Richard,

Thank you for your thoughtful analysis. You confirm my impression that this approach is also fraught with potential inaccuracies.

Of course the pragmatic answer is not to worry about the pool volume, but rather empirically build a data set by direct observation (which I have done), on the effect of adding different amounts of different chemicals . However I confess to now being entertained by the challenge of finding a better estimate of my pool volume for its own sake!

A few further thoughts and comments for your consideration:

As you suggest, it certainly makes sense when using proportions to represent ‘before’ and “after’ in the same units and also measure the concentartion ‘before’ and “after’ with the same test. The bleach I used was a supermarket's own brand (Shoprite), which is probably from the makers of Chlorox anyway, but presumably your point about it not being 6% still applies. At least for bleach it should be possible and probably be worth the effort to accurately measure the amount of bleach, and its strength with the FAS-DPD before it is added. I imagine this will involve taking a sample with insulin syringes and dilution etc.

I wonder if using another dilutional agent might provide more data to help triangulate this elusive pool volume estimate? Using calcium hypochlorite would seem to be a better choice than other possible compounds which are readily available, apprpriate to add to the pool and easily measurable, such as sodium bicarbonate, cyanuric acid, muriatic acid etc. What do you think?

I don’t know the selectivity and accuracy of Ben’s test for Ca but in theory it could offer more data....or more uncertainty!(assuming I could get Ca shock fully dissolved quickly). I’m also not sure how close the amounts of calcium hypochlorite represented on the package are to what is actually inside. Confirming this with measurment might be much harder with Ca shock before addition.

It looks like I need to go back and do a better estimte with dimensional analysis. Unfortunately i don't have the original architechural drawings of the pool to get an accurate surface area, but I think I could get a better idea using google earth to generate an accurate shape.

Finally I would be fascinated to see the results of your experiments with your own pool.

Thanks again for the stimulating response