I've been lurking for a while, enjoying the common sense approach espoused in this forum. With a background in academic chemistry until my health collapsed in 1993 due to chemical exposure from working in an organic lab with defective fume extraction, I've particularly enjoyed some of the deeper threads here.
It is probably little surprise that with chemical exposure related health problems and an indoor pool, I won't add chemicals to the pool without wearing a 3M respirator with ABEK1 (organic gas, inorganic gas, acid gas and ammonia) filters stacked with P3 particulate filters. Obviously, my interest is really in the inorganic gas, acid gas and particulate protection when working with the pool. I encourage everyone to consider respiratory protection when working with pool chemicals - modern half masks such as the 3M 7500 series are pretty comfortable and inexpensive, though non-Europeans will have different coding for the filters as the ABEKP system is a European standard. The mask really does make a difference - you notice the smell from the recently treated pool when you take the mask off!
I have a small indoor fibreglass pool with a counter-current swim unit - there's some details of the equipment in my signature. This pool is kept very warm (nearly 31oC - that's 87.5oF) due to the arthritis of one of the users, also the water volume is very small for a swimming pool at 9000 litres (2400 US gallons). When not in use, the pool is kept covered to keep evaporation under control and to minimise heat loss, which are both especially important due to the eye-watering electricity prices in the UK. Though the pool is heated using supposedly cheap off-peak electricity, it still costs us well over ┬ú1000 a year in electricity bills, making an air source heat pump a likely upgrade during 2012. You can see a picture of the installation on the manufacturer's data sheet for the pool shell (click) - ours is the pool pictured at the bottom right, though that picture was taken very shortly after installation when the plaster on the conservatory walls hadn't been painted and the protective tape is still on the plastic skirting board.
Because of the high temperature and small water volume, in some respects the pool is more akin to a spa when it comes to thinking about water chemistry, despite being used for serious exercise. The key differences I believe there are compared to the outdoor pools more commonly discussed at TFP are:
- limited opportunity to outgas because the pool is usually covered[/*:m:2f9ayivs]
- no UV destruction of free chlorine[/*:m:2f9ayivs]
- significant agitation from the counter-current unit (though the pump can turn the entire pool volume over in about 45 minutes - a 0.33HP unit might be a better match to the pool size and save power)[/*:m:2f9ayivs]
- lots of aeration when the air injector is enabled on the counter-current unit[/*:m:2f9ayivs]
- the small volume means that an exercising bather represents a substantial bather load[/*:m:2f9ayivs]
As I have a colour vision defect (I can't visually read phenol red reliably, nor, for that matter, many of the other indicators used in quality test kits) and test 'gadgets' aimed at the residential market tend to be unreliable, I have a commercial colorimeter and associated chemistry from Palintest. The initial 'sticker shock' of such a setup is horrendous, but over time it doesn't work out too bad as the bulk reagents are reasonably priced, the shelf life is long (being tablet based) and you order each reagent individually. This means that I can test free chlorine, total chlorine and pH frequently, whilst only testing the other parameters every few weeks.
The test results below use the Palintest setup.
FC 1.64 ppm Cl2
TC 2.10 ppm Cl2
Alk 75 ppm CaCO3
CH 335 ppm CaCO3
Sulfate 180 ppm SO42-
Salt 4500 ppm NaCl
No bromine products, borates or cyanurates have ever been added to the pool. By way of confirmation, a cyanuric acid test has no visible cloudiness and reads as 1 ppm CYA in the colorimeter (which is merely a small zeroing error).
