Before going ahead with my plan, I'd like to present it here to see if anyone sees any obvious flaws in my design or reasoning.
I'd like to dose Carbonate when the pH drops below 8.2 and Bicarbonate when it is equal to, or greater than 8.2. (Actually, the Bicarbonate will contain a small amount of Carbonate to avoid the temporary pH drop that occurs when dosing 100% Bicarbonate. This ratio is about 1 : 4.1 (Carb:Bicarb) when the pH is at 8.2 (verified by @JimWelsh). So there will be two reservoirs, one for Carb. and one for Bicarb., each controlled by separate dosers.
EDIT: @Randy Holmes-Farley just informed me that the initial pH drop from bicarbonate is only about 0.06 for an addition of 1.4 dKH, so this step is probably not essential.....(but I'm thinking, probably can't hurt either now that I know how to do it).
Since the dosing will be controlled by a (GHL) Timer (24 doses/day) the two solutions must yield equal dKH because the Controller function will only select which reservoir (Carb or Bicarb) that will be dosed, based on the pH. The same volume will be dosed because that amount is preset in the Timer so it is important that x mls of each buffer supplies the same amount of dKH.
For anyone interested in making the two solutions that will result in equal dKH, you would use 190.5 gm Soda Ash and fill to 1 gallon; and, 302 gm Bicarbonate and fill to 1 gallon. (also confirmed by Jim Welsh). (This relationship is also nice to know even if you just want to use one reservoir, and not the system I am describing. You could pour unequal amounts into the one reservoir without altering the dKH yield.....e.g. if your pH is lower than you want, you could just pour a random amount of carbonate in the reservoir without having to worry about altering the dKH dosage.)
I will begin by randomly selecting the nominal value of pH-1 = 8.2. Programable Logic would be used as follows:
P.L.-1 "AND" Function with Inputs 1 (Timer-1) and Input 2 (pH Downward) and would control Doser #1 (Carbonate). (Note: I have to review this and make sure it shouldn't be "pH Upward". When the pH DROPS to 8.2, I want this Doser #1 to activate.)
P.L. 2 "AND" Function with Inputs 1 (Timer-1) and Input 2 (pH Downward-INVERTED) and control Doser #2 (Bicarbonate). (This hasn't been tested yet so, again, I have to check and make sure these are the correct settings).
I'm choosing to use {"pH Downward" and "pH Downward-INVERTED"} instead of {"pH Downward" and "pH Upward"} because I think this will avoid the hysteresis interference that I would get if I used the latter. (I believe there would be no reaction at all over a narrow range around pH 8.2 if I set it up that way).
So, does any body see any problem with:
1) The concept of using Carbonate when the pH is "low" and Bicarbonate when it is "not low"? The outcome will most likely be less of a daily pH swing. I guess the big question is, "Is there any benefit to that?"
2) The actual programming that I described?
Thanks for any ideas.
Jim
I'd like to dose Carbonate when the pH drops below 8.2 and Bicarbonate when it is equal to, or greater than 8.2. (Actually, the Bicarbonate will contain a small amount of Carbonate to avoid the temporary pH drop that occurs when dosing 100% Bicarbonate. This ratio is about 1 : 4.1 (Carb:Bicarb) when the pH is at 8.2 (verified by @JimWelsh). So there will be two reservoirs, one for Carb. and one for Bicarb., each controlled by separate dosers.
EDIT: @Randy Holmes-Farley just informed me that the initial pH drop from bicarbonate is only about 0.06 for an addition of 1.4 dKH, so this step is probably not essential.....(but I'm thinking, probably can't hurt either now that I know how to do it).
Since the dosing will be controlled by a (GHL) Timer (24 doses/day) the two solutions must yield equal dKH because the Controller function will only select which reservoir (Carb or Bicarb) that will be dosed, based on the pH. The same volume will be dosed because that amount is preset in the Timer so it is important that x mls of each buffer supplies the same amount of dKH.
For anyone interested in making the two solutions that will result in equal dKH, you would use 190.5 gm Soda Ash and fill to 1 gallon; and, 302 gm Bicarbonate and fill to 1 gallon. (also confirmed by Jim Welsh). (This relationship is also nice to know even if you just want to use one reservoir, and not the system I am describing. You could pour unequal amounts into the one reservoir without altering the dKH yield.....e.g. if your pH is lower than you want, you could just pour a random amount of carbonate in the reservoir without having to worry about altering the dKH dosage.)
I will begin by randomly selecting the nominal value of pH-1 = 8.2. Programable Logic would be used as follows:
P.L.-1 "AND" Function with Inputs 1 (Timer-1) and Input 2 (pH Downward) and would control Doser #1 (Carbonate). (Note: I have to review this and make sure it shouldn't be "pH Upward". When the pH DROPS to 8.2, I want this Doser #1 to activate.)
P.L. 2 "AND" Function with Inputs 1 (Timer-1) and Input 2 (pH Downward-INVERTED) and control Doser #2 (Bicarbonate). (This hasn't been tested yet so, again, I have to check and make sure these are the correct settings).
I'm choosing to use {"pH Downward" and "pH Downward-INVERTED"} instead of {"pH Downward" and "pH Upward"} because I think this will avoid the hysteresis interference that I would get if I used the latter. (I believe there would be no reaction at all over a narrow range around pH 8.2 if I set it up that way).
So, does any body see any problem with:
1) The concept of using Carbonate when the pH is "low" and Bicarbonate when it is "not low"? The outcome will most likely be less of a daily pH swing. I guess the big question is, "Is there any benefit to that?"
2) The actual programming that I described?
Thanks for any ideas.
Jim
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