creighton;23306 said:
Vaughn,
Is their anything that could be used as an indicator with a faster reaction time, or is it best just to stick to the eyeball method? The whole problem with the pH controller method it that the water's hardness changes as the plants uptake nutrients and the KH:CO2 relationship will be off?
I don't really know, but somebody's probally done something that works.
The processes that make a drop checker work are relatively slow: CO2 diffuses from the tank water to the air gap, CO2 diffuses from the air gap to the drop checker water, CO2 diffuses from the drop checker water to the air gap, CO2 diffuses from the air gap to the tank water: an iterative process that takes time to reach an equilibrium.
There are two problems with the pH controller used as a CO2 concentration controller: First, the pH of the tank water is affected by more things than CO2, so the relationship between pH, KH and ppm CO2 is not the simple one the tables show. Second, the tank water pH at a given concentration of CO2 in the water is not the same as the tank water changes its chemistry, due to tannins, KH changes, and other changes such as the presence of some phosphates added by the water company to raise the pH.
If you want to use the pH controller to automate the CO2 bubble rate I suggest first adjusting the bubble rate so the ppm of CO2 in the tank, as indicated by a drop checker, is stable at the ppm you want. Then read the pH controller indication of tank water pH, and set the controller to control to that value. If you want a faster rise in CO2 concentration, adjust the bubble rate to a slightly higher rate. Then monitor the drop checker color occasionally to be sure this is working as you want it to.
Always present is the problem that we cannot accurately measure the ppm of CO2 in the tank water. First is the obvious difficulty of judging the color of the drop checker water. That leads to a possible +/- .2 pH error, which causes about a +/-60% error in CO2 ppm! If we use a good pH meter instead of judging the drop checker water color we are still likely to only be able to measure pH only to a +/-.1 pH accuracy, which causes a +/-25% error. Don't ever believe someone who tells you they have 23 ppm of CO2!
A faster drop checker reaction time is possible, but only by getting rid of the air gap and greatly reducing the amount of water in the drop checker. This means using a gas permeable membrane to separate the drop checker water from the tank water. The difficulty of judging the drop checker water color is much, much greater as the amount of water gets smaller, so the improvement that can be made isn't great. To get a big improvement in response time requires using a small diameter pH probe to measure the drop checker pH, and a gas permeable membrane to eliminate the air gap. This is possible to make, but isn't something an average planted tank keeper can easily do, plus being expensive. (Most pH probes seem to be about 1/2" in diameter, with a water "depth" of about 1/4" being needed to cover the sensor bulb. As those dimensions get smaller the cost goes up rapidly.)