pH Test Variations = Confusion + Frustration

Rackel

Member
Aug 21, 2018
23
Medford, OR
Hey ya'll -- as mentioned a few days ago, I purchased the fancy-dancy pH meter this spring in an attempt to have greater confidence in managing our pH. However! It has actually incurred less confidence and more confusion for me! :-\

We resurfaced our pool this past December, so I've been watching pH in particular, knowing it would drift higher and quicker than last season. That said, I feel like I've been adding a lot of acid -- I worry it has been too much. The water is beautiful, however it feels drying to the skin. I related that to pH in my mind and so was adding acid at "normal" levels in an effort to improve water feel at a lower pH.

Here are test results/acid add history since the beginning of April:

April 3 - pH 7.8 (drop test), TA 100, added 32 oz acid
April 12 - pH 7.7 (drop test), added 32 oz acid
April 16 - pH 7.7 (drop test), TA 80, added 16 oz acid
April 23 - pH 7.4 (drop test), TA 80, zero acid added
April 24 - pH 7.5 (drop test), added 16 oz acid
April 25 - pH 7.4 (drop test), added 8 oz acid
April 27 - pH 8.0 (meter - 1:06pm), TA 80, added 30 oz acid
April 27 - pH 7.6 (meter - 2:59pm), added 16 oz acid -- this is the day I read about adding salt to soften the water feel. Of course I read it AFTER I had already added the acid.
April 28 - pH 7.4 (meter), zero acid added
April 29 - pH 8.1 (meter), panicked, added 64 oz of acid

This morning the pH reads 7.5 (meter), TA 70 (not surprised with that huge acid add).

Through reading the forum I have learned that to improve the water feel issue I will need to add 40 lbs of salt (to start). Will do that this week before our next swim.

So here is my confusion: is the drop test really that unreliable? At the beginning of the month we had quite a bit of rain, but the pH readings were averaging much lower that the past few days when it has been sunny (though a little windy). The drop test / pool math app says pH readings can be +/- .4. This seems quite significant! If you read the drop test at 7.6 it could actually be 8.0 at which point you would normally add acid, but you wouldn't because it looks "normal". Conversely, you could get a reading of 8.0 when in actuality it is 7.6; you would add your acid and potentially drop your pH quite low (too low?).

Additionally: Is the meter really that much more reliable? The (poorly edited) start-up instructions mention an error rate of +/- .1, which is obviously much better than the drop test. I was careful to calibrate it with distilled water, etc., so I am confident in the results it is giving me, but I do have concerns about long-term meter reliability because there are no instructions for cleaning/ongoing maintenance. I have been rinsing the meter after each use with distilled water, but is that even necessary?

I've seen some folks here advise to use both the drop test and meter as a dual-control, but with the variability of the drop test, would it be best to do one or the other? Or is that just good advice for neurotics like me who want the most consistent, accurate results possible? 🤪

Finally, whyohwhy would pH fluctuate so much over the course of 5 days from the 25th to 29th???? Is this really a "fresh plaster" thing, drop test vs. meter thing, or do you think something else is going on?

Thank you, in advance, for help with this.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
22,414
Laughlin, NV
From Pool School:
"The precision of the measurement varies with your skill at distinguishing colors. With practice it is possible to read the level to +-0.1 or better, though many people will get results that are +-0.2 or worse."

I find if I put 4 drops of R0014 in the comparator I can better match the colors. Use a white background.

The pH meters are fine, but must be calibrated.