Hi. I'm using a tablet-based comparator test kit, on a 13,500 US gall pool. My inital tests read FC 5, pH 8.2, CH 220, TA 100, CYA 90. I'm trying to get the pH and CYA into line.

I've dumped (and replaced) 12,500 litres of water and the CYA has fallen to around 75, so I'm on my way with that. But I can't get the pH to budge. I've tested my mains water (which is actually spring-fed) and the pH is 7.2 which I would expect. I've also added three doses of Sodium Bisulphate in 21 oz, 18oz and 18 oz amounts. Still the pH reading is 8.2.

My modest test kit only reads pH up to 8.2. Could it be that the pH was and is significantly higher and will need more PH- to get it down? How high can pH be, is a pH of over 9 possible in a pool? We haven't swum in the pool this year - still getting frosts - but we have never noticed any discomfort in previous years.

I notice from the Pool Calculator that 72 oz of PH- is required to lower pH from 9.2 to 8.2, so maybe I need to keep plugging on. I am worried because the pool store tested my pH at 7.8, but looking at the grubby state of their test kit I'm rather inclined to trust my tests more.

I've added four litres of 5% bleach, by the way. I'm coming off tri-chlor tabs. My FC hardly ever shifts either, but that's another story. And muriatic acid is unknown in the UK. Water is cloudy.

It's possible that your chlorine may be higher than you can test. 5ppm is the top-end for most kits. Higher chlorine levels interfere with the pH test and can give falsely high readings.

Muriatic Acid is hydrochloric acid, and may be called Spirits of Salt in the UK. Check with someone who knows before substituting though.

Good old Hydrochloric acid AKA Spirit of salts or Brick cleaner in builders merchants. Ask for the MSDS (material safety data sheet) to verify.

Your cyanuric acid is way too high for the UK so you may want to start by dumping 31,000 litres of water to bring the level down to around 40ppm then you will certainly lower the PH too and require far less chlorine to accomplish the correct levels.

Thanks everyone. John, I thought that a high chlorine level would bleach the pH reading, but mine is a resolute red, the shade of the 8.2 reading. I don't think that my FC level is more than 6-8 ppm (adding 4 litres of 5% bleach raises the FC 4 ppm).

I have plenty of sodium bisulphate, so I think I'll leave other acids alone. I have enough pairs of spotted jeans already.

Teapot (now that's a UK name) I am dumping/refilling in an attempt to get the cya down. I'm dumping 5,000 litres and refilling every two days. I know that's not as efficient as dumping half the pool in one go, but it is a liner, albeit a pegged liner, and our water is free.

The question remains: is a high pH (above 9 as my tests seem to indicate) often found in pools, or never? If never, then what I'm testing, or interpreting, is incorrect.

High PH can occur, but should be avoided if at all possible. The highest most people will see is around 8.4 to 8.5, but now and then it can go higher, especially if you add the wrong things to the pool.

Kes hi,
If your dumping 5000 litres a day I wouldn't worry too much about the PH for now as the supply water will be around 7 so wait until you have reduced the CYA before spending money adjusting the PH.

Sodium hypochlorite has a PH of 11-14 depending on the concentration and it's worth remembering the PH scale is logarithmic, that is to say each 1 PH move on the scale is 10x the prevoius so Pool schools' Pool calculator is very handy indeed.

Good luck with the refilling and keep us posted

The high pH of sodium hypochlorite is only relevant when adding a lot and is temporary, such as when shocking, because as the chlorine gets used up, the pH drops back down since chlorine usage/consumption is acidic. Over time, using bleach or chlorinating liquid is close to pH neutral if you use products with a minimal amount of excess lye in them such as 6% Clorox Regular bleach.

That's probably good advice, Teapot, so I'll get the cya down first and then attack the pH. It will probably take another week to get the cya down to around 40. My supply water has a pH of 7.2 so that will help in that area.

The pool was relined and refilled just under two years ago, and the fitters left a half-filled can of stabiliser behind. I have used tri-chlor tabs ever since (up to joining TFP) and just measured FC and pH with the simple tablet comparator, as we do in the UK. We keep the same water in more or less forever, with only backwashing and vacuuming causing a top-up. The cya was probably high when the fitters left, and has been increasing ever since. Needless to say no tab has gone in since April 24th This site has been a godsend, and an eye-opener. Thanks to everyone.