FC 0.13 ppm Cl2
TC 0.65 ppm Cl2
Alk 200 ppm CaCO3
CH 219 ppm CaCO3
Sulfate 114 ppm SO42-
Salt - I don't have an up-to-date test result, but it is going to be very low
From this I deduce:
- the pool pH is on the low side at present, as I try not to drop it below 7.2. This time, the pH dropped from 7.35 to 7.0 when using sodium thiosulfate following a recent chlorine shock. Because of the SWCG, the pH will soon drift up to 7.35 or 7.4 where it typically sits[/*:m:2f9ayivs]
- the combined chlorine level of 0.46 ppm is not unusual for a SWCG based pool with limited opportunity to outgas - if the combined chlorine level gets above 0.5 ppm and the persulfate level is almost 0, the pool gets shocked using potassium monopersulfate. The use of a chlorine shock is rare as the free chlorine level takes forever to drop afterwards, necessitating the use of sodium thiosulfate to bring the free chlorine level down. (N.B. this total chlorine test was carried out with Palintest's DPD┬*Oxystop reagent added, so there's no monopersulfate interference)[/*:m:2f9ayivs]
- the total alkalinity is fine as it is[/*:m:2f9ayivs]
- the calcium hardness is getting a little high, but with relatively hard fill water, it is going to be hard to lower the calcium hardness significantly, unless I import fill water or find a way to collect rainwater, add a touch of sodium hypochlorite bleach, and add that to the pool. The pool system is pretty much closed, with splashout and condensate from the dehumidifier all directed back into the pool[/*:m:2f9ayivs]
- the CSI is acceptable (-0.53 according to the pool calculator) - indicating the calcium hardness really isn't an issue unless it climbs much higher[/*:m:2f9ayivs]
- the sulfate level is fine - as the structure of the pool is fibreglass, the sulfate level isn't going to be a huge issue anyway. The only cement based products near the pool are the mixture the coping is bedded down on and the reinforced concrete under the rubber flooring. Again, with the relatively high sulfate level in the fill water, I have limited scope to reduce the current level. I only test the sulfate level infrequently in order to get an idea of how much water I'm turning over, bearing in mind that most of the chemicals I use add sulfate (pH control is with sodium bisulfate as hydrochloric acid isn't easy to get in the UK┬*and the amounts of concentrated hydrochloric acid I┬*would be adding would be tiny in any event, shock is with potassium monopersulfate, chlorine reduction after a chlorine shock is with sodium thiosulfate) and the filter needs very little backwashing[/*:m:2f9ayivs]
- the salt level is fine - though it's getting towards the bottom of the acceptable range for the current SWCG unit (4000-7000 ppm NaCl), the cell is leaking and the entire SWCG installation is likely to be replaced with a more modern unit that needs a lower chloride level (and uses polarity reversal so it won't require acid descaling several times a year), so I don't want to add salt to increase the level only to struggle to dilute it back down in a few months[/*:m:2f9ayivs]
So, in many ways, all is well with the pool.
However, I┬*would appreciate people's views on:
- Is there anything I┬*have written so far that appears to be wrong?[/*:m:2f9ayivs]
- Is a free chlorine level of ~1.75 ppm Cl2 really acceptable, bearing in mind that the pool, if unused, loses less than 0.1 ppm in 18 hours and I have never seen it lose more than 1.00 ppm on the heaviest used normal days? My fear is that on exceptional days and/or in some places within the water volume, the free chlorine level is dropping too low for safety. Should I┬*target a slightly higher figure instead, such as 2.50 ppm?[/*:m:2f9ayivs]
- Bearing in mind the small water volume and consequent heavy bather load whenever the pool is used, is it worth considering an ORP controlled SWCG when I┬*go shopping for the replacement unit? I know these units have significant drawbacks - keeping the ORP probe in good order may well turn out to be an expensive nuisance and potassium monopersulfate shock has to be used with extreme caution in an ORP controlled setup as it raises ORP but is not a sanitiser. However, there is the possibility of minimising the swings in free chlorine level using an ORP┬*controlled SWCG unit. (If only someone made an amperometric controlled SWCG, but I guess the expense would be prohibitive).[/*:m:2f9ayivs]
- Is it worth considering a small cyanurate level, say 20 ppm of cyanuric acid? It is very unusual to use cyanurates in an indoor pool in Europe, but it may be an answer to my concerns about free chlorine dropping too low.[/*:m:2f9ayivs]
- Does anyone want to suggest target values for my pool, bearing in mind the given pool and fill water test results. I am aware of the usual TFP values, but am unsure whether they are directly applicable to my small indoor pool.[/*:m:2f9ayivs]
The only chemicals I use are:
- salt - for the SWCG[/*:m:2f9ayivs]
- sodium bisulfate - to reduce pH when necessary[/*:m:2f9ayivs]
- sodium bicarbonate - to increase total alkalinity when necessary[/*:m:2f9ayivs]
- potassium monopersulfate - non-chlorine shock[/*:m:2f9ayivs]
- sodium thiosulfate - to reduce free chlorine levels following on the rare occasions that I use a chlorine shock[/*:m:2f9ayivs]
- United Chemical's Pool Stain Treat (in other words, oxalic acid) - control of metal staining[/*:m:2f9ayivs]
I may well switch to ascorbic acid once I've used up the oxalic acid. Overall, my strategy is to add as few chemicals as possible to the pool, and, with the exception of the Pool Stain Treat, I avoid proprietary products.
If anyone has any advice that does not directly relate to the questions I've asked, I'm listening!
My apologies for such a long post. I felt it was more useful to explain where I was and my questions in one structured post rather than a long meandering thread.
With best wishes,