How long for muriatic to take effect?

[BillyBumbler]

I tested my water this morning:

18000 gallons, gunnite pool/spa combo w/ aggregate finish
FC 1.0
CC 0.0
Ph 7.8
TA 90

I have to assume the CYA is at a good level until my new testing kit arrives.

I added a quart of muriatic acid to knock down the Ph a couple of points and left the pump running. 6 hours later the test result is pretty much the same, a reading of 7.8 with 2 drops of acid demand solution to bring the color into the 7.2-7.4 range. Does muriatic acid take a long time to lower the Ph?

[waterbear]

The acid demand test is only a guideline to how much acid you need to add. It sometimes works and it sometimes doesn't becuase it is only an approximation of what will happen in the pool and is based on 31% muratic acid. Some muriatic acid is more dilute. This is why the best way to adjust pH is to add small amounts of the pH adjusting chemical and restest. Is your plaster new or fully cured? New plaster can require more acid to shft the pH. You can get an idea of how much the pH is moving after an hour or two of circulation but it can take as long as 24 hours to see where it really is. Brushing after adding the acid (or any chemical) will help to distribute it through the pool. If the pH didn't move a few hours after adding a quart try adding mabe a cup or two more and see what happens until you get the pH where you want it.

[BillyBumbler]

I added another couple of cups of acid and the Ph is finally lowered into the right range, however my TA has lowered from 90 (where it was all week since I first filled the new pool) to 80. Does Muriatic lower TA? Is a TA reading of 80 low enough to worry about adding baking soda or should I leave it alone for now?

[waterbear]

The reason the TA went to 80 is a bit technical but in a nutshell you have shifted some of the carbonates/bicarbonates that make up the TA to carbonic acid which the TA test doesn't measure. As long as the carbonic acid is still dissolved in the water it will reconvert to carbonates/bicarbonates if the pH rises. If it is driven off by airation then the TA has been lowered. What is your calcium hardness? TA of 80 is certainly within range if the CH is in proper range.

[BillyBumbler]

My Spa does run over into the pool. I also have some sheer decents that I run once in awhile. I suppose that qualifies as airation. Assuming that I will be continueing to use these features, will I be dealing with low total alkalinity issues? I have a SWC that will go online in a couple months after the pebbletech finish cures. I am expecting that I will have to add muriatic acid regularly, about once a week, to keep the Ph balanced. That seems to be the standard expectation when running SWC units from what I've gathered. If this results in low TA issues, could regular addtions of baking soda also be in my future? As for Calcium Hardness I do not yet have this test, should have it soon.

[waterbear]

I have a spillover spa, 2 waterfall/bubbler deck pots by Fiberstars and deck jets so I have a lot of airation but I find my TA tends to stay constant at around 100-110 ppm. My fill water has a TA of 110 so that might be the reason. I have to add acid about every 3 weeks to bring my pH from about7.8-7.9 down to about 7.2 because of the SWG and my TA is around 100 when the pH is low and around 110 when it is on the high side. The only time I had to add baking soda was after a metal seqestering treatment which lowered my pH and TA signifigantly.

Every pool is different so there are no hard and fast rules. Also I have fiberglass and yours is gunite so your pool surface is going to be more reactive with the water. What happens in my pool might not happen in yours.

Just keep tabs on things and see where your pool goes. Make small adjustments and retest if needed. Don't try to rush and make big changes at once. You will soon find the perfect levels within the recommend ranges for your pool that are fairly stable.

If you calcium levels are not too high you might want to consider raising your TA a bit. This might create less of a change when you have to lower the pH.

For right now I would keep all levels at the range that your builder recommends for the new pebbletec finish until it is cured! If that means monitoring and adjusting the pH, TA, and CH levels then do it. Once the pool is cured things should settle down somewhat.