How do I stop ETCHING, CH @425!

[Jd5996]

Hi everyone, as usual let me thank you in advance for your replies,,,your help here has been priceless!

HISTORY:
-Purchased home w/pool in Mar 17. As far as I can tell pool was redone relatively not long ago...gonna guess '07-'10 l-ish.
-Took control of pool w/TFP June 17
-Pool has been incredibly stable since then, pool had some scaling on walls prior to me taking it over but nothing major (approx 6-8 instances approx 3-4" long by 1/16-1/8" high)
-Scaling has pretty much disappeared BUT etching has been happening and getting worse and is leaving "holes" on the pool floor-see pics.

TESTING: (TF100) 10/1/17 @8:45am
FC=5.5 (floats from 7ppm in am after adding 24oz LC to 5.5 pm =daily FC loss of 1.5ppm...was 3ppm during peak summer months)
CC=0
TC=5.5
PH=7.8 (usually 7.7, have made zero adjustments here in 2 months)
TA=60 (found my pools happy place, again no adjustment in months)
CH= 425 raised from 200ppm at time of takeover, adding 50ppm per week hoping to stop etching!
CYA= 40 (was 120 at takeover, drained pool 40% reduced to 70ppm then slowly stabilized at 40

These numbers have been incredibly stable since mid July and the only modifications I have been making is the daily dose of liq chlorine and the addition of calcium as I raised it in hopes of stopping the etching.

NOTE: During initial takeover in June and early July I was running FC a little higher l, between 10-13 range, then dropped to 7-10 range now at 5-7ppm range. CH was down to 175 for a week or two after draining pool.

Pool has been crystal clear since with no issues other than the etching!

QUESTIONS:

- Could the higher FC levels have caused this? But it is still happening now that FC is 5-7ppm
- Should I continue to raise the CH levels?
- How high can I bring CH? At the higher levels over 300 I have seen zero scaling BUT etching continues.
- 500?, 600? Is it safe? Will it stop the etching?
- Should I crank up the CSI? CSI has NEVER been below -.15 and is mostly in the -1 to 1 range, usually close to 0. I'm assuming neg is going towards etching and pos is towards scaling?
- Am I missing something? Is something else causing this that I'm not understanding?

Sorry for so many questions, I guess bottom line is how do I stop this? LOL

Thanks again!!!

Jim D

EDIT: Plz note second pic is just showing clarity of water

 

[JoyfulNoise]

QUESTIONS:

- Could the higher FC levels have caused this? But it is still happening now that FC is 5-7ppm
- Should I continue to raise the CH levels?
- How high can I bring CH? At the higher levels over 300 I have seen zero scaling BUT etching continues.
- 500?, 600? Is it safe? Will it stop the etching?
- Should I crank up the CSI? CSI has NEVER been below -.15 and is mostly in the -1 to 1 range, usually close to 0. I'm assuming neg is going towards etching and pos is towards scaling?
- Am I missing something? Is something else causing this that I'm not understanding?

- No, FC has nothing to do with etching of plaster

- No, you should not raise your CH

- Absolute CH values are somewhat irrelevant. Mine is 1250ppm at the moment and I can still keep my water balanced without scaling. CSI is what you need to focus on

- If you are chlorinating with manual bleach additions, then keeping your CSI near zero or slightly negative (-0.1 to 0) will be just fine

It's very hard to tell from those pictures but those are likely plaster defects and not etching. Etching of plaster would be uniform all over the pool surface and you'd have to have your CSI very negative (-0.7 or lower) for months, even years, on end to notice anything at all. This is simply not a case of etching.

Let me ask you this - how are you performing the CH test? What steps are you following? Please list exact details of how you do the test.

As I said, this is likely just defects in the plaster due to workmanship issues from the previous resurfacing and not a chemical issue.

 

[JamesW]

If the pool was operated on tabs for a long time, there could be some chemical related problems.

The CYA starting at over 100 is a bad sign.

What were all of the readings when you got the pool?

Do you know when the plaster was redone?

Are you getting aggregate or plaster flakes when you clean the pool?

What do the holes feel like?

 

[Jd5996]

JoyfulNoise: Thank you that is reassuring to know. As far as testing goes I am doing exactly as the TF100 instructions call for...no different.

JamesW: AFAIK the pool was on tabs for yrs as the previous owners were snowbirds and only here Jan-Mar from '03-'16.

Yes CYA was a problem and 2 pool guys could NOT keep the pool clear, hence why I am so grateful to have found TFP!!!

Readings were approx as follows when I took over FC=2-4, CC=0, Ph=7.6, TA=120, CH=200, CYA=120.

A guess is the plaster was redone between '07-'10 as that is when the house was completely remodeled. I have no proof of this however. Looks relatively recent though...not like it was done 20yrs ago.

Im getting neither aggregate nor plaster...its just being "eaten" by the pool. Much like the scaling it is just disappearing and I find no sign of it in the filter.

