Follow advice above. Here is some more general novice advice regarding PH and TA from the "what it's worth file" that has worked for me since learning from my mistakes and experiences in the past. TFPC pool school does not really get in to all of this, but I learned it on the forum from experts with a PH creeping issue:
1. Make sure that the FC is not very high when you test PH, as high FC levels, irrespective of my CYA level make my test results inaccurate; and also, I don't make PH adjustments based on one test at one particular time as a general rule, especially in a situation where your TA/PH balance has served you well for a long period like is your case. You don't want to fix something that ain't broken and to suddenly put more work on yourself and initiate a troublesome pool that had been trouble free based on a one-time result that may or may not be right.
My pool has been solid all year at 7.3-7.4 with my TA at or about 60 all year long; but there have been a few instances where my PH reading was showing on the high end, yet still in range (7.8ish), but it turned out those readings were false due to high FC levels at or about 7. This situation occurs to me due to an overshoot of chlorine dose the day before relative to that day's consumption. I'm sure others have done this...you're dosing every day; the pool is consuming chlorine every day, but you'll add one day and the next day it's cloudy all day or maybe you just shot above your target or whatever the reason, the rounding we do or whatever, but when you go to test the next day, you find that you're still at or near target and this is 24 hours or so after your last dose, yet you still don't need any chlorine for the next day. This is not the best time for trusting a PH reading. This is one of the reasons I prefer CYA at the lower end (because it gives me better management / accuracy with respect to my PH operating a pool at 30-40 CYA, as opposed to 50 or higher). With a CYA at the upper end of the recommended range, I'm always more skeptical of the PH reading, and what I'll do in that situation is to occasionally let my FC drop down below the recommended minimum just so I can get a good PH reading.
2. Another point I'd like to make just as a general guide, and I've learned this the hard way, I never, never try to adjust TA to any kind of recommended level just for the purpose of getting it to any kind of arbitrary level. At least I don't anymore. TA adjustments should be made only out of necessity to keep PH more steady and that's it, especially for vinyl pools. But when you start adjusting PH, it's going to change your TA level. What I'm saying is don't get hung up in that new TA level and try to change it, because this new TA level is what you need. So let's take your example where one can mess up with respect to TA. Let's say hypothetically that this happened mid season when you're leaving the pool open for a while, and so you've got a pool that the PH suddenly began creeping up out of range. You tested it at least two times on two different days, with Taylor drop test kit, with FC relatively low, and so you know that your PH is 8 or above; but you've been good all year, so something has changed things. You don't know what it is, but you know you've got to lower your PH. Test your TA and record it. Do the pool math to bring PH down to at least mid-range and then begin testing every day just to see what's going to happen. Make sure you test the TA a day or two after the PH adjustment to see if it dropped as well (it should have). Let's say hypothetically, your PH crept back up four more times, and you adjusted with acid down to mid range all four times, and that after the fourth adjustment it quit drifting or creeping, but now your TA is at 50...wait a minute, TFP says that my pool needs to have TA from 70-90. Traditional pool care says it should be even higher (100-150)...You may think, "I've got to raise my TA back up to at least 70--right". No you don't. That is unless you want to keep making adjustments forever. For whatever reason, you now have a new TA level that is the new perfect level, and that level is 50 no matter what any expert is recommending. Don't mess with the TA.
Let's now say that hypothetically, two months later, your PH starts drops down out of range. And if that happens, you'll add a little borax and test, borax and test, borax and test if necessary to keep PH up in to range. You can handle this two ways. You can just keep adjusting up PH over and over via borax doses until the TA rises enough to reach your new perfect TA level, or you can raise TA up 5 via baking soda and then observe and test several times to see if the issue is resolved or if the issue has slowed and then proceed until you know longer have a PH drop problem. But you do this slow and easy and with certainty. In other words, it's okay to raise your TA upward if necessary as long as you do it incrementally, so as not to overdo it, and for the purpose of getting PH more steady; not to try and reach any specific number. However, in the opposite scenario, where you're trying to lower PH, you don't try and lower TA specifically via acid. You just keep adjusting PH until the TA gets down to the correct level for your current pool balance need. But in the situation where your PH is continually dropping down out of range, you should try to reach a new perfect TA number via baking soda in small incremental changes with a lot of careful, incremental adjustments and monitoring of your pool until the PH becomes steady.
3. One other observation I've made with respect to my pool and PH is that sometimes I'll get a movement in PH that is temporary and that rights itself on its own, so be careful about unnecessary PH adjustment that can cause more work by getting in a hurry to keep things perfect all the time. If I've had excessive outgassing due to lots of aeration, e.g. swimmers splashing in a one-time event, that upward PH move tends to right itself at least in my pool after a day or two. Also, this year I've added CYA twice and noticed my PH predictably dropped down towards the low end of the range after adding that acid. Last year, when I did this, it actually dropped down to 7.0 on one occasion. I don't adjust it right away. I tested and retested before I took action. For the last two seasons, these drops in PH due to adding CYA corrected themselves and I was back to normal within three days. So my pool has not seen muriatic acid or dry acid or borax or baking soda all season and for most of last season. That doesn't mean that it won't start drifting or dropping out of range tomorrow. Something may change in the chemistry or the water and the PH may start moving one way or the other. But if it does, I'm going to start doing a lot of testing, minor adjusting, and observing to try to get back to a trouble free pool.
thanks to you all...
