What is the best HOCL (as ppm CL2) number to aim for

[chem geek]

You should ask questions regarding the PoolEquations spreadsheet in The China Shop (and this thread should be moved there) as it is not for general users.

The correspondence between the various columns in Ben's Best Guess CYA chart and in my chart (at TFP) compared to HOCl concentration (and approximate FC as % of CYA) are as follows:

Minimum FC column corresponds to an HOCl of: 0.03 ppm (7.5%)
Target FC column corresponds to an HOCl of: 0.05 ppm (11.5%)
Maximum FC column corresponds to an HOCl of: 0.07 ppm (15%)
Yellow/Mustard Min FC column corresponds to an HOCl of: 0.07 ppm (15%)
Shock FC column corresponds to an HOCl of: 0.3 ppm (40%)
Yellow/Mustard Shock FC column corresponds to an HOCl of: 0.7 ppm (60%)

The correspondence with Ben's Shock FC column is only approximate -- his chart has a higher FC at lower CYA and a lower FC at higher CYA than based on HOCl concentration.

Richard

[smallpooldad]

chem geek,

My apologies I will go to China Shop if a question of this sort arises again and now request that this post be moved to the China Shop as I do not know how to facilitate this operation.

Your answer was clear and is very helpful with balancing the water especially to combat marine algaes found here in Hawaii.

Aloha

[Water_man]

I know that curiosity killed the cat, but I wonder, since the goal is to combat algae, why would you care about HOCL levels, if all the charts, the calculations, and the tests are based on FC?

[chem geek]

The easiest way to look at it is that the industry has never developed the appropriate chemical test to determine the true amount of disinfecting and oxidizing chlorine (that is, hypochlorous acid). The closest tests are expensive sensors with selective membranes that measure hypochlorous acid level and ORP sensors that are a rough proxy for it (though they vary by sensor manufacturer).

The Free Chlorine (FC) level is a simple chemical test, but it really just measures the total reserve of chlorine so will tell you whether you have enough total quantity to not run out if there is a certain rate of consumption. It does directly not tell you the rate of disinfection, algae kill, or oxidation.

Fortunately, the chemistry of chlorine, Cyanuric Acid, and their combinations is well understood (has been since at least 1974) so one can use the FC/CYA ratio (or FC percentage of CYA) as a reasonable estimate of the hypochlorous acid concentration. Though this ratio relationship varies somewhat with pH, it only varies by about 25% between a pH of 7.2 and 7.8.

Richard

[Water_man]

Granted the hypo-acid is the active species that actually does the job, but as you said, it's not simple to measure it. Do you think the original poster has a viable means to measure HA level? If not, what's the use? It won't have a significant practical implication and it can only cause confusion when using the "regular" FC charts, tests and calculations.
Just my 2 Cs.

[chem geek]

He was just using my spreadsheet where you enter in the pH, FC, CYA and other parameters and I calculate the HOCl concentration. He isn't measuring it directly. He just wants to know how the spreadsheet calculated HOCl concentration relates to the tables of FC and CYA that Ben and I did (I based mine roughly on Ben's, but made it consistent in HOCl concentration for each column).

[smallpooldad]

Yes what chem geek described in the last post was exactly what I was trying to acheive.

The reason being that even though it is recommended that the Pool Pilot Total Digital SC-60 be mainted at 3 ppm measured chlorine I have found that to be too low as small spots of yellow algae start to form.

If I up the ORP to 700 it maintains around 5.5 FC at a pH of 7.4 and Alk at 70 (to reduce acid usage), calcium is about 580, borax 50, and cYA 35, salt 3000. This gives a calculated HOCL of about 0.075. 10,000 gals and pump runs 3.5 hours per day, from noon to 3:30 pm at 55 GPH (too reduce energy cost) as KWH is $0.27 in Hawaii and going up steadily.

This gives a algae free pool it might be that being at a lower altitude algae develops faster as the UV index is stronger.

It seems that having a bigger unit such as the SC-60 allows for stronger chlorine production and thereby allows lower run times, in Hawaii that is. It might be that different parts of the country can get away with less powerful units. I would be interested in any comments.

So far I have been very happy with the Pool Pilot Total Digital performance 10 out of 10, manual is not that clear or well written but adequate with calls to tech support 5 out 10, tech support is weak at level one they seem to be robotically answering questions and quickly become frustrated if one gets at all technical. At higher levels it gets better and if you drive them crazy enough you will finally reach the top level support which is excellent albeit that it can take you a frustrating 30 minutes of back and forward "that is not what I am saying and I need to know", 3 out of 10 at lower levels and 9 out of 10 at the top level ( one mark off for sounding a bit irritated that they had to answer the call).