FAS DPD test - What am I doing wrong - 4 tests in a row - 4 different results

sws1

Well-known member
Aug 13, 2010
55
Neptune City, NJ
I just got a batch of new chemicals. Trying to SLAM my pool. (CYA of 50, PH ~7.2, TA 150-160)

I woke up this morning and tested and saw that I was not at my target FC of 20. So added liquid chlorine and then tested 1 hour later.

4 test results:
FC of 15, CC of 1
FC of 12, CC of 4
FC of 5, CC of 12
FC of 14, CC of 3

In each case, I added the drops at a "less than 1 second rate" in order to keep in moving. HOWEVER, the only difference between these tests was: Amount of powder, and how long I let the powder dissolve before adding drops. In one case, I added 4 scoops just to make sure.

The ones with the higher FC / lower CC had more powder and I let it dissolve for 30-60 seconds so it got very pink.
The one with a FC of 5, I followed instructions literally. 1 round scoop, quick dissolve, then the drops.

I understand the powder needs to bind to the FC, but I don't know if I should let it wait to make sure it's nice and pink. OR, if waiting is causing the powder to bind the CC as well.
Note that in all those tests, the TC is about the same. So something about how I do the test is driving the results.


(On a side notes, I think my liquid chlorine has lost alot of potency, since I can't seem to get the numbers up (FC, CC, or TC), even after sundown. Some of the bottles were bought last year, and some were recently bought, but I suspect they were sitting on the shelf at the pool store.)
 
Last edited:

P3Jason

Member
May 17, 2017
17
Jacksonville, FL
You only need enough powder to turn the solution pink, normally one scoop. Start adding drops immediately. If conducting the CC test, start immediately after the solution goes clear. It will turn pink again if allowed to sit.
 

sws1

Well-known member
Aug 13, 2010
55
Neptune City, NJ
You only need enough powder to turn the solution pink, normally one scoop. Start adding drops immediately. If conducting the CC test, start immediately after the solution goes clear. It will turn pink again if allowed to sit.
When I do that, I get CC results over 10 and very lower FC numbers, which seem unreasonable is a spotless pool.

Another interesting note...

I followed up with an OTO chlorine test. After adding the yellow drops, and shaking a bit, it gave me a reading of about 3. I list it sit another minute, and then the color was yellow enough to be off the high end. i.e., >5
 

sws1

Well-known member
Aug 13, 2010
55
Neptune City, NJ
What is sodium bic? And how much did you add?
Sorry. I meant Sodium Bisulfate.



New FAS/DPD test just now. Using exactly 1 heaping scoop and stirring/dropping fairly quickly.

FC - 3
CC - 11

Added 2 gallons of 12% chlorine that I bought last year, so likely reduced in potency. Will check again in 30.