Taylor FAS-DPD inconsistent results?

[ImMikeJones]

Trying to SLAM my pool and I've been measuring readings throughout the day and adding liquid chlorine as needed. Tried to do an overnight test and saw that I lost 3.5ppm FC overnight. Dang.
Shock some more and then test last night to get the baseline for OCLT and got 15ppm FC (target is 16). Just out of curiosity I tested it again from the same cup of pool water and I got 13ppm... What am I doing wrong?

Obviously it needs to be repeatable for me to trust the results. I did this in my kitchen with plenty of light and I was VERY careful to make sure the measurements were correct. The only thing I can think of is that the DPD powder doesn't seem to ever dissolve fully? I mean it's like 99% dissolved but I can still see a few specks floating around no matter how much I stir. I am manually stirring btw.

 

[mknauss]

If you see a few specks of undissolved DPD powder, you have added enough.
Clean your test vial with alcohol or plain bleach.

There will be some variation of FC results. There is inherent error in the test. Do not stress over it.

 

[Bperry]

Trying to SLAM my pool and I've been measuring readings throughout the day and adding liquid chlorine as needed. Tried to do an overnight test and saw that I lost 3.5ppm FC overnight. Dang.
Shock some more and then test last night to get the baseline for OCLT and got 15ppm FC (target is 16). Just out of curiosity I tested it again from the same cup of pool water and I got 13ppm... What am I doing wrong?

Obviously it needs to be repeatable for me to trust the results. I did this in my kitchen with plenty of light and I was VERY careful to make sure the measurements were correct. The only thing I can think of is that the DPD powder doesn't seem to ever dissolve fully? I mean it's like 99% dissolved but I can still see a few specks floating around no matter how much I stir. I am manually stirring btw.

Make sure you put enough powder in the sample. If it all dissolved, then it’s probably not enough. Then make sure your dropper bottles are vertical and the drops aren’t being squeezed out artificially fast.

 

[RLP]

Get the speed stir. It's totally worth it. Takes a LOT of inconsistency out of the process.

 

[ImMikeJones]

If you see a few specks of undissolved DPD powder, you have added enough.
Clean your test vial with alcohol or plain bleach.

There will be some variation of FC results. There is inherent error in the test. Do not stress over it.

So it's okay that there are some specs at the bottom? I have just been trying to get the little scoops perfectly level so I know I have enough. I stress because two "identical" tests showed 2ppm difference and one of the criteria for a SLAM is no more than 1.0 ppm loss overnight right? I am trying to figure out if what I have going on in my crystal clear pool is algae or not. It does seem to accumulate when the pump is running even with FC at 13ppm minimum for multiple days with good CYA and PH. I get that stuff could start growing again after the chlorine level gets low, but this has been pretty high. I would have thought it would kill ANYTHING growing in there.

 

[mknauss]

So it's okay that there are some specs at the bottom?

yes.

I have just been trying to get the little scoops perfectly level

Do not stress over it. Just get a scoop of powder and put it in the water sample.

I stress because two "identical" tests showed 2ppm difference and one of the criteria for a SLAM is no more than 1.0 ppm loss overnight right?

Sure. Your SLAM FC level is low (12ppm?) and thus the error is low.

I would have thought it would kill ANYTHING growing in there.

Algae is very good at creating a biofilm to protect itself. You must disrupt it to kill it. That is why brushing, etc is necessary.

 

[ImMikeJones]

If you see a few specks of undissolved DPD powder, you have added enough.
Clean your test vial with alcohol or plain bleach.

There will be some variation of FC results. There is inherent error in the test. Do not stress over it.

Didn't think to do this. I was just rinsing it in the sink after. I've been doing some reading and apparently you don't need to be that precise with the powder. You can do one big heaping scoop or two small scoops. As long as it turns that color pink it's good to go. I think the second test might not have had enough powder? I don't know.

 

[ImMikeJones]

Algae is very good at creating a biofilm to protect itself. You must disrupt it to kill it. That is why brushing, etc is necessary.

I have been brushing pretty regularly, usually when I add chlorine. There are a couple spots I know to look for that seem to get this stuff at the bottom regularly. Those spots I scrub the Crud out of with the brush and yet it still returns.

 

[wireform]

Didn't think to do this. I was just rinsing it in the sink after. I've been doing some reading and apparently you don't need to be that precise with the powder. You can do one big heaping scoop or two small scoops. As long as it turns that color pink it's good to go. I think the second test might not have had enough powder? I don't know.

Rinse with the same pool water you will be testing otherwise residual tap water will change the outcome.

 

[PoolStored]

I stress because two "identical" tests showed 2ppm difference and one of the criteria for a SLAM is no more than 1.0 ppm loss overnight right?

Get a smart stir:

TFTestkits.net

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tftestkits.net

My brother, who owned a pool for 30 years came to visit. We did back to back FC test comparison, me with smart stir, him swirling. His results were 1ppm higher. Smart stir makes the mixing of chemicals consistent.