Have seen a couple of references to "6-8 hours" to allow this test to run. Should I assume from this that an hourly free chlorine loss rate of 1/8 to 1/6 ppm per hour (or less) is a passing result?

As already said, we can't test with such a precision anyway, so there is no point in defining such criteria.

That aside, it's not that simple, you can't just divide the total loss by the number of hours to get the loss rate. You have to look at chlorine loss in the same way as calculating changes to a bank account due to interest rates.

Chemical reaction rates are proportional to the number of available reaction partners, that means that relative chlorine loss (∆FC/FC) is a better measure.

You could for example say that a certain UV intensity results in a chlorine loss rate of ∆FC/FC = 5% per hour (just an arbitrary number).

You start for example with FC 10ppm. After an hour you will have lost 0.5ppm and end up at 9.5ppm. In the next hour you will lose 0.475ppm and end up at 9.025ppm and so on. The FC after a time t can be calculated with the formula:

FC = FC

_{0} (1-p)

^{t}
With FC

_{0} the starting FC, p the relative loss per hour (e.g. p=5%=0.05) and t the time in hours.

This is already a bit academic, because the UV intensity is not constant throughout the day, and it will be a lot more beneficial for the pool owner to just work with a chlorine loss to UV per day, like ∆FC/FC = 20% over a whole day caused by the total UV load over that period. That number will change with cloud coverage on a daily basis, so you may even be more interested in a weekly average. Then this number changes with the seasons.

With algae it gets even more complex, because algae reproduces, you therefore have two competing rates, an algae reproduction rate and an algae kill rate. Only if the kill rate is faster then the reproduction rate you have a chance of winning the battle.

Let's say you have a swamp, and at normal target FC levels your reproduction rate is faster than the kill rate. Then, after an hour, you will have more algae than you started with, and therefore your hourly relative chlorine loss rate ∆FC/FC will speed up.

Then you have to consider all the chlorine losses due to chlorine oxidising things that are independent from algae. And draw a line somewhere. This line will of course be in a grey area. If you pass with flying colours with no or maybe just 0.5ppm loss, then you have a clear pass. If you fail with losing most of your FC then you have a clear fail. And inbetween? If in doubt, slam a bit longer and see if things improve.

I hope this will be enough to show the motivation behind TFP's simple 1ppm criterion.