001TO4QVA

Both of these should greatly improve the response time.

Hoppy
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