The holes feel like "pitting". If you ever had bad acne or know someone who has and have seen the pitting it can create it is similar to that. The edges are rough but the inside is smooth and "new" ... like virgin surface,,, and completely white! The larger ones are slightly smaller than a dime and approx as deep as a dime. They are starting to pop up alot and were not there prior to me taking over the pool.

 

[JamesW]

It's hard to know what the chemistry was before you got the pool. The TA might have been below zero with the pH below 4.5. No way to know for sure.

Typically, if the plaster is dissolving, you get aggregate like marble or quartz.

Any chance that the pool is painted or was resurfaced with fiberglass?

Sometimes the plaster delaminates in small shallow spots and sometimes it delaminates in big deep spots down to the gunite or previous plaster.

If you rub your hand on the surface, is it chalky or does it leave white residue on your hands?

If you brush, do you get a cloud?

 

[JoyfulNoise]

To close on my testing question of CH -

Are you counting drops until the solution first turns from purple to blue OR are you counting until the blue color stops changing hue?

 

[JamesW]

Google spot etching in swimming pool plaster for some opinions on what is happening "on balance" has some good information. The National Plasterer's Council" also has some opinions on the subject.

 

[Jd5996]

JoyfulNoise: I am counting from until it turns blue then I stop, because I swirl while putting in the drops its only about one drop from purple to blue. I do not continue until it gets "bluer".

JamesW: Of course I cannot be certain how pool was maintained but I know that it was serviced regularly and according to my neighbor who used the pool consistently over the yrs even while the snowbirds were not here( they were close friends) there were never any major issues other than an occasional shock or two by the pool guy. Of course that means nothing BUT this issue did not start until I took over the pool in June.

TBH, I have begun to suspect it may be the calcium powder itself as this problem really began when I started raising the CH levels. I have noticed that a bit of the powder does not fully dissolve and will rest on the pool floor until I vacuum it every Sunday. Is it "burning" the plaster? I have zero issues on the walls, just the floor.

In any event, last week I decided that I would dissolve the powder prior to putting it in the pool despite the manufacturers recommendation not to do so... I just cant help to think that is what is happening,,, the calcium itself is "eating" the plaster as it lays on the floor undissolved.

Ever heard of that before? Do you think it's possible? The correlation of the "holes" with the addition of the calcium is striking to a T!!! The more calcium I put in the more "holes" I get!!!

 

[JoyfulNoise]

Jd,

The correct way to do the test is to count until the blue color stops changing hue, not at the transition. This is complexation back-titration test, so in order to get the correct concentration you keep adding drops until the color stops changing, not at a specific color transition.

In reality, your test is probably 2-3 drops short of the correct value. So your CH is a bit higher than what you think it is.

 

[Jd5996]

Thank you JoyfulNoise ( love that user name BTW . So then my CH may be closer to 450 or 475 rather than the 425 I record it at. Still though, I cant seem to get away from the correlation of the etching and the time I began adding the calcium, so from here forward when I have to add calcium it will be dissolved first in a bucket of pool water for a few hrs. Might be time to look into liquid calcium

 

[JoyfulNoise]

Calcium chloride can be a bit of nightmare to mess with. It is very exothermic and generates a lot of heat. So one should never let granular calcium chloride sit on plaster as it can create hot spots (its not going to boil but it can get pretty hot). Then you have the problem of locally high calcium concentrations driving the CSI very positive.

Best to pre-dilute any calcium sources.

 

[jontomosicz]

I had a new pool built last year with pebble sheen. A month or so after the build I started to see the white pits like you're describing. The plaster guy knew exactly what it was when he saw it. He explained it as clumping in the plaster mix. It creates hard spots in the plaster that eventually break away. They sent a diver to repair the spots. There were about a dozen. No problems since then

 

[peirek]

JD - I know this is focused on your plaster issue but want to make you're aware of some other points. What is your fill water CH level? Remember CH does not leave the pool other than by way of splash out or overflow. I think you can stop adding CH else you will be in the same boat as you were with CYA, draining a portion of your water. Have you plugged your numbers into PoolMath? With water temp of 80, I get CSI of 0.16 (assuming salt of 1000ppm).

 

[Jd5996]

Just wanted to thank everyone for thier input on this thread. I apologize for taking some time to get back to the post but just wanted to submit an update.

Ok, the last time I added calcium I decided to dissolve it first rather than broadcast it in the deep end as called for in the instructions ( and pool school too). So I dissolved 1lb in a 5 gallon bucket for about an hr, stirring occasionally. That was approx 3-4 weeks ago and what I have noticed is that the etching has STOPPED. Completely!!!

Now, the damage done is done and that has not changed, improved, or worsened, but no new "spots" have formed and at its peak I was getting 3-5 new spots per week.

Conclusion is that for sure, IMO, the undissolved calcium was "burning" holes in the plaster floor.

So just a word of caution for anyone that may be experiencing similar issues...funny thing is the directions warn against pre-dissolving due to extreme heat generation :-(