Help Constistenty Different Readings TF-100

[mare49]

Hi, Im new to this site, and sooooo glad i found it. To make a long story as short as possible......

Pool: 20x40 (8ft deep end) Ph and TA have always been within range, CH 150, (pool store told me my phosphates were high range)

High CYA 120+ (to late in season to drain and fill, pool will be closed soon, well water a pain too. I did try draining/fill about 6in, then had to deal with metals mixed with high chl )

Green algae tackled and hopefully I won ( 21 gallons of 12.5% bleach added at 3-6 gallons at a time over 2 weeks, very pretty and clear today)


My problem is I tested the FC (FAS-DPD) twice, and got different readings so i tested again and again and still... all different readings. I did this in the evening and the early morning. I am using the TF-100, (thank you for fast delivery). My FC readings go from 17 to 24, which when im doing the overnight test, i dont know which one is right or what to compare. Though after each reading when i add the 5 drops to test for CC, the water stays clear, which is good right, but if i keep getting different FC numbers when i triple check myself, how will i know if i am truly in range of FC 9-15 (Is that correct range for CYA of 120) What am i doing wrong??

 

[Beamup]

The usual reason for FC readings to be inconsistent (especially at high values) is an insufficient amount of the R-0870 powder. You should add enough of that so that the sample turns hot pink and there are some undissolved crystals at the bottom.

Essentially, as I understand it, a particular amount of R-0870 can only test for a particular amount of FC. If your FC is higher than that, the test result will anyway be "capped" by the amount of powder used. The undissolved crystals are a good signal that there's enough, because if there wasn't then the FC and powder would react and the crystals dissolve.

Edit: Fixed R-0870 vs. R-0871 typo, and some additional info from the Extended Directions thread:

The exact amount of R-0870 powder used is not critical. The goal is to add more than you really need rather than using too little. Using too little R-0870 powder can throw off the results of the test. You need to use enough to bind to all of the chlorine that is present. Adding extra, within reason, has no effect. At very high FC levels it is likely that you will need to use more than the normal amount.

 

[mare49]

Hi, thanks
I did add enough R-870 powder as I did read that too in the extended direction. The "TEST KITS COMPARED" said the TF100 tests up to 50ppm, which i thought was great and why i bought it.
Well, i tried testing again this afternoon and this time I tested 4x (R-8071 # of drops, 28, 29, 31, 29) So, i am goin with the 29 since i got that # twice (29 x .5 = 14.5) CC was still zero.
Now, am i correct in saying that since I want to maintain my FC between 9-15, and that since im now at FC of 14.5 , (2:30pm and the temp is sunny and 95) and if by tonight when i test again, and say for example the FC tests at 11, then I want to add about 3qts 3 cups of 12.5% Liquid Chlorine to get my FC back up to 15?? Im trying to make sure im using the Pool Calculator correctly.

NOTE: I did turn off and empty my in line tab feeder since my CYA 120

 

[Beamup]

The uncertainty is 1 drop or 10%, so results between 28 and 31 are completely consistent. The 17 to 24 you mentioned in your first post is more difference than one would expect, and what I suspected as inadequate powder.

For the numbers, I put in 20'x40'x6' (guessing at that as an average depth) which is about 36k gallons. For which raising FC by 4 ppm requires 1 gallon 2 cups of 12.5% (143 oz). That's close enough to your figure to be accounted for by a different guess on average depth. If I guess 5' and therefore 30k gallons I get what you do.

 

[mare49]

Thanks so much,

Now, am i correct to say...as long as i keep the FC within 9-15 range, i will only have to shock if my CC is 1ppm or greater, and then i would have to bring it up to at least 46? Im basing that number on the Chemgeek chart using the CYA of 120. I would use the Calculator to determine how much to add.

And..., though my water is clear, i still still a thin film on top, which i didnt even notice till I looked in the skimmer. Is that dead algae or existing??

 

[frogabog]

The film could be oils (sunscreen, body oils, etc.).

There are a number of reasons to shock without bad test results, but normally you'll see one of the following testing values that will tell you to shock:

• 5ppm or greater CC[/*:m:x51qisht]
• An overnight chlorine loss of 1ppm or more[/*:m:x51qisht]
• Consistently or regularly falling below your minimum when dosing to the high target the previous evening[/*:m:x51qisht]
• A 0ppm FC result[/*:m:x51qisht]

Your eyes also will tell you to shock:

• Cloudy water not related to calcium or simple dirts/pollens[/*:m:x51qisht]
• Any green tint to the water not related to copper[/*:m:x51qisht]
• Slimy walls and floor (this can also form a film)[/*:m:x51qisht]
• Green staining[/*:m:x51qisht]
• Green in niches and corners, lights, skimmer, pump, stairs/ladder, etc.[/*:m:x51qisht]

Insofar as your test results, to save yourself from stress just do the FAS-DPD test once and go with your first result. There's a margin of error, but if you never bother to test it you won't know what it is and can ignore it.

ChemGeek's Chlorine/CYA chart is a little lower than the pool calculator's levels at the 120ppm CYA level so in this case, yes use the more conservative values of 9 and 15. That way you'll have less risk of falling below minimum.