FAS/DPD doing crazy things

[Rocket50]

My FAS/DPD tests is showing crazy numbers. Tested this morning and FC was 17.5. Doing a SLAM so want the FC at 20. Put in 1/2 gallon this morning to bring FC to 19. Tested again when I got home from work. FC was 14.5. Added 2 gallons of 10% chlorine to bring it back to FC 20.5. Just tested and it is FC 27.5. Been using pool math and have everything dialed in correctly. What am I missing? What could be going on?

 

[jblizzle]

Stronger bleach than expected? Smaller pool than expected?

 

[jblizzle]

Are you holding the reagent bottle vertically when doing drops and adding them one per second or so.?

 

[Frankhugus]

How many scoops? 1 or 2?

 

[Rocket50]

Probably dropping quicker than 1 per second. Holding vertical now. Wasn't for the first few days of the slam but old chemistry class memories came back.

- - - Updated - - -

1 scoop.

 

[Rocket50]

10 % bleach from Walmart in the pool section. 20x40 foot rectangular pool, 3 feet to 8 feet. 32,900 gallons. Unless the bleach is stronger than what is advertised on the label... I don't know. This happened a few days ago as well.

 

[jblizzle]

You might need a little more powder. The higher the FC, the more you need. You can not use too much, but too little can cause difficulty getting the end point.

 

[Rocket50]

OK. Tested at 20 this morning. The pool is sparkly with minimal debris. No way it went through 7 FC's last night. Must be user error.

 

[jblizzle]

What was the CC? It is certainly possible to lose that much overnight. Although I may not rule out user error yet.

 

[JoeSelf]

That is why I went with the sample sizer and calibrated cylinders. To get consistent results. Also, speed stir is the BOMB.

 

[mytmouse]

You might need a little more powder. The higher the FC, the more you need. You can not use too much, but too little can cause difficulty getting the end point.

this was the solution for me with for consistency during a slam (also was trying to hold at 20 with CYA of 50). 2 scoops.

 

[Rocket50]

CC has been 0-.5 from the beginning. I have been losing less and less FC each day and never lost more than 5 FC when I first started on 5/13. Kind of cloudy and rainy today and tested out at 19.5 tonite. So a .5 FC loss today during the day. Sounds like its time to order more DPD powder.