How is my CH going down?

[Enumjon]

Hi all-

New pool owner that recently did a pool drain and refill thanks to high CYA levels from previous owner. Fill water has crazy high CH and TA levels. My research here on TFP indicated that if I just lowered the TA level and kept pH lower the CSI would be tolerable and keep scale from forming. So I jumped in and started lowering TA with the muriatic acid and aeration method. The strange thing is, my CH levels have been dropping as well! I thought that there was no way to lower CH, other than draining water and refilling.

A few facts. So far I have put in 19 gallons of muriatic acid. TA has dropped from 650 to 220. CH has dropped from 1200 to 675. For the first month of this drop we were not swimming in the pool and the temperature was low, so there was no water added to the pool, and in that time the CH dropped to about 800. In the past few weeks I have started topping off the pool with softened water, so I can see that lowering the CH a bit. Can high amounts of acid drop the CH in the pool? Any other ideas on why the CH has lowered?

 

[mknauss]

Only way is testing error for those large drops. Adding acid does nothing to CH levels.

When you backwash your sand filter and then replace that with softened water your CH will come done slowly.

 

[Enumjon]

But it was a gradual drop over a month. Testing remained the same - I have a speedstir. Now that I am backwashing filter and adding it makes sense to come down slowly, but not before.

 

[Enumjon]

I can see testing error account for a small part of the change. But a drop from 1200 to 700 is 20 drops of reagent - that is a huge amount. In the high levels I am dealing with the color change is very slow, so there is quite a bit of subjectivity that enters the picture when trying to figure out the color change-but may a drop or two, not 20.

When the pool was drained I cleaned old scale off using acid. Could the acid wash have removed too much calcium from the plaster, and now the plaster is taking calcium from the water?

 

[Enumjon]

Regardless of the reason, I am very happy that the calcium levels are starting to approach normal.

 

[mknauss]

Could the acid wash have removed too much calcium from the plaster, and now the plaster is taking calcium from the water?

No.

At high CH levels, testing becomes much more difficult. Using the Fading Endpoint method, going very slow. Turning on and off the Speedstir, etc improves the accuracy.

 

[Enumjon]

Thanks for the input. So a testing question that responds to both CH and TA testing. When they say it is supposed to turn red, or blue. How strong of a color should it be? The color will do a gradual decreased in intensity, go murky or clear depending on which test is happening, then start to build up the new color. At what point do you say YES - that is the level?

 

[mknauss]

TA test - once the color stops changing, then subtract the last drop.

CH test -- much more subtle. You are looking for sky blue. I use 4 or 5 drops of R0011L to make the color more intense.

 

[Enumjon]

Thanks for the tips. Love the idea of adding more R0011 to make the color more intense - I am going to run out of R12 much before R11!

 

[mgtfp]

I am just guessing here, maybe someone like @JoyfulNoise will have a better idea if that is just nonsense: With CH and TA in these high areas there must have been a lot of calcium scaling happening, i.e. calcium leaving the water and depositing on surfaces where it doesn't get picked up any more by the CH test. That could explain CH going down. But I have no experience how much of a CH change you could explain like that.

Water will aim to reach equilibrium (that's why it's called equilibrium), which in terms of calcium means that the number of calcium molecules leaving the water each second will eventually equal the number getting back into solution. That means CSI will trend towards 0 by forcing calcium out of the water to deposit on surfaces (opposite for negative CSI: in a plaster pool, the water will draw calcium out of the plaster into solution until CSI=0). The question is, how fast do these processes towards equilibrium happen, are they measurable on a time scale of a month? Reactions certainly occur faster at higher water temperatures in summer, which will also further decrease the calcium solubility, i.e. increase the CSI. That's why you see calcium scaling built up very quickly in a water kettle.

 

[JoyfulNoise]

The Taylor CH test isn’t intended to work at those levels. It’s really not suited for CH values over 800ppm. It has to do with the way the chemistry of the dye indicator works. If you are over 800ppm, you would be better off carefully diluting a pool water sample 1:1 with distilled water (you can buy it in the supermarket) and then just doubling the final result.

Also, your water is obviously coming from a highly alkaline source with a TA that, for all practical purposes, is off the charts. If you have that much CH and TA in your fill water, then you also probably have a large amount of magnesium and possibly iron in the water. Both of those are going to heavily interfere with the CH test.

Acid additions do not lower CH. In your case, it was just testing error due to water balance being so far off.

 

[Enumjon]

Thanks joyfulnoise-that makes good sense. My CH continues to drop about 50 points a week - now in the acceptable range at 600 in my most recent test. But the current CH drop makes sense, I am adding water from the water softener to make up for evaporation, backwash, and kid loss of water. TA is proving more of a struggle, reached a plateau of about 200. MA drops it each addition, but then the fill water brings it back up. I should probably get the numbers where I want them right when it is time to close the pool for the year.

