Testing high FC levels during Algae treatment

[cood60]

Hi all, here the details of my latest algae fight:

Have been keeping pool balanced all year
FC between 3.5-4.5
Ph stable at 7.5
TA stable at 50
CYA had dropped to 30 ish due to lots of rain, evaporation and consequent replenishing with city water
Last week was very hot ie upper 90 s all week
I added Liquid CYA to bring CYA to 60 This was no more than two days before the algae showed up

Yesterday, I brought FC to 12 (forgot to lower Ph first) Also added some cellulose fiber aid to help clearing process, pressure never did rise!

Did OCLT last night, went from 12 dwn to 9 overnight (it did rain a little during the night)

I added 9 more gallons of bleach this morning to get to algae shock level of 36…brushed again, Re-checked FC after 3 hours, and the test is not behaving right.
When I added the powder to the vile, it only turned a light shade of red but I continued by adding drops and as I did, the water turned dark red. After 64 drops, the water turned to a slightly yellowish tint but would never get clear even after adding 15 more drops.
Ph is off the chart now, TA is 60, not sure what the FC is due to discolored water instead of clear
Lowered Ph to 7.2, then added 3 gallons of bleach to get to shock level, brushed again. Water is much more clear already but I am concerned about this light yellowish tint to the water in the vial at the end of the drop count. (FYI, the water is clear in the vial when beginning the FC test

Anyone know what this light yellowish tint might be? And do you think the FC test is valid at the point the red is gone, just not clear water

Thanks

Cush Dobbs

 

[Richard320]

It sounds like the FC is too high for the amount of powder. Try it again with two scoops. If you can see undissolved crystals before you try the drops, you have enough powder. Don't even bother testing pH when FC is above 10. The reagent reacts with the high FC and reads artificially high. If you try to lower it, you'll go way too low.

 

[cood60]

oops, I did lower it to 7.3 or at least that is what the reading is.

FC now reads 46 after using two scoops....but the transition to "clear" is still yellowish

The OCLT directions say the standard is no more than 1 PPM change over night. But at high levels, when shocking (in my case around 40 PPM would the standard change?
Example: If I do the first OCLT with an FC of 4, and it is 3 in the morning, that is 25% reduction in FC
If I do an OCLT with 40 PPM and the morning test is 37, that is appx 8% reduction.????


I also noticed that my CH is 700. That was unusual. I think the city has done something to their water supply. I checked the city water today and it has a CH of just over 600!
What to do with fill water that comes at 700?

 

[JasonLion]

Performing the FAS-DPD chlorine test when FC is high is a bit more difficult. If you have properly mastered the techniques required for using the test, then the overnight loss limit is still 1.0, as the test is very repeatable even at high FC levels. One of the keys to testing high FC levels is to move along quickly, adding drops rather rapidly especially near the start. If the test takes to much time the result can read higher than actual.

High CH fill water is manageable, up to a point anyway. You can keep PH and TA lower to compensate for the high CH levels. If you have a lot of evaporation, you might want to think about topping off the pool with water run through a water softener.

 

[cood60]

Thanks for the CH refill tip, I was wondering how I can get the fill water with less CH
It's hard to count so fast as I miss a number when I blink...LOL