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Idea for measuring CO2

GlassWalker

GlassWalker

Member
Joined
30 Jun 2014
Messages
205
Location
Swindon
I installed a drop checker not that long ago... and decided I hate it already. Well, I know I don't have enough CO2 until I install a better diffuser, but looking at a blue-green colour and watching it change to a slightly greener blue-green isn't exactly telling me much.

I'm aware that calculation of CO2 from pH and KH has difficulties due to other substances altering pH in tank water, plus not knowing the actual KH accurately.

I thought about the a variation of the drop checker but replacing the indicator with an pH probe, on a cheaper scale than the one proposed by plantbrain. That lead me to the thought, couldn't we work out the effective KH of the tank?

My proposed method is as follows:
Take a reference 4 dKH solution (or similar)
Take a sample of tank water
Put both into separate open containers and allow them to reach CO2 equilibrium with the air. Temperature may have an effect, so it may be best to do this at tank temperature.
Measure the pH of the 4 dKH solution. Use this to calculate the CO2 it has reached.
Measure the pH of the tank water sample. Knowing the CO2 above, calculate back to KH.
From then, you can use that KH value for CO2 calculations.

Probable weaknesses:
This is not a measurement of KH, as it still includes other factors that influence pH
I don't know if this effective KH value scales with other pH altering substances in water. I would assume it doesn't, and the question then becomes, how much error is there?
Tank water is unlikely to remain stable in this measure. Question is, how much change is there likely to be? This process may need to be repeated regularly. For starters I might try this before and after a water change.

Any thoughts?
 
1. How do you know CO2 has reached equilibrium ?
2. CO2 solubility highly affected by temperature ?
3. KH can vary day by day as things dissolve and/or consume the hardness. Gets "reset" at water change.
4. Probe measures pH not KH. KH may or may not be related to pH dependant on what else is in the water.
5. Calibration and contamination of pH probe will be an issue.

Drop checker doesn't involve messing around, is completely passive and if set up correctly is relatively immune to error.
 
1 - wait long enough. I don't know how long that is, but overnight should be more than plenty for example, although shorter is likely enough e.g. run an airstone. Strictly speaking, they don't need to be at equilibrium, but just need to equalise with each other.
2, 3, 4 were acknowledged as possible problems in my original proposal. To repeat, I'm not trying to measure actual KH, but get an effective KH to use for calculation, which takes in those other factors.
5 - pH measurement calibration is not a problem, at least not on something better than a pH pen off ebay. From my observations of using one so far, calibration is stable enough over long periods.

A drop checker is a very crude tool. For sure, what I propose isn't perfect either. The question is, is it "good enough"?

I would propose another measurement phase at least as a one-off, which can verify this. This is to insert a pH probe into a drop checker with reference solution, and compare results between the two methods.
 
Why not build a pH probe sitting in 4 dKH water isolated from tank water, similar to drop checker ? Then it won't matter what on earth the KH of your water and pH of your water is that day, as it will be isolated from the great unknown of your tank water.

Remember the pH of you tank will vary by itself, as fish poo, plants consume oxygen and give out CO2, plants give out oxygen and consume CO2, that hidden pile of detritus rots away, that dead fish rots a bit more, you left your lights on too long, its a cold day, its Tuesday.....

You cannot rely on your tank water for any calculations as its parameters are not fixed.
 
Plantbrains probe is probably the best way to do the electronic drop checker on the cheap, I don't have one and haven't seen one but when I pondered the very same question my thinking led to the same device. Then I looked online and discovered all the extensive previous testing by various people with different membranes and liquid gaps. The membrane is probably not cheap (not paper cheap) and the construction of the probe I imagine is fairly laborious.

The problem with drop checkers is not the colour reaction but the response time, same goes for plonking a ph probe into a modified hang-on drop checker even though it appears to be the obvious solution. It's the liquid volume that determines response time to available co2, hence the need for a reduced reference cell solution and reactive enough membrane (which eliminates the variable of the air gap in a drop checker) to increase the measured response times to something more meaningful than the current 2hrs the drop checker gives us.

:)
 
The pH probe in 4 dKH in tank is what I propose to verify (or otherwise) my described method by. It sounds too cheap, but I found a probe for about £8 delivered, which if it calibrates and remains stable, I'll modify into such an assembly somehow. That's a disposable enough price compared to the many tens of pounds they usually go for.

I know the tank water will change, but without quantifying those changes, we will never know if there's any merit to my method. My hope is any such changes are gradual enough that they will still leave this method usable, but again that will have to be proven or not as the case may be.

So, putting asides the question of why I want or even need to do this, I'm not seeing any good reason it couldn't work in principle.

I need to see what's going on with the OCO project on Kickstarter too... if that works as advertised, all this messing around with pH probes is pointless :) I didn't sign up for it, but know others who have so will wait to see how that turns out.
 
The pH probe in the drop checker does work in principle but it still only works as quick as the drop checker.

Once a reproducible consumer method for near realtime co2 measurement hits the market then it's probably only a small step to produce a method for controlling it automatically, then we can all holiday without fear of gassing fish.

:)
 
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