Need Help with Inconsistent FC Reading

[Feernot]

First of all, I haven't added chlorine since May 12, 4:45pm. The pool is pretty and clear and I've just been waiting for the SLAM levels to drop. However, I need help with my inconsistent FC readings.

I bought my 2024 TF-Pro refill kit at the end of March and have been using it since opening on April 30th. I also bought another bottle of R-0871 FAS/DPD reagent on 5/10/2024. I keep all my reagents indoors in my TF-Pro case and use one of the old plastic containers they come with to pull my water sample from an arm-length down in the water. I've been doing the same routine since 2021, but this week I noticed very inconsistent FC readings. I've listed my details below. In summary, yesterday I got a FC that was 13.0 (a drop from 13.5 the previous day), then a retest of 16.5, and another retest of 14.5. Today I got a result of 13.5 and then did an "insanity" pool store check that said 10.32.

Is there any chance my TF-Pro refill R-0870 DPD Powder is causing a problem with my tests? I did note it was not as "powdery" this year and had some bigger chunks than I remember last year. Any other ideas? I use the Smart Stir every time and give it plenty of time to dissolve. I guess it is possible during the SLAM I would pour my R-0871 drops too quickly. But during today's test I was very careful and deliberate to ensure I did everything accurately.

Today's Numbers
FC = ???
CC = 0.0
CYA = 30
pH = 7.7
TA = 60
CH = 25
BOR = 40

Timeline of Inconsistent Readings:

Monday May 13, 5:23pm: FC = 13.5

24 hours later

Tuesday May 14, 4:53pm: FC = 13.0

It was a cloudy overcast day, but I was still surprised by only a .5 drop in FC. So I tested again using the same process.

Tuesday May 14, 4:56pm: FC = 16.5

Wait...what? How did I mess up doing this? So I opened my newest bottle of R-0871 FAS/DPD reagent I purchased on 5/10/2024. I bought a new one just because I used a lot during my 11 day slam. So I tested again.

Tuesday May 14, 5:02pm: FC = 14.5

I thought perhaps I waited too long after collecting my sample to test and something was off? So I waited to test again the next day.

Wednesday May 15, 12:15pm: FC = 13.5

I used my new bottle of R-0871 and this time was getting 13.5. The sun is shining brightly and I was again confused by only dropping 1ppm. So I went to the pool store just to see their results for a sanity check.

POOL STORE Wednesday May 15, 12:45pm: FC = 10.32

 

[Bperry]

First of all, I haven't added chlorine since May 12, 4:45pm. The pool is pretty and clear and I've just been waiting for the SLAM levels to drop. However, I need help with my inconsistent FC readings.

I bought my 2024 TF-Pro refill kit at the end of March and have been using it since opening on April 30th. I also bought another bottle of R-0871 FAS/DPD reagent on 5/10/2024. I keep all my reagents indoors in my TF-Pro case and use one of the old plastic containers they come with to pull my water sample from an arm-length down in the water. I've been doing the same routine since 2021, but this week I noticed very inconsistent FC readings. I've listed my details below. In summary, yesterday I got a FC that was 13.0 (a drop from 13.5 the previous day), then a retest of 16.5, and another retest of 14.5. Today I got a result of 13.5 and then did an "insanity" pool store check that said 10.32.

Is there any chance my TF-Pro refill R-0870 DPD Powder is causing a problem with my tests? I did note it was not as "powdery" this year and had some bigger chunks than I remember last year. Any other ideas? I use the Smart Stir every time and give it plenty of time to dissolve. I guess it is possible during the SLAM I would pour my R-0871 drops too quickly. But during today's test I was very careful and deliberate to ensure I did everything accurately.

Today's Numbers
FC = ???
CC = 0.0
CYA = 30
pH = 7.7
TA = 60
CH = 25
BOR = 40

Timeline of Inconsistent Readings:

Monday May 13, 5:23pm: FC = 13.5

24 hours later

Tuesday May 14, 4:53pm: FC = 13.0

It was a cloudy overcast day, but I was still surprised by only a .5 drop in FC. So I tested again using the same process.

Tuesday May 14, 4:56pm: FC = 16.5

Wait...what? How did I mess up doing this? So I opened my newest bottle of R-0871 FAS/DPD reagent I purchased on 5/10/2024. I bought a new one just because I used a lot during my 11 day slam. So I tested again.

Tuesday May 14, 5:02pm: FC = 14.5

I thought perhaps I waited too long after collecting my sample to test and something was off? So I waited to test again the next day.

