confused on FC reading

[run53]

So, I got my new TF-100 kit from Fedex yesterday and of course has to try it out right away.

So last night, I took a TC reading using the block and it was 0.5 (I opened the pool last weekend and just turned on the SWG yesterday morning to full power). The TC was 0 before that.

So, I decided that it would take too long for the SWG to boost the FC and I was worried about low FC as the air temp and sun were getting more intense (actually hit 90 deg in Massachusetts last weekend). So I decided to shock the pool using the pool calculator and bleach. Based on my numbers as measured by the TF-100.........

TC = 0.5
FC=0.5
CYA = 60
TA = 140 (this seems high)
PH = 7.6
CH 260

the pool calc suggested a shock FC of 18 and suggested to add (11) 96oz jugs of bleach to get from 0.5 to 18.

I wasn't sure I wanted to add that much Chlorine, so I only added (6) 96oz jugs.

I tested the chlorine in the morning (6:30am) and got........

TC=FC=15.5


Based on the calculator and adding only the 6 jugs, I was only expecting FC=10.5, assuming no FC loss over night. I am scratching my head to figure out how the heck FC still reads 15.5 in the morning. I'm kind of glad I didn't put the suggested (11) jugs in otherwise the FC probably would have been double. I am wondering if something went wrong either in the tests or using the calculator.

Also, this raised another question for me......what is a safe FC level for swimming (assuming CC=0)? In other words....how long after a shock should we not use the pool?

 

[mynewpool]

Are you sure you were using the correct pool gallons in the calculator?

 

[PaulR]

Was the SWG running overnight?
--paulr

 

[run53]

Are you sure you were using the correct pool gallons in the calculator?

I used 29,000 in the calculator. I got that number from the pool builder. I suppose it could be incorrect.

 

[run53]

Was the SWG running overnight?
--paulr

No, I turned it off altogether so it wouldn't interfere with the reading.

 

[mynewpool]

Are you sure you were using the correct pool gallons in the calculator?

I used 29,000 in the calculator. I got that number from the pool builder. I suppose it could be incorrect.

I would think it would be correct, but you could do your own measurements to figure that out.

I guess the way to tell is you could add bleach at night and if possible wait an hr and test to see what your real FC is. Then in the morning you can test again and get your number. I have only had a few issues with the pool calculator, but could be my fault as it is a saver and use it daily. So this would give you an actual number to work with when testing after the bleach was added.

I did not see or may have missed, but how is your water clarity? Another issue is, that if you have any type of organics in your pool, you could have hit the 18 that night and then the organics used up some of your FC. That is why you could do what I said above to get an exact number of where your FC is.

 

[Butterfly]

One gallon of bleach will raise 10,000 gallons of water approximately the percentage of the bleach in FC (i.e, one gallon of 6% ultra bleach will raise 10,000 gallons approx. 6 ppm FC).

You could use this to get an estimate and help confirm your gallonage.

 

[Butterfly]

Also, this raised another question for me......what is a safe FC level for swimming (assuming CC=0)? In other words....how long after a shock should we not use the pool?

Anywhere between your minimum and shock level based on your CYA level is fine for swimming.

TA = 140 (this seems high)

Yep, it seems high considering you have a SWCG.
When testing, did you wipe the dropper with a damp cloth in between drops?

 

[run53]

I did not see or may have missed, but how is your water clarity? Another issue is, that if you have any type of organics in your pool, you could have hit the 18 that night and then the organics used up some of your FC. That is why you could do what I said above to get an exact number of where your FC is.

The water is crystal clear.

 

[run53]

Also, this raised another question for me......what is a safe FC level for swimming (assuming CC=0)? In other words....how long after a shock should we not use the pool?

Anywhere between your minimum and shock level based on your CYA level is fine for swimming.


[quote:2npptceh]TA = 140 (this seems high)

Yep, it seems high considering you have a SWCG.
When testing, did you wipe the dropper with a damp cloth in between drops?[/quote:2npptceh]

I did read about wiping between drops and I did after "most" of the drops. But I'll admit that I missed one or two. Could it be that sensitive? I'll try it again.

 

[Butterfly]

I'm not sure, but I do not think that it is that sensitive.

Have you had probs with rising pH?

 

[run53]

One gallon of bleach will raise 10,000 gallons of water approximately the percentage of the bleach in FC (i.e, one gallon of 6% ultra bleach will raise 10,000 gallons approx. 6 ppm FC).

You could use this to get an estimate and help confirm your gallonage.

Yep....that's the problem. Using that math my gallonage is 18,500. But I doubt the builder would give me a figure that was off by more than 10,000 gallons. That leads me to believe I goofed up on the test somewhere.

 

[Butterfly]

Testing can take a little getting used to. :-D

Couple years ago I was testing my FAS/DPD thinking I had 10 ml.... nevermind.

Wanna walk us thru it and see if we spot something?

ETA: btw, the guy that replasterd our pool said it was about 24K. Our water meter later told us it was closer to 18K
Not saying .......just making conversation

 

[run53]

I'm not sure, but I do not think that it is that sensitive.

Have you had probs with rising pH?

Yes...it's a constant battle and I needed a lot of muriatic Acid last season to control it. This year, my pool store had me add 20 lbs of Alk-up because it started low (50 - based on pool store reading). Now it's reading 140 on TF-100. Doesn't make sense that it went that high with 20 lbs of Alk-up as the pool calc says it should take 38 pounds of Alk-up to go from 50 - 140.

Hmmmmm.....between this reading and the FC reading, I'm starting to think my gallonage is off. But if that's true, then the pool store (also pool builder) has been way over prescribing me for the last 3 years since 29K is what they have on record for me.

 

[run53]

Testing can take a little getting used to. :-D


ETA: btw, the guy that replasterd our pool said it was about 24K. Our water meter later told us it was closer to 18K
Not saying .......just making conversation

I'm starting to think you're on to something here....