bad acid or bad test?

Oct 30, 2016
7
NW Georgia
#1
Hello,

I have a 900 gallon vinyl AGP - new set up.

I added water, bleach, and CYA.
Pump on for 48 hours, and the CYA is 40 with FC of 4. Excellent.
I added borax and muriatic acid.
Pump on for 48 hours, and the borates are 50 with a pH of 8.0. TA was 110-120.
So I added 8 liquid ounces from a measuring cup of 31.45% muriatic acid.
Pump on for 48 hours and retest. TA is still 110-120 with pH of 7.4

I'm using HDX brand 5-way drop test. I know it can be hard to discern the shades of yellow for FC, but it was immediately available to at least get the pool started.
Since the pH came down, it makes me think the acid did something.
Could the acid be old or could the test be way off for TA - or something else?
 

YippeeSkippy

Mod Squad
LifeTime Supporter
Jan 17, 2012
9,087
Evans, Georgia
#2
Actually, you may have this idea backwards. When your FC is higher than 10ppm, it can skew pH tests and make the pH appear higher (usually) than it actually is.

Can you provide us with a full set of results:
FC
CC
pH
TA
CH
CYA

Maddie :flower:
 
Oct 30, 2016
7
NW Georgia
#3
Actually, you may have this idea backwards. When your FC is higher than 10ppm, it can skew pH tests and make the pH appear higher (usually) than it actually is.
FC has never been higher than 5. It's a new set-up.

FC - 4
CC - 4
pH - 7.4
TA - 110-120
CH - didn't test
CYA - 40
Borates - 50

My question is why would TA stay at 110-120 despite adding enough muriatic acid to drop the pH?
I didn't test pH immediately after adding acid. I added enough muriatic acid according to pool math to bring my pH down to 7.0 and then was aerating via the pump return for the past 48 hours.

(ETA borates)
 

JamesW

TFP Expert
Mar 2, 2011
12,837
#4
Are you sure about the volume of the pool?

The TA test can be +/- 10 ppm under good conditions, so it could be within the precision of the test when accounting for test variability.
 
Oct 30, 2016
7
NW Georgia
#5
It's an easy set pool so it's narrower at the top than at the bottom. Intex specifications for 80% capacity and pool volume calculator by measurements are both pretty darn close to 900 gallons, so I'm going with it.

So let's say the high TA reading was 120 and the low TA reading was 110.
Shouldn't dropping the pH from 8.0 down to 7.0 do more than change TA by 10ppm?
 

JamesW

TFP Expert
Mar 2, 2011
12,837
#6
If the acid label says 31.45% muriatic acid, then it’s probably good. I wouldn’t be overly concerned unless you use the acid several times with no TA drop.

Make sure that you’re doing the TA test the same each time. Hold the bottle vertically, wipe the tip if the drops seem off size, make sure that the sample volume is the same and allow for some change if fresh water is added.

What’s your fill water TA?