Getting ready to close - Shock FC measurement question

[zamazing]

I'm waiting for the water temperature to get below 60 before closing but getting all the chemicals ready. It's a pretty big pool (53K gallons) so I've picked up 3 quarts of Polyquat 60 and 15 gallons of 12.5% liquid chlorine. Because it's an SWG pool, I keep my CYA between 70 and 80, so according to Pool School my Shock FC level is between 28 and 31. I want to make sure that I've poured in enough chlorine to reach Shock FC level, but using the TF-100 kit, that means that I have to count up to 62 drops.

I'm bound to lose count by the time I get to 40 drops, so here's my question: Does it make sense to reduce the amount of water in the FC test to 5 ml instead of 10 ml, and count each drop as 1 ppm instead of .5 ppm? I know accuracy of the FC measurement will be reduced but is it enough to matter given that the pool is being closed and the FC level is going to need to drop anyway for the Polyquat 60 to go in?

 

[Texas Splash]

Your thoughts are correct. You could modify the measurement portion as described, or some might actually just add the amount of bleach required by the Poolmath calculator and press-on assuming the bleach will take the FC up to (or very close to) the estimated SLAM level anyways. Since you're not actually doing a true SLAM or performing an OCLT, you have some wiggle room with accuracy. But you do want to keep this in mind ... after you add all the bleach to get to your SLAM FC (i.e. 30), and let it mix for 24 hours, then you must let is drop naturally halfway (i.e. 15) before adding the Polyquat. Too much FC will breakdown the Polyquat. As for losing track during the high counting .... it's happened to me as well. Have a nice evening.

 

[zamazing]

Thanks Pat. I like your idea. I'm going to add the amount of liquid bleach suggested by Poolmath, wait 2 days, and then test for an FC of 15 before adding Polyquat. No point wasting TF-100 drops...

 

[MarianParoo]

If you are the over-kill type like me, 24 hours after adding the polyquat, I raise my FC back up to shock level and then close.

 

[zamazing]

That's interesting. I've read that adding Polyquat to water with high FC lowers it's effectiveness. Are you thinking that raising the FC after the polyquat has already been added doesn't? If this is the case, wouldn't it make sense to avoid the initial step of getting the FC to shock level, and just add the polyquat first, wait 24 hours, and then raise the FC to shock level?

 

[Texas Splash]

Zamazing, here's a Chem Gook quote from a previous Poly thread:

If you want to add Polyquat 60, do so after the chlorine level has dropped from SLAM levels. If you add it with high chlorine levels, then the chlorine will just oxidize the Polyquat faster. Though that doesn't make it completely ineffective (it mostly breaks up the polymer into smaller pieces), it would be better to wait until the chlorine level is lower.

A previous thread discussion: Polyquat algaecides: Mode of Action

Since the closing instructions simply say to add Poly 60 after the FC has dropped, I would simply stick to that.

 

[MarianParoo]

One reason for raising FC to slam level first is that polyquat will reduce your FC. You don't want to risk being at low FC levels even for a bit before closing. Another reason is that you want slam levels prior to polyquat to make sure that there is not a trace of potential algae in the pool. Some people don't wait the 24 hours after adding polyquat to raise back up their FC. The benefits are minimal, but I will take any benefit I can! I love opening clear and balanced! Some people also don't bother raising their FC back up to shock level. I do. Like I said, over-kill for me!

 

[zamazing]

One reason for raising FC to slam level first is that polyquat will reduce your FC. You don't want to risk being at low FC levels even for a bit before closing. Another reason is that you want slam levels prior to polyquat to make sure that there is not a trace of potential algae in the pool.

That makes a lot of sense! Thanks for the clarification. If I have any liquid chlorine left after getting the FC to SLAM level the first time, I might just throw it in after the Polyquat for extra insurance. The stuff is going to degrade over the winter anyway.

 

[MarianParoo]

Sounds like a great plan! Good luck!!!