FC test results - Taylor K-2006

[kjbarth]

Tested hot tub water yesterday morning 8:45.
FC - 1.4
CC - .6
pH - 7.2
TA - 40
CH - 70
CYA - 70

Added bleach to increase FC, baking soda to increase TA, calcium chloride to increase CH (not at the same time, TA and CH about 4 hrs apart).
Tested again at 2:30.
FC - 3.0
CC - .4
pH - 7.9
TA - 80
CH - 140
CYA - 70

Then decided to shock the water and added 12 oz bleach to bring FC to 20 per "The Pool Calculator".

Here's my question... When I tested the water last night to see what the FC level was, I had some trouble getting an accurate reading. I normally add only 1 dipper of DPD to do FC test. When I added the 1 dipper of DPD, the water (25ml) in the tube turned pink but after swirling for about 10 secs, water turned clear. So I added a 2nd dipper of DPD and water turned and stayed pink. Was a little concerned about this but kept testing. Began adding R0871. Expected FC to be below 20. Added 100 drops of R0871 but water in tube never got clear - slight pink tinge remained from 80 drops R0871 and color didn't change after adding 20 more drops (so I stopped). How accurate is FC test in kit when FC level is fairly high? How high can I accurately test FC level with this kit? I'm not sure what the FC level was and I wanted to make sure the FC level stayed high so I probably did a stupid thing by adding another 8 oz bleach.

Tested water again this morning and I couldn't get an accurate FC reading. Again had to add 2 dippers of DPD to 25ml water to get test sample to stay pink and added over 100 drops of R0871 and water never got clear. Since water sample never got clear, couldn't do CC test.

 

[FamilyGuy]

It will test reliably up to 50 ppm. Try using the 10 ml sample it will save reagent and give accurate enough results

 

[duraleigh]

added over 100 drops of R0871 and water never got clear.

Based on what you post, I would strongly suspect your FC is over 50 ppm. Yikes! (how old is the reagent you are using?)

 

[kjbarth]

Just received reagent R0871 from TFTestkits about 1 month ago. Purchased DPD powder about 3 months ago.

 

[duraleigh]

They should certainly be valid then, unless thay have been left in the sun a day or so.

The key ingredient in your story is that you had to add 2 scoops to keep it pink......that always indicative of very high chlorine.

Any plausible explanation how it got there?

 

[kjbarth]

I added 12oz bleach at around 9am yesterday to shock water. Per Pool Calculator, for 450 gal hot tub, it recommended to add 12 oz bleach to raise FC from 3 to 20 (the shock level) - which I did. When I tested again around 9pm yesterday to see what the FC level was and determine whether I needed to add more bleach to keep shock level up, I first had this issue of not being able to get an accurate FC reading. I didn't think the FC level would be over 20 after 12 hours shocking so I added another 8 oz bleach at about 9:15pm last night (probably a stupid decision on my part). But I assumed FC level would go down after 10hrs in sun and I wanted to keep shock level in water overnight.

 

[kjbarth]

Oops, sorry, mistake in my timeline.

Added 12oz bleach to shock at around 2:30pm yesterday, not 9am. Tested at 9pm after about 6 1/2 hrs of shocking and hot tub cover open and in sun for most of that time.

 

[kjbarth]

The key ingredient in your story is that you had to add 2 scoops to keep it pink......that always indicative of very high chlorine.

Question on this. I thought the DPD powder turned water sample pink only if there was chlorine in the water sample. I've read in other posts on this site that if 1 dipper of DPD powder turns the water sample pink, a second dipper isn't necessary even though the instructions call for 2 dippers in the 25ml water sample. I had no idea that if there is too much chlorine in the water sample that the sample can turn pink initially and then go back to clear. Am I reading and understanding this correctly?

 

[chem geek]

You are correct that additional scoops are only needed if you get a "flash of pink" that goes clear and that this means the chlorine level is high. Basically, at high FC levels, the chlorine bleaches out the indicator dye, but if you add more dye then there will be enough to show the pink even at higher FC levels. Technically, even with one scoop that bleaches out at high FC, you could add FAS-DPD drops and the color will return at some point before eventually going away again (you count all the drops, including those when there is no color before color returns), but since that's confusing to people the instructions are to add more powder until you get a stable color.

 

[kjbarth]

Just an update. After letting hot tub sit uncovered in sun all day Saturday, I tested FC on Sunday morning. FC now 6.2 so water in hot tub is fine.

I tested on Sunday morning twice, once using 10ml water sample, 2nd time using 25ml sample. FC was 6 with 10ml sample and 6.2 with 25ml sample. When doing 25ml sample test, added 2 DPD dippers, water turned pink then turned a little paler pink. When I added the R0871 reagent, I added 5 drops of R0871 and the water sample turned bright pink again (which I didn't expect) and then as I added more drops of R0871 the sample turned progressively paler (which I did expect). Could this be an indication that the DPD power is stale or bad?? I bought it 3 months ago as part of the Taylor K-2006 kit though I have no idea how old the kit was when I received it.

 

[FamilyGuy]

The dpd powder may turn darker and clump up but wont affect the test (to the best of my knowledge) edit: read the post directly above your last. It explains your post

 

[chem geek]

Also, the 25 ml sample generally needs about twice as much powder so normally two level scoops (I usually use one heaping scoop). And as noted in my post before yours, what you saw is normal when the FC is high. If you don't like seeing the sample get darker before it gets lighter, add another scoop or two of powder.