Calcium Hardness

[Ciscokid]

Hello, I have a 3 year old in ground plaster pool. It is 30000 gallons. I am running a Autopilot dig-220 SWG. My problem is a constantly rising CH.About 1 year ago I had the harness at 600, so I decided to drain the pool and refill. When I was done and got the chemicals back in balance it was down to 200 CH. Now once again I am at 550 CH. Any ideas as to what can be causing this? My fill water is fine. A whole other issue is my constantly rising PH. The only chemicals I add to the pool is Muratic Acid for the PH and occationally I add bleach to get my Chlorine levels where it needs to be. Thanks for any information you can provide me.

[chem geek]

You say you occasionally add bleach to get your chlorine levels where it needs to be, but what is your normal source of chlorine? Are you using Cal-Hypo either in an inline feeder or a floating feeder or adding it manually? Your CH is probably rising due to using Cal-Hypo which for every 1 ppm Free Chlorine (FC) it adds, it also adds 0.7 ppm Calcium Hardness (CH).

Richard

[Ciscokid]

The normal source of Chlorine is coming from the SWG. I use no other chemicals what so ever.

[JohnT]

Well, only two likely causes. One is that you are adding a chemical with calcium. That's out, since you don't. The other is that the calcium is coming from the pool itself. That would be bad. A third option would be that you have a test issue. Have you tried having a pool store test your water to see what they get? I'm concerned that you may be overdoing the acid and dissolving your pool.

[Ciscokid]

That is interesting John.There is one thing I just thought of that might be the source and that would be the salt I added to the pool. I will have to go to home depot where I purchased it and see what other chemicals they may add to the salt.
Now as for it coming from the plaster itself. Is there a way to tell if this is true? Are there tell tail signs? Also should I be concerned with my CH at 550 if it stays there and does not continue to rise? Thanks for your information.

[chem geek]

Sorry I didn't read your post thoroughly the first time -- you had said you had an SWG and I missed that. As for knowing if the calcium is coming from your plaster, the way to know that is to calculate the saturation index for your pool so to do that we need to know the pH, TA, CYA, CH, and water temperature. However, unless your TA and pH are very low, I would be surprised if your water is that corrosive.

For example, with your CH of 200 then if your pH were 7.2 and your TA were 80 and your CYA 70 and temp 80F, then your saturation index would be -0.8 and would probably be corrosive. However, with an SWG it's hard to keep the pH low and in fact you mentioned a frequent battle of rising pH so that implies you probably don't get it much below 7.5 or so anyway.

It's possible that with your frequent acid additions you've gotten the TA so low as to be a problem or that locally acidic conditions dissolved some plaster where you poured it, but why don't you give us the full set of numbers and we can go from there.

The CH level of 550 is nothing to be concerned about, especially if you keep your TA lower to prevent the pH from rising as quickly. By the way, if there WAS corrosion, then this would make the pH and TA rise (whereas outgassing of carbon dioxide makes the pH rise without any change in TA). As for tell-tale signs, it is possible for the plaster to feel more rough and less smooth IF the corrosion were somewhat uneven, but that's speculation on my part.

Richard