testing water right after shocking?

LeeM2

Well-known member
Jul 3, 2010
96
Which parameters are likely to get messed up (other then chlorine) if you test your water 30-60 min after shocking?

Today I found some algae in the pool on the lamp, behind the plastic cover. As I removed the plastic cover and wiped off the lamp with the cloth soaked in bleach, the algae ended up on the pool floor. I didn't have time to vacuum, so I shocked the pool and left automatic vacuum in it and left.

About 1hr later this water was tested at different pool stores (I didn't have time to test it myself, so I asked my brothers to take water to 2 stores). This time to my surprise the test results were nearly identical between the 2 stores (except for PH, 7.6 vs 7.8), however they both showed CYA at 60 and CH at 370. My TF100 always has CYA at 30-35 and CH~~600. Could shocking temporarily change these two values so drastically? If not, then I really wonder which results are more accurate, mine or the 2 stores where the water was tested.

Thanks.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
Shocking tends to affect the PH test, but it doesn't normally cause problems for the other tests. As a general rule, you shouldn't trust pool store test results.
 

svenpup

LifeTime Supporter
Nov 18, 2009
835
Sacramento, CA
High chlorine (over 10) can affect pH measurement. CYA and CH are not affected.

Since you were surprised that the pool stores numbers were identical, I assume you know that pool store testing is unreliable. The fact they they were both wrong by the same amount is a coincidence (or maybe your brothers only went to one store and fibbed?).
 

LeeM2

Well-known member
Jul 3, 2010
96
svenpup said:
High chlorine (over 10) can affect pH measurement. CYA and CH are not affected.

Since you were surprised that the pool stores numbers were identical, I assume you know that pool store testing is unreliable. The fact they they were both wrong by the same amount is a coincidence (or maybe your brothers only went to one store and fibbed?).
No, they went to two stores. In fact, these two stores always had very different CH results which was the reason why I joined this site and got TF100. Now both stores have the same CH result of 370, while my TF100 is at 600.. and both have the same CYA result of 60, while I can see the dot up until 30-35. It really is either a weird coincidence or the pool employees are reading these forums and communicating :)
 

anonapersona

TFP Expert
LifeTime Supporter
Nov 5, 2008
2,598
More likely they were told to test until the color changes, so when it goes to purplish they stopped counting, instead of letting it get to blue with no further changes in color with more drops.
 

LeeM2

Well-known member
Jul 3, 2010
96
anonapersona said:
More likely they were told to test until the color changes, so when it goes to purplish they stopped counting, instead of letting it get to blue with no further changes in color with more drops.
That's how I test it :). I did not know that I need to wait until it turns blue with no further changes in color with more drops. thanks for clarifying this.
My CH changes color at around 550-600, but turns blue at around 630-650.
 

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