I have a 1,893 L (500 US Gal) tub. Over time, I have gotten to know how much chlorine my tub needs with regular usage so I just keep a set of plastic measuring cups (1/2 cup, 1/3 cup etc. size) handy beside the tub so when I get out of the tub I pour out roughly what I need into one of those cups, toss it in the tub, circulate a bit and call it good. On days I don't use the tub, I just go out and throw in a different amount which I know from experience is approximately what's usually needed to get the FC level up to 6 or 7 ppm on non use-days and again, call it good. Once in a while, I do a test to see what my FC levels are at just to make sure I'm keeping it in the good range and it's almost always is in the 3 - 6 ppm range where I want it to be. If I need to be more precise with my measurements, I have a set of graduated cylinders on hand to precisely measure out liquid chlorine or muriatic acid doses.
More interestingly, I've been working on a system where I use a speed controllable dose pump to inject chlorine directly into the circulation/heater line just downstream of the heater at the point where the ozonator used to inject its ozone into the system. The dose pump is controlled by a WiFi controlled Sonoff switch that I can program to run for a set number of seconds and then automatically shut off. By varying the length of time I program the WiFi switch to run the dose pump and by varying the speed at which the dose pump runs, I can fine-tune the system so it will inject enough chlorine to raise the tub's FC level by 2 ppm each time I cycle the dose pump. The Sonoff switch has an app that allows me to control it from my mobile devices so when the system is operational, I'll simply have to press a button on my cell phone or tablet screen to inject enough Chlorine to raise the tub's FC level by 2 ppm. If I want to raise it by 4 or 6 ppm I just need to cycle the dose pump another time or two.
The Sonoff switch unit I'm using actually has 4 separate switches that I can control from the app so I'm using the other two to control the two jet pumps by sending a 12V control signal to the pump relays. The app will be programmed so that when I push a button on the screen, one of the pumps will be turned on for 3 minutes (or however long I choose) and then automatically shut off. Pushing another button will do the same thing with the second jet pump.
The beauty of this system is that I'll be able to operate it from anywhere I have cell phone reception. When I want to dose the tub with some chlorine, I'll just open the app and push the dose pump button however many times I need to (e.g. twice to raise FC by 4 ppm). Then, after a few minutes I'll just have to push the two jet pump buttons once each so they'll run and circulate the newly injected chlorine for 3 minutes and then turn off.
