I manage multiple commercial pools (30k-220k gallons) and have been adding 50lb bags of calcium flakes to balance calcium hardness. After adding calcium, the chlorine level drops to 0ppm and takes days to recover to 1-3ppm. The rest of the pool chemistry is balanced: pH 7.4, alkalinity 80-100ppm, cyanuric acid 50-70ppm.
Any ideas on why calcium is affecting the chlorine like this? Looking for suggestions or solutions.
@AquaAdventurer What other reason could there be? It seems like TFP doesn’t know, so they just make things up. I’ve seen them do that before. I guess they’re doing it again.
@CurrentCraze I’ll support your point. The issue is chemical because everything has a reason. There is a reason, but it may not have been explained clearly, so TFP just shrugs it off. That’s fine; they’re skilled at what they do, but they aren’t chemists.
If I had to guess, a few things might cause this. First, calcium is usually added using calcium chloride, which takes time to dissolve. If you test the free chlorine (FC) right after adding it, you might see a lot of cloudiness, which can confuse electronic and photometric testers.
Second, calcium chloride has chlorine in it. In water, it breaks down into one calcium ion and two chloride ions. Those chloride ions create hypochlorous acid, just like other chlorine sources. Adding 50 pounds of dry chlorine will likely raise your chlorine levels for a while, which can affect your FC test. There might be “hot spots” in the pool until it fully dissolves and mixes. High concentrations can interfere with test strips, mess up electronic testers, and give wrong results with other test kits. This chemical also impacts temperature (it releases heat as it dissolves) and pH.
Lastly, and I think this is the least likely reason, the powder could have its own organic material that uses up chlorine locally. It would take a lot for that to happen, and you would still get a high total chlorine (TC) reading even if the FC is low. That seems unlikely.
More likely, the problem is that you’re testing before the calcium chloride has had enough time to dissolve and mix well. The extra chlorine is reacting with the DPD in your test kit, using it up in a way that prevents the color change from happening.
@AquaAdventurer In the first link, it says, “There isn’t a chemical reason for this, but it is happening.” Then, it offers a possible chemical explanation:
@CurrentCraze Are you a chemist? Can you explain it?
Read through. Pure calcium chloride shouldn’t lower the free chlorine (FC) levels. Still, since many people notice this happening, they think an anti-caking agent or some extra ingredient in the calcium increaser might be the cause.
Other than criticizing TFP, what does your post contribute to this discussion?