Calculating shock level - conflicting chart / calculator?

Steverino

LifeTime Supporter
Aug 18, 2009
73
Central PA
#1
Hi all,

Opened our pool last weekend & found very green walls & floor, but fairly clear 54 degree water. Usually when I close I throw in the floater with chlorine pucks, but last fall I tried Polyquat instead. Maybe a bad move... in the past I've never had much algae on opening. :confused:

Anyway, I got the pump fired up, walls & floor brushed, and began SLAM with 10% Walmart bleach. My CYA was/is about 50, and I figured I shouldn't bring it up to SWG level until the SLAM is done. So the Pool Calculator says with CYA at 50, shock level should be 16. However the Pool School Chlorine/CYA Chart shows a shock of 20 for CYA of 50 (non-SWG; the SWG chart doesn't go down to 50).

My question is - should I be trusting the Pool Calculator shock of 16 for my circumstances? Or do I need to go higher?

The pool is looking really good, and I did pass OCLT Thursday morning but ran SLAM one more day for good measure and failed it this morning... So that made me question my process a bit.

Any thoughts appreciated... Thanks!
 

tim5055

Mod Squad
TFP Expert
LifeTime Supporter
May 11, 2014
10,094
Franklin, NC
#2
Hi all,

Opened our pool last weekend & found very green walls & floor, but fairly clear 54 degree water. Usually when I close I throw in the floater with chlorine pucks, but last fall I tried Polyquat instead. Maybe a bad move... in the past I've never had much algae on opening. :confused:

Anyway, I got the pump fired up, walls & floor brushed, and began SLAM with 10% Walmart bleach. My CYA was/is about 50, and I figured I shouldn't bring it up to SWG level until the SLAM is done. So the Pool Calculator says with CYA at 50, shock level should be 16. However the Pool School Chlorine/CYA Chart shows a shock of 20 for CYA of 50 (non-SWG; the SWG chart doesn't go down to 50).

My question is - should I be trusting the Pool Calculator shock of 16 for my circumstances? Or do I need to go higher?

The pool is looking really good, and I did pass OCLT Thursday morning but ran SLAM one more day for good measure and failed it this morning... So that made me question my process a bit.

Any thoughts appreciated... Thanks!
First, you are correct - do not make any upward CYA adjustments until the is over.

Use the [FC/CYA][/FC/CYA] for calculations. For now the SWG section is not applicable as you are manually chlorinating the pool.

When you say Pool Calculator, are you using our or the old calculator named Pool Calculator (not controlled by TFP)?
 

Jezza

LifeTime Supporter
TFP Guide
Jan 18, 2015
969
Bunbury, WA, Australia
#3
That's right. You need to use 20FC for a CYA of 50 to SLAM.
Not sure if you have done it yet but it is best to turn off the SWG for a SLAM and it absolutely must be off to successfully conduct an OCLT.
 

Steverino

LifeTime Supporter
Aug 18, 2009
73
Central PA
#4
Thanks Tim. Actually I was using the old Pool Calculator - I didn't realize there was a difference between that and PoolMath, as they look very similar. HOWEVER, I just put my CYA, etc. in PoolMath and it says I should be at 20. Aargh, blast and tarnation. I guess I've been somewhat under-shocking for at least some of my SLAM. (Since we both work full time, I've been taking the level a bit higher in the mornings (over shock) as I can't monitor during the day.)

Guess who's going back to WM for yet more 10%? Me!

Thank you for setting me straight.

- - - Updated - - -

Jezza - I wondered about turning off the SWG, however had figured that with the water barely at 60 it wasn't generating any significant chlorine - especially overnight. But I will make sure it's off tonight. Thanks!