CYA/Chlorine Chart vs Pool Calculator

[frogabog]

Split from shocking-and-fc-t38274.html Butterfly


Yet another case where the pool calculator and the chlorine/cya chart disagree. Yet now that I look closely it's a 6ppm discrepancy. Which is significant. At 40ppm CYA it's 2ppm off but at 60ppm CYA it's 6ppm off.

For some, the pool calculator values work fine for shocking. For others such as the OP here it doesn't. My question is (and this should probably be asked in a different thread...) how to know when to use the chart instead of the calculator's values? And, why is there such a huge difference in shock levels? If 1ppm below shock level can make or break a pool, 6ppm discrepancy seems like too much to say use one or the other, but the chart is more aggressive. It's a LOT more aggressive as CYA level goes up. Any plans for a happy medium between the two, or perhaps merging one with the other so there's less confusion?

 

[chem geek]

The shock levels in The Pool Calculator come from Ben's Best Guess Chart from The PoolForum/PoolSolutions. That chart was from his experience whereas this chart I made was designed to have the same active chlorine level in each column. The Pool School Chlorine / CYA Chart is based on my chart.

When it comes to shocking, higher FC/CYA ratios are faster. In most cases, any FC/CYA ratio above 20% will likely kill algae quite a bit faster than it can reproduce, but if one has a significant bloom then the recommended 40% is better since you have to have the chlorine get into all the algae clumps to kill off algae everywhere, not just at the surface of such clumps. Also, when there is a lot of algae, the chlorine level drops rather quickly so starting off higher makes it less likely to get low enough for algae to grow too fast. There is no one magic number for shocking and the charts are a rough guide where higher is faster. The higher yellow/mustard algae levels are not only because that algae is harder to kill, but the higher shock levels make it more likely for at least some chlorine to get into harder areas and to kill off the most resistant members of the population.