Affordable CO2 sensor?

[mistergreen]

I set the co2 to turn on an hour a head of the lights. It's 2 hours in and it's reading 30 ppm. My target is 40 ppm. I'll set it for 2 hours before the lights.

I could bump up the bubble rate but it's safer and the ph swing is gentler if I leave it be.

[Hoppy]

I suspect that the sensor time lag is the reason for the still low ppm reading after one hour. Unless the sensor can reach the equilibrium reading in just a few minutes, its time lag will keep you from seeing the actual ppm of CO2 as it rises. That is why I think you need a "faster" membrane. I also think that the actual ppm in the water will reach equilibrium in less than an hour, probably a lot less, unless you have really poor water circulation in the tank. (Too bad I can't prove this!)

[mistergreen]

Or too much surface agitation. I've pointed my outtake straight up which is fine.

The lag is a possibility as well since I had to put in a rigid screen under the membrane which further reduce the membrane surface area.

[mistergreen]

Ok, I bit the bullet and bought .010" silicone from http://www.sspinc.com/ssp_store/Thin..._to_040_21.htm

I could have gone thinner but I'm afraid it might be too fragile.
We'll see.

[mistergreen]

Originally Posted by Hoppy

unless you have really poor water circulation in the tank. (Too bad I can't prove this!)

I think you're on to something with the poor water circulation. I stuck it in the corner behind plants to get it out of sight. It's more reactive out in the open by water movement. It's ugly but it works better.

[UDGags]

Non-porous Teflon lets gas pass through but not liquids. It's used for this application when you bag and autoclave a composite.

The math at the beginning brought me back to Transport Phenomena class from undergrad. We always had to do those stupid gas diffusion problems.

[audioaficionado]

Originally Posted by mistergreen

I think you're on to something with the poor water circulation. I stuck it in the corner behind plants to get it out of sight. It's more reactive out in the open by water movement. It's ugly but it works better.

Maybe a small powerhead or recirc pump blowing on it would help.

[mistergreen]

From reading on the web, silicone seems to be the best material for gas permeability. I decided to take out the sensor and wait for my silicone membrane to arrive.

While inspecting the sensor, I noticed a nice slimy feel to the membrane. Bio-film had formed. I can see that building up and blocking gas exchange. If you were to design a housing, it's best to keep in mind that you'll need to wipe the membrane clean periodically. A weekly routine should work.

[UDGags]

Yeah, I was suggesting something that's fairly common that could work. Other than having rolls of it at work, I know some came with my food dehydrator so figured I'd throw it out there. I agree the silicon is probably the best long term.

[mistergreen]

I'll test out the Teflon sheet. I saw some Teflon heat transfer sheet on amazon for cheap. I'm going to use it to make silicone membrane

It should be non-stick too so it's good making the membrane.

[Hoppy]

The .010 thick silicone membrane will be very interesting! My bet is that the response will be about 10X faster with it, which makes the sensor very useful.

[mistergreen]

Yeah, hopefully the new membrane will work great.

Hey, I just started a forum focusing on these tech tank projects. I'll be finishing up the rest of the experiment over there. Tpt is too large to learn technical stuff and it focuses on plants. It's a place for beginners to learn and for advanced people to innovate.

http://aquatictechtank.net

There's a lot of work still to fill up the knowledge base. Come join.

[mistergreen]

The silicone membrane arrived! I just glued it to the box and waiting for things to cure.
I could have gone thinner with the silicone. It's pretty sturdy material.

[mistergreen]

The co2 controller is a lot more responsive. I set the solenoid to turn off if greater than 42 ppm and turn back on at 40 ppm. I wish I have a clearer log of what's going on but it seems to be on initially for 2 hours, then off an hour then on for .5 hour and off .5 hour subsequent hours.

The fishes are a lot happier. No more late pearling but that's ok as long as the plants get their CO2.
There is slight pearling so the plants are happy too.

[Hoppy]

Is the sensor noticeably faster in response with the new silicone membrane? A .001 thick membrane should make it respond about 10x faster, right?