How much chlorine is safe?

[roger888]

At what point during or after a shock process is it safe to use the pool again? Once the shock is complete, and FCL is coming back down from around 12-14 ppm, at what FCL level is it safe for the pool to be used?

[Théière]

This question is a good one, it depends on the cyanuric acid level (CYA) as too how much of the chlorine is free active chlorine.  That is to say 30 ppm CYA and 3ppm chlorine is roughly similar to 100 ppm CYA and 10 ppm free chlorine.

From my perspective I also want to see a low level of combined chlorine before letting people back into a pool.

 

[roger888]

Yes, I'm assuming by this point that TCL is equal to or only slightly above FCL, in other words that CCL is low or zero. So would there be any risk in using the pool at, say, FCL = 10, 6, 5?

[Théière]

To give an accurate answer I need to know your cyanuric acid level (CYA).

Indoor communal pools don't generally have CYA in them and frequently run at 1.5-2ppm chlorine, that is a lot more powerful than an outdoor pool with 10ppm chlorine and 30ppm of CYA by a factor of x4 or equivalent to a pool with 24ppm of chlorine and 60ppm CYA. Just don't drink the water [:)]

[roger888]

As of this morning, I have:

FCL: 10.76

TCL: 10.9

PH: 7.87

CYA: 35

[Théière]

There you go Roger, from my previous post 10ppm chlorine in 30ppm CYA (5ppm extra won't make much difference)  is safe as it's less active than an indoor pool at 0.6ppm chlorine and no CYA, still don't drink it though [:)]

EDIT: don't worry about your slighlty high Ph that should drop down into the normal range (7.2-7.6) as the chlorine lowers.

[roger888]

Thanks. Most helpful. For future reference can you give a simple rule of thumb relating FCL, TCL, CYA and active equivalence (assuming that you're not going to drink the stuff!)?

[NickP]

With all due respect to the incredibly well informed Théière and the Pool guy who 's advice myself; and I'm sure lots of others seek regularly and indeed follow, when I read these things it makes me wonder how I survived childhood ? Swimming in the streams and rivers of south London, the green slime covered swimming pool of the boy scouts camp at Sunbury on Thames, or the Wimbledon slipper baths (Latimer Road) where they used to put soda etc.into the baths, the chlorine in the swimming pool at Latimer Road was so strong that the smell almost made you gag. Happy days, but please keep the good advice coming guys my grandchildren are very precious. [B] [B]

[Théière]

Nick I know exactly where you are coming from, the old school trips to the baths when you could smell the chlorine byproducts a mile down the road and your eyes would sting as soon as you entered the building [+o(]

The thing is that is not chlorine, chlorine has remarkably little smell until it reacts with something. The eau de javel 9.6% has barely any smell in the bottle, try it. On the skin after swimming you can smell it for hours. The thing is the bad stuff actually eats the swimming pool buildings away, whole roofs have and do collapse every year through stress cracking caused by chlorine byproducts and through poor ventilation. 

What are these byproducts? cyanogen chloride, chloroform and trihalomethanes. It is reckoned that the poor air in UK swimming pools could be compared to a child smoking 1 cigarette every 2-3 minutes. So it is important and change albeit very slow will happen as the HSE have a duty to make the workplace safe for pool staff and that will also mean pool attendees.

How did we ever make it [:)]

[Théière]

[quote user="roger888"]Thanks. Most helpful. For future reference can you give a simple rule of thumb relating FCL, TCL, CYA and active equivalence (assuming that you're not going to drink the stuff!)?[/quote]

The combined chlorines should be 0.5 ppm or less.

The range I gave earlier should be sufficient as you can see there is considerable scope with that.

10ppm chlorine and 30ppm of CYA  OK

24ppm of chlorine and 60ppm CYA. OK

Roughly 40% of the CYA is ok

Just don't drink the water [:)]

[roger888]

Great. Keep CCL under 0.5ppm, FCL 40% CYA. Bingo. That's a good rule of thumb. Thanks.

[roger888]

Nick

I have some sympathy with where you're coming from. I'm from south London too, probably around the same age, and we all no doubt took risks we shouldn't have. Perhaps we were hardier then (a peck of dirt and all that).

But... there are too many instances of people catching bugs from badly-maintained pools and ending up in hospital. If you're maintaining a facility for family, friends or guests it's your responsibility to ensure it doesn't pose a risk to them.

Basically, it's just as simple and no more expensive to get it right, once you understand what you're doing. This is my second full year of pool owning, and I'm more than happy to acknowledge the invaluable information I've picked up from this board. Thanks again to all who contribute.

[Théière]

[quote user="roger888"]Great. Keep CCL under 0.5ppm, FCL 40% CYA. Bingo. That's a good rule of thumb. Thanks.[/quote]

That is for shock level only, your maintenance level should be 5% of CYA........That you could drink [;-)] but I prefer something more refreshing [:)]