Free chlorine

[slepax]

Hello,

I have just received my Taylor kit a few days ago and ran some tests. Now I did the test at night so I only had artificial colour, which may explain why some of the results looked a bit off to me (TA and CH), I will repeat the test again when there's still daylight but for now this is what I have:

CYA   70
FC    8
CC    0.5
TA    80 (this looked more pinkish red)
CH    230 (this looked more purplish)
ph    7.4 - 7.6

What interests me most is the FC being too high, should be half of that based on everything I've read so far. Also I understand that the TA and CH results look wrong because of the high FC value.

My SWG is running at 75% for 8 hours, and based on the specs of my SWG (thanks aussieta) at maximum capacity it can generate 18gr an hour, so at 75% it generates 108 grams of chlorine a day. Obviously I can just lower the SWG settings or reduce pump run time, but before I do that I was wondering if there is any way to back calculate the expected FC based on my SWG settings, just so I can get some validity in the results?

Many thanks!

 

[Isaac-1]

No, since FC is lost to sunlight as well as organics it is not a constant and can build up over time if your SWG is set too high, etc. For sanity checking get a cheap OTO drop based color matching chlorine test kit (the one that shows chlorine as shades of yellow), it will read in TC not FC and is only accurate up to 5 ppm, 8 ppm will be sort of bright yellow, which would turn to more burnt orange then brown at even higher levels.

Ike
ps for extended idea on OTO colors see
http://www.troublefreepool.com/threads/58831-Extended-OTO-color-comparator

 

[slepax]

Thanks Ike. I also have this test kit, when I used it even before I received the Taylor kit the colour looked very dark.

I am not chem expert, but can I assume that any chlorine generated by the SWG is free chlorine?

If I may turn this into a hypothetical question, so regardless of sunlight and other organics. Imagine a situation where the pool has no chlorine at all and then the SWG generates 108gr of chlorine instantly, is there any way of converting this amount to FC ppm?

 

[jblizzle]

PoolMath will do that in the effects of adding chemicals section. You select chlorine gas and input the amount added.

 

[chem geek]

If the SWG generated 108 grams of chlorine instantly in 27,700 liters, then that would be 108*1000/27700 = 3.9 mg/L FC (ppm is same as mg/L). The grams chlorine units given in SWG production is the same chlorine unit used for chlorine measurement in test kits (technically, it's ppm Cl₂, or equivalent weight concentration of chlorine gas). This is why your chlorine level is too high because a more usual daily chlorine usage in the peak season, which is January for you, is usually around 2 ppm FC per day. So you are generating about 2 ppm FC more chlorine per day than is needed. What happens is that this builds up the FC level higher such that this production equals the loss from sunlight which proportional to the FC level. So while you might only lose 2 ppm FC if the FC were at 4 ppm, you lose closer to 4 ppm FC with the FC is at 8 ppm (with the same CYA level in both cases).

So you can probably turn down your SWG on-time roughly in half, so why don't you try 40% and see how that goes. The FC should slowly drop and settle in at around 4 ppm FC. If it goes lower, then bump up the on-time a bit (say, to 50%).

 

[slepax]

So while you might only lose 2 ppm FC if the FC were at 4 ppm, you lose closer to 4 ppm FC with the FC is at 8 ppm (with the same CYA level in both cases).

Thanks for the detailed answer! Most likely I will have to read it a few more times to digest it completely

Can we take one step backward .. from reading various posts on this website my understanding was that FC being 5% of CYA will be retained and the rest lost (this is assuming just UV, if we include organic matter that should result with more loss). So considering my CYA levels, I should have been left with 3.5 ppm FC at the end of the day which is when I took the test, now although the SWG had been running for ~2 hours before I took the test, FC should have been in the 4-5 ppm range.

Am I missing anything here?

I will turn down the SWG for sure, but I first want to get the numbers sorted in my head, just for the sake of learning and complete understanding.

 

[jblizzle]

At a given CYA level, you will lose a % of the FC every day just due to the sun. The higher the CYA, the lower the % FC loss. If you are not adding any FC each day, then the FC will drop by a similar % each day until it is all gone.

I am not sure where you got the 5% number I think at SWG CYA levels the % FC lost daily is closer to 30-50%.

 

[Isaac-1]

That test kit you linked to appears to be a DPD (shades of pink) chlorine test, the DPD test does have some advantages over the cheaper OTO (shades of yellow) test I referred to in some cases being easier to read and distinguish between levels within its range, however it makes a less ideal sanity check as the DPD color can get bleached out at high FC levels potentially making high FC look like NO FC. By comparison the OTO test will just get darker and darker at any potential pool FC level, I am not sure what it would do with straight bleach, but will try it tonight and find out.

Ike

 

[jblizzle]

If the FC gets high enough the OTO ends up having brown floating particles ... but that is still well below bleach level ppm.

 

[slepax]

So I have lowered the SWG from 75% to 50% in the morning. The pool is never covered during the day, so plenty of sun/UV. I checked again tonight and it still came up as 8 ppm (7.5 but that's almost the same).

If daily FC loss is 30-50%, wouldn't that mean I should have had my FC closer to 6 than 8?

 

[chem geek]

At 70 ppm CYA the daily FC loss is probably around 30% or so but it depends on other factors in addition to sunlight such as bather load and pollen and leaves blown into the pool and water temperature. The FC isn't going to come down very fast. Give it another day or two to see if it drops. You may need to lower the SWG on-time even more. If you are testing daily, you could lower the on-time to 30% and just test to make sure it doesn't drop too low. You'll want your FC to stay at least at 3.5 ppm FC given your 70 ppm CYA, but you can just target 4 ppm FC to keep things simple.

If your pool had reached steady-state at 75% on-time, then the SWG was adding 3.9 ppm FC every day and that was how much you were losing every day at 8 ppm FC, but I don't think you were at steady-state and that the FC may still have been on its way climbing higher. If it was truly at steady-state, then the loss was 3.9/8 = 49% which is higher than normally seen at a CYA of 70 ppm though not unheard of.