Wednesday May 15, 12:15pm: FC = 13.5

I used my new bottle of R-0871 and this time was getting 13.5. The sun is shining brightly and I was again confused by only dropping 1ppm. So I went to the pool store just to see their results for a sanity check.

POOL STORE Wednesday May 15, 12:45pm: FC = 10.32

You should have a few undissolved grains of powder for the test after mixing. If its all fully dissolved, theres not enough powder being added. Dont wait any extended time for things to dissolve. 10-15 seconds should be plenty.

 

[duraleigh]

The most common error in that test is not getting a consistent endpoint. You must continue to add drops until there is no FURTHER color change. Subtract that last drop from your total and that is the endpoint of your test.

The next most common is drop size inconsistency by the operator. Let each drop form on the tip and allow it to fall from the tip from gravity.....don't force it (it takes a little practice)

As long as the solution turns pink, you have added enough R-870 regardless if there are undissolved crystals or not.

There is no value in pool store results (as you found out).......save your gas.

Trust your own testing.

 

[Feernot]

@duraleigh Then I'm at a loss. I'm certainly not an expert, but this is my 4th season using these tests. I do follow the process as you've outlined it. With the Smart Stir light it is very easy to see when the pink is gone and you're clear. Bummer...

 

[Bperry]

@duraleigh Then I'm at a loss. I'm certainly not an expert, but this is my 4th season using these tests. I do follow the process as you've outlined it. With the Smart Stir light it is very easy to see when the pink is gone and you're clear. Bummer...

The test themselves have +\- 0.5ppm tolerance so tests within 1ppm variation os normal.

The oddball one is the 16.5ppm result (and the pool store of course). Maybe test you didn’t count the drops right or didn’t clean the vile pool water or had a little less water in that sample vs the others?

 

[mgtfp]

How certain are about the sample size? If you get that wrong by x%, then the FC result will be wrong by x% (on top of all other test uncertainties). Did you use the same test tube? If not, then check that the graduations are consistent on both. You can try using a syringe to measure sample sizes to rule out that effect.

 

[newdude]

Then I'm at a loss

Actually it was a gain.

Did you use the same test tube?

+1. I learned right away that my original vials were a skootch off. I could consistently hit with any of them but when I rotated which one I used it got wonky.

 

[Feernot]

I did order a new "chlorine only" test vial that I believe I swapped in, that might be the discrepancy. I didn't think about it because I assumed the 10ML would still be accurate between the two.

This morning I got a 10.5 reading.

I'm careful getting my eye level with the 10ML mark and being spot on each time. I'm guessing I got so used to dropping the R-0871 FAS/DPD reagent at high intervals from the SLAM that I was not being as careful and added too much per drop.

Anyways, from what I'm hearing there is no reason to believe my reagents are the problem so it MUST be user error.

 

[Feernot]

The test themselves have +\- 0.5ppm tolerance so tests within 1ppm variation os normal.

The oddball one is the 16.5ppm result (and the pool store of course). Maybe test you didn’t count the drops right or didn’t clean the vile pool water or had a little less water in that sample vs the others?

I appreciate the suggestions. I always wipe the vial with a fresh paper towel between tests.

 

[Feernot]

How certain are about the sample size? If you get that wrong by x%, then the FC result will be wrong by x% (on top of all other test uncertainties). Did you use the same test tube? If not, then check that the graduations are consistent on both. You can try using a syringe to measure sample sizes to rule out that effect.

I actually do think I used a separate test vial, as I ordered a new one recently. I didn't even think there might be a variance between those vials, but that is a variable I introduced. Thanks for the suggestion.

 

[proavia]

• Clean the test vial with rubbing alcohol.
• Go to the local drug store and get a 10ml syringe for giving kids medicine. Use this syringe to do the test and to verify the markings on your test vial.
• Hold the reagent bottle perfectly vertical, allow the drops to fully form on the tip - do not force them off the tip. Dispense one drop about every second.
• Be sure the pump is running for 30 minutes or so before grabbing your water sample.
• Take the water sample from the same location on the pool perimeter each time.
• Be precise and consistent.

 

[newdude]

so it MUST be user error.

None of us are a machine. The sample sizer helps with the volume, but you still have to nail repeated drops, which gets harder the higher the FC, especially during high CYA slams. Did you really make 56 perfect drops ? Every last one ?

If given the choice, I will take discrepancies in the teens any day of the week. Being nowhere near minimum, we have all the time in the world to figure it out or watch it eventually correct itself.

 

[Feernot]

Thanks again everyone!