Why would FAS-DPD test give different results

[Bperry]

For the last two days, when I do the FC test, I got a much lower than expected FC result, but when I do the test again the result is more inline which what I expected.

Yesterday first test was FC 2, second test was 4.

Since 4 is my bottom number, I turned up the SWG and added a bag of cal hypo.

This morning, first test was 2ppm, second test was 7.5. The 7.5 was more what I expected.

So what would cause such varied test results only taken a few minutes apart?

 

[mknauss]

Did you use the same water sample for both tests? Have you cleaned your test vial recently?

 

[Bperry]

Did you use the same water sample for both tests? Have you cleaned your test vial recently?

Cleaned every day but not the same water sample. But the samples were taken from the same spot at the edge of the pool about 5 min apart.

 

[mknauss]

Hard to say. Was the SWCG on? How close to a return? How long was the pump running before. Etc. My method is I take the sample at the same spot each time. I also do it BEFORE the SWCG starts in the morning.

 

[Bperry]

Hard to say. Was the SWCG on? How close to a return? How long was the pump running before. Etc. My method is I take the sample at the same spot each time. I also do it BEFORE the SWCG starts in the morning.

It was early enough that the SWG wasn’t on yet, but the pump runs 24x7. The first time it happened, I chalked it up to sampling from a dead spot. But since it happened two days in a row, has got me thinking I may be testing it wrong.

 

[yann]

Are you using enough FAS-DPD powder to saturate your sample?
Not enough powder will give you false low FC results.

 

[Bperry]

Are you using enough FAS-DPD powder to saturate your sample?
Not enough powder will give you false low FC results.

I suppose that’s possible. I try to use two level scoops. I see there’s some bigger flakes of stuff in there, maybe my scoop was light or had an inert piece of powder in it. I’m getting towards the bottom of the container.