FAS-DPD test consistency

[joninnj]

I would have thought that I would know and understand this concept by now having (successfully) owned a pool and used the TFP methods for 5 years, but my recent SLAM/OCLT efforts have led me to question how to use the FAS-DPD test with consistency. During my most recent OCLT attempt (last night), I did three FAS-DPD tests, all using the Speedstir, at both sunset and sunrise with the following values (CYA 60):
Sunset: 33.5, 31.0, 29.0
Sunrise: 24.5, 29.0, 27.5

From looking at old threads here, people have brought up different variables that could affect test consistency:

1. Volume of water tested: this clearly affects results, but I have been pretty consistent to carefully use 10 ml.
2. R-0871 bottle orientation and pace of drops: this is where I have not been consistent. I recently started to make sure I was holding the bottle vertically, but it is not clear how to do the drops, particularly when I have to do so many with each test. I have seen comments on not squeezing at all, but that doesn't seem to work even with a full bottle. Is there something magic about 1 drop/second? It is hard for me to be patient with that many drops! What are clear best practices for this?
3. Amount of DPD powder used: I have been using a heaping scoop but have seen some posts that suggest using 2 full scoops.
4. OCLT test level: does it matter if and how much I overshoot the SLAM level?
5. Starting CYA: are there any different considerations when testing at FC levels of 30+?

I am headed out of town for a few days but want to try the SLAM/OCLT again when I return (will turn on SWG again, pool is completely clear). Exactly what should I do to ensure consistent FC tests?

 

[Texas Splash]

One thing I learned years ago from one of the chemist experts here when I did my first SLAM is that if you over-shoot your SLAM FC level too much, it can influence the results. In your case with a CYA of 60, your FC SLAM level should be 24. Once you go too high, testing may get a little skewed. That's exactly what happened to me. So the first thing I would say is let the FC fall to the normal FC SLAM level of about 24 next time you do the tests. The amount of powder is not that critical. A generous, heaping single scoop for the 10 ML sample size is fine. The 2-scoop language comes from the Taylor 25 ML sample size. If you've done everything else correctly such as getting the pool water from the same location, water has been mixing well before getting the sample, etc, then everything should work fine. Let us know how it goes.

 

[Jimrahbe]

Jon,

Are you using the speed-stir???

One large scoop of powder is all I ever use..

Thanks,

Jim R.

 

[joninnj]

Yes, forgot to mention that I use the Speedstir.

And @Texas Splash , thanks for the comments on overshooting SLAM level. I had meant to ask that and will add to my OP.

 

[joninnj]

One thing I learned years ago from one of the chemist experts here when I did my first SLAM is that if you over-shoot your SLAM FC level too much, it can influence the results. In your case with a CYA of 60, your FC SLAM level should be 24. Once you go too high, testing may get a little skewed. That's exactly what happened to me. So the first thing I would say is let the FC fall to the normal FC SLAM level of about 24 next time you do the tests. The amount of powder is not that critical. A generous, heaping single scoop for the 10 ML sample size is fine. The 2-scoop language comes from the Taylor 25 ML sample size. If you've done everything else correctly such as getting the pool water from the same location, water has been mixing well before getting the sample, etc, then everything should work fine. Let us know how it goes.

@Texas Splash , do you think the drop rate issue would be partially or mostly mitigated by starting the OCLT at FC at or closer to the SLAM level?

 

[Texas Splash]

do you think the drop rate issue would be partially or mostly mitigated by starting the OCLT at FC at or closer to the SLAM level?

It shouldn't be an issue. We're careful not to drag it out too long between drops, and of course don't want to force or squirt the drops in too fast. We also don't over-analyze the process with a stopwatch or anything, but a simple, steady flow of vertical drops about every .75 to a second or so is the general timing of drop application.

 

[JoeSelf]

Dave has always said that you want the drops to fall off the tip.

 

[joninnj]

Dave has always said that you want the drops to fall off the tip.

I found that even with a new full bottle vertically held, I needed to apply a little pressure. I suppose a drop might eventually come out, but with a FC of 24+, that would take forever!

 

[joninnj]

It shouldn't be an issue. We're careful not to drag it out too long between drops, and of course don't want to force or squirt the drops in too fast. We also don't over-analyze the process with a stopwatch or anything, but a simple, steady flow of vertical drops about every .75 to a second or so is the general timing of drop application.

For more data, here are the OCLTs I have done this week when I have confidence that the pool is fulls scrubbed and clean (I cleaned behind the light on the first day of this series):.
FC sunset and sunrise
Day 1: 34, 27.5
Day 2: 27.5, 25
Day 3: 28, 25
Day 4: the 3 tests each I listed in the original posts

Would you think the 27 and 28 are high enough (beyond the 24 target FC for CYA of 60) to affect the test?

 

[JoeSelf]

I found that even with a new full bottle vertically held, I needed to apply a little pressure. I suppose a drop might eventually come out, but with a FC of 24+, that would take forever!

Yes, slight pressure. So the drop forms, then falls.