 

[mgtfp]

You don't really lower CH by replacing evaporated water, because the calcium stays in the water. If you are replacing evaporated water with softened fill water (so more or less replacing H2O with H2O), then CH should stay fairly constant.

The problem usually goes the other way. If you are replacing evaporated water with high CH fill water, then CH will rise over time.

If your CH keeps falling, I would think that it is creating scaling somewhere. Or you have that much loss due to back washing, rain overflow, etc. In this case you remove calcium from the pool. That is probably more likely, but try to estimate if the CH drop is consistent with your water losses not caused by evaporation. In a 30k gal pool at CH 600, you needed to replace 2500 gal of water to get CH down by 50ppm. Just check if that amount is reasonable, sounds like a lot to me.

 

[Jimbarstow]

Something must be going on since Ca can't disappear on its own.

I successfully kept our pool balanced with almost no scaling up to CH 1200. (I saw no problem with the Taylor Ca test at the upper range.) The TA was pretty consistently around 100 so I had to keep the pH in the low 7's. There were some minor Ca deposits at the water line and I had to vacuum occasionally since there were some Ca flakes but I was the only one who noticed. Frequent cleaning of the SWG cell was the biggest pain.

The key is to maintain the CSI within the acceptable range.

Adding softened make-up water will not, and cannot, reduce Ca since the Ca doesn't evaporate.

The only thing that can reduce Ca is replacing the water or reverse osmosis. I've done both. Where I am, trucking water in is VERY expensive so RO is much cheaper. If you replace the water, make sure you know the quality of the water you will get. I've found that I can get water low in Ca or up to 350 ppm for the same price. With reverse osmosis, you basically run it until the Ca level is where you want it. (RO will also lower the alkalinity.)

 

[Sandi DC]

TA test - once the color stops changing, then subtract the last drop.

I am fairly new to TFP so just want to be crystal clear on this... so... drops 5 & 6 change from green to red but go back to green. Drop #7 stays red. Does that mean my reading is a 60? (I have been recording this as a 70)

Thanks so much!

 

[mknauss]

I am fairly new to TFP so just want to be crystal clear on this... so... drops 5 & 6 change from green to red but go back to green. Drop #7 stays red. Does that mean my reading is a 60? (I have been recording this as a 70)

Thanks so much!

You should actually use a Drop #8 and if that shows no change, you subtract that drop and your TA is 70.

 

[Enumjon]

Now I am confused about the CH drop. I am not adding that much water each week, maybe 500 gallons. I am keeping my CSI around .4, and not noticing any scaling in the pool. My thought was that the softened water is "oversoftened" and the potassium in the water is binding up the calcium in the pool, just like it does in the water coming in to the softener. I will keep a look out for scale, but am not noticing any buildup on tiles. Maybe a little on the floor of the pool, but hard to tell since it is not a nice even color.

 

[Jimbarstow]

I’m also confused about the Ca drop. An old research advisor once told me that, “When you start blaming your instruments, your know you’re really confused“. That said, I question the testing. Ca can’t just disappear unless you are splashing an enormous amount of water out of the pool or lose water from something other than evaporation. I wonder if the astronomical TA could some how interfere with the Ca test? I don’t know the specific chemistry of the Ca test but it is interesting that as you lowered the TA, the Ca came down. A lot of these tests are accurate within a specific envelope of conditions; you may be outside that envelope.

Water softeners don’t work the way you describe. The hardened water passes through a water insoluble resin charged with either K or Na ions. The Ca is preferentially bound and the K/Na released. These non-Ca ions in the softened water can’t go on to bind other Ca ions.

The fact that you don’t have scale doesn’t surprise me that much. I was really careful with the CSI when I had Ca above 1100 ppm and I had minimal scale. My issue was the weekly (or more often) cleaning of the SWG cell. I did see some minor flakes on the pool floor presumably Ca from the SWG cell.

 

[mgtfp]

If your water really ever was at CH 1200 and TA 650, then your CSI would certainly have been in the scaling range. With CH 600 and TA 200, you can get CSI below 0.6, where scaling is still possible but becoming less likely, by keeping pH below 7.5 (at 90F, assuming CYA 40 and TDS 500). But if there are any hot spots, like heater, pipes/equipment exposed to sun or hot concrete/bricks especially while pump is not running, etc), your CSI can be much higher there, at 120F the CSI would be above 0.8. When having hard water, then you don't see scaling in a glass of water happening, but surely in the kettle. You don't have a SWG, right? Is your gas heater running at the moment?

Have a think if there are any hot spots where scaling could still occur where you don't see it. As mentioned by Jim, high CH is manageable with careful pH and TA control, but scaling can still occur in SWG cells and heaters or other hot spots.

 

[mgtfp]

How's your CYA, is that stable?