One annoying little number...

[UnderWaterVanya]

Well, looks like not enough powder is looking very suspect! I am still very surprised no one else has run into this problem, especially since my levels are based on a CYA of 50, which granted is at the high end, but still considered "normal".

See post number three in this thread: extended-test-kit-directions-t25081.html?hilit=extended test

It recommends 1 heaping or two level spoonfuls of the powder.

 

[ecowolf]

Hmm. Well, I did two level spoonfuls on all tests. Still, considering the massive amount of FC in my samples every time, maybe that still wasn't enough.

 

[jblizzle]

Hmm. Well, I did two level spoonfuls on all tests. Still, considering the massive amount of FC in my samples every time, maybe that still wasn't enough.

That is for normal FC levels. You need more at high levels (maybe this should be made more clear):

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.

 

[UnderWaterVanya]

Hmm. Well, I did two level spoonfuls on all tests. Still, considering the massive amount of FC in my samples every time, maybe that still wasn't enough.

Doh - sorry I forgot to point out that was for normal FC not high FC. I also thought you were saying you had only used 1 level spoonful - my mistake.

 

[ecowolf]

Yes, if I had read this article before testing, I might have avoided this. I didn't know the article existed, though. =/

 

[UnderWaterVanya]

Yes, if I had read this article before testing, I might have avoided this. I didn't know the article existed, though. =/

Don't feel bad - it isