Kalk for my ATO, with high PH as a shut off safety, and using a peristaltic pump that’s only capable of delivering slightly more than my daily evaporation volume as a secondary safety.
For many years I ran a kalk reactor with once daily stirring. Recently I (followed Randy’s long standing advice) changed to a barrel of settled kalk instead of the reactor. (Barrel gets topped off and stirred about every 2-3 weeks).
I found that the barrel method is far more stable in terms of potency. With the reactor I was seeing a constant up and down in potency because the constant (daily) stirring keeps it in over saturated mode for at least some of the time, and that is very sensitive to how frequently I would add kalk powder. Mixed in the barrel, kalk settles, then stays pretty much the same until the kalk mix is nearly depleted. The reactor created a constant up and down in potency I would then have to correct for.
The only negative about the barrel method is that the kalk solution goes through the dosing pump (vs pushing RODI into the reactor, and then that pushing the Kalk solution out of the reactor), so I have to watch for clogging.