hoppycalif

I have been thinking about the electronic version - the "requirements" for the solution in it are much different from that of the indicator solution version. The latter needs water with a KH of 4-5 dKH so that the color will be green at the CO2 ppm that is desired. But, the electronic one doesn't have that need, so for that one it is best to set the KH to where the pH will be about 7 at the desired ppm of CO2, and that is at about 13 dKH. (Because most pH probes are calibrated to be right on target at 7.01 pH.) I don't understand how a pH probe works well enough to be able to figure out why your reading seems to be off. My pH probe has always been so erratic that I don't trust it at all, and it is too bulky to be used for a drop checker anyway. I just wasted my money when I bought it.

dennis

I'm not sure about that. A kH of 13 woudl mean you inject lots of CO2 before getting a pH change. That means you would OD you tank on CO2. I think better would be to use a very low kH so that the pH change happens fast and easily, allowing the controller to be more in control. I do not think it matters if the probe reading is at 7 or at 5, it will still be as accurate as the probes accuracy. What I mean is, I do not think that having a lower pH as the setpoint will compound any probe inaccuracies. Therefore, a lower kH will mean a faster response time... I think.

hoppycalif

With a 13 dKH water reference you get:
pH - 6.6 = 100 ppm CO2
6.7 = 80
6.8 = 60
6.9 = 50
7.0 = 40
7.1 = 30
7.2 = 25
7.3 = 20
7.4 = 15
7.5 = 10

The amount of CO2 introduced into the tank wouldn't be affected by the KH of the drop checker water, nor would the amount of CO2 to be absorbed by the drop checker be affected. The goal should be to make the KH be such that around the ppm of CO2 you want, the changes in pH per change in ppm of CO2 are a maximum. At a KH of 1dKH you get:

pH - 5.5 = 95 ppm CO2
5.6 = 75
5.7 = 60
5.8 = 50
5.9 = 40
6.0 = 30
6.1 = 25
6.2 = 20
6.3 = 15
6.4 = 10

So, I conclude that the only effect of KH is to shift the pH range that is of interest, and it seems to me that the closer that is to 7.01, where you do a one point calibration, the more accurate the pH probe/meter is. (All of the above ppm values are rounded off to the nearest 5 ppm)

I also manipulated the equation for ppm vs KH and pH to find at what KH the change in pH vs ppm is a maximum - and, as the tables above show, the change in pH vs ppm is not dependent upon KH.

wiste

How long do you leave the drop checker in the tank? I understand there is a minimum time that you have to leave it in the tank but I am curious to know if you leave it in the tank over a long period, days or even weeks and use it like a thermometer.

epicfish

Yes, it's held in there with a suction cup indefinately so that you can always glance over at it and know approximately what your CO2 levels are.

hoppycalif

Yesterday I learned that Dennerle makes this device, complete with reference solution with indicator reagent in it. They call it "CO2 Long Term Test Correct" and it is made of plastic. The reference solution (distilled water with bicarbonate in it?) has a KH of about 3dKH, to give a green indication at 20-25ppm of CO2 in the water. The configuration looks to be about that of the teardrop shaped glass one that is sold on ebay. So far I haven't seen this device for sale in the US, but it is in the UK and Europe.

Based on Dennerle's instructions, the solution in the device needs to be replaced about once a month because the indicator, which is a dye, will fade with exposure to light over that period of time.

I suppose in a few weeks someone will point out that Leonardo da Vinci first invented this device back a few years ago!

wiste

Try a web search for ‘Dennerle CO2 Visual Indicator’. If this is the same product it is available.

hoppycalif

Yes, that is it! For some reason I couldn't find those suppliers when I googled it yesterday. $17 plus shipping - not too bad unless the shipping cost is exhorbitant.

hoppycalif

Here is a simple way to DIY this, using clear acrylic tube in 1 inch diameter and 1/2 inch diameter, plus two acrylic discs 7/8" in diameter and one such disc 3/8" in diameter. Tap Plastics has all of these shapes.



The advantages are:
The air gap has a maximum area and minimum length to decrease the response time.
The tube of indicator solution is small enough to take very little solution, but is easily viewed in the aquarium.
It can be mounted in the tank with a "heater holder" suction cup device made for a 1" diameter tube. The clip can be easily cemented to the device.
It is small enough not to be overly obtrusive in the tank.

But, the disadvantage is that it takes a syringe with a bent needle to squirt in the solution.

I haven't made this yet, but will do so shortly.

Laith

hhmmm... if one could add threads or a perhaps an o-ring to make a tight fit the top part of the unit could perhaps be removable?