Anomaly in pH testing

[Jetjohn6]

Since assuming ownership of our pool last fall I've done my of my water sampling from the same area of the pool...near a return in the deep end. I get the sample roughly elbow deep. Our pH tends toward the high side (7.8). I'm typically adding acid at least once a week.

On Friday we had a few swimmers over. I did add some acid a few hours before they arrived after a test of ~7.7. On Saturday I happened to take my sample closer to the middle of the pool. pH tested 7.0-7.2. I've never experienced a result this low. I was quite surprised because we even had the waterfall running for several hours on Friday. Looking back I wish I went back and took some more samples but did not. Sunday I took samples from both the deep-end return and the spot I got the low pH reading from the day before. Both samples showed ~7.7

What's the explanation for this? All samples were taken when the pump was running.

 

[setsailsoon]

"I did add some acid a few hours before they arrived" sounds like a mixing issue. If you want to confirm try to duplicate the conditions and sample a few times. I wouldn't mess with it though. Where are you adding acid?

 

[mknauss]

Also -- how are you chlorinating the pool water?

 

[Jetjohn6]

"I did add some acid a few hours before they arrived" sounds like a mixing issue. If you want to confirm try to duplicate the conditions and sample a few times. I wouldn't mess with it though. Where are you adding acid?

I added 16 oz of acid which would have brought my pH down to 7.4-7.5, not the 7.2 result I measured. The 7.2 result was measured roughly 24 hours after the acid was added so I would think acid would have mixed by then considering how many folks were in the pool and the hours of the pump running. I add acid in the deep end in front of the return.

 

[Jetjohn6]

Also -- how are you chlorinating the pool water?

I primarily use chlorine but have been using pucks recently to increase CYA. CYA wasn't increasing fast enough so I have used some granular CYA recently. It was most recently used Thursday in a sock hanging in front of return.

 

[mknauss]

Trichlor is very acidic. Also, the CYA will lower your pH a bit depending on how much you add.

 

[Flying Tivo]

Pucks are acidic and will lower your Ph.

 

[Jetjohn6]

Trichlor is very acidic. Also, the CYA will lower your pH a bit depending on how much you add.

The pucks have been in the water for over a month. I've still had to add acid. Less than one full puck remained in the floater as of Thursday. However, i don't recall the proximity of the floater to the location I sampled the water that day.

 

[setsailsoon]

I primarily use chlorine but have been using pucks recently to increase CYA. CYA wasn't increasing fast enough so I have used some granular CYA recently. It was most recently used Thursday in a sock hanging in front of return.

Granules are very acidic. When I add I do it about 3-4 lb at a time. Next day my pH is usually .4 pH units less almost exactly what Pool Math predicts. This could easily explain the pH issue you are experiencing. As Marty and others mentioned tablets are acidic as well. Usually eliminated the need for acid when I used to use them on vacation travel before switching to salt.

Chris