Quote:
Originally posted by yardboy
Why wouldn't a controller prevent excess CO2 accumulation in the system?
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The controller only measures pH in the system, pH can be driven by other substances in the water column besides the concentration of CO2, as carbonate species, level of buffering, and even the addition of phosphate and extra calcium may adversely affect pH control in closed systems.
Another example of problems that may occur in systems where water column parameters are used to control the solenoid rather than reactor effluent: You own a home built since 1992 in the US. It is an excellent example of energy efficiency, high R value ceilings, walls, floors. You use a high SEER heat pump and your water heater runs off waste heat from the unit in the summer and gas in the winter. In addition to all these proactive mods to the home, you've made sure that the house is tight to drafts and exterior air, to the point where you've closed off the chimney and installed indoor gas logs. As a consequence of this, the pCO2 in your home climbs to near 500 (just from the "tightness" of the house alone), undetectable to the animal inhabitants in the home (including you), but strongly affecting the pH of the tank. As a consequence, the pH detected by your monitor and controller tells the solenoid to stay closed, and your alk and Ca suffer. This has nothing to do with the rate of CO2 delivery, obviously, but is totally due to the efficiency of the home in terms of atmospheric exchange at the surface of the tank and the affinity of SW for CO2 in general. The solenoid stays off due to low water column pH, but it is not acidic enough to affect resonable dissolution of the reactor media within the reactor chamber(s). Such a system would benefit tremendously from the use of outside air to the skimmer air intake to degas CO2 (outside air would have a lower pCO2 than the house and the skimmer has a much greater surface area for air exchange).
There are several other issues that could also skew the solenoid function that have nothing to do with the functioning of the pH controller, but the presence of the probe in the main water column leaves it open to biological growth and contamination as well, skewing the function of the probe as it looses sensitivity and calibration. Better to leave it in a (relatively) acidic, lightness, monospecific medium where growth is inhibited somewhat, although periodic check of the pH function with borate buffer is in your best long-term interests.
Although pH is related to pCO2 and the alkalinity of the tank, these are not the only factors that do, and using pH as the feedback only works if it is the only factor that would be affected by CO2 levels, and visa versa, as this is NOT a closed feedback system. If there were no other mechanisms that would affect the pH in the system besides the partial pressure of CO2 in the water column, then it would be the perfect means of controlling the rate of delivery for the reactor's CO2 and acidity. Unfortunately, this is not the case.
Sorry to drone on so, hth.
