Hi, another new pool owner. I ordered the TF100 test kit and tried it out today on CYA. I agree with other posters that it is very difficult to tell when the dot disappears. I put in the recommended amount of CYA to bring my freshly filled pool up to a CYA level of 40. In doing the test the dot mostly disappears at around 40, but really if you look very carefully you can see it ever so slightly even when the tube is completely full. I'm not sure how this subjective decision as to when the dot disappears is any more accurate than the subjective decision as to what color the test strips are. It appears that different lighting levels greatly affect the results, as does asking different people to read it. My wife sees it disappear completely at around 40, yet if she hands it to me I can see the dot.
I wonder if someone could make a little blacked out tube with an LED in the bottom, and a light sensor in the cap that would give precise light transmission readings based on how occluded the solution is. This could be calibrated before testing to adjust for differences in LED light output due to battery drain or aging of the electronics by using a calibration solution that has a known occlusion level (either premixed or mixed by the consumer as needed). I know this would take some work, but these days programmable microchips would make it pretty cheap to produce. Anyone here have more free time than me?
Am I putting too much weight on this test? Is it really not that important? Should I shoot for a higher CYA level just to be on the safe side?
In case someone wants to double check my math on the CYA I added, I added 2lb of dry 96% pure CYA to 6300 gallons of fresh well water.
Thanks for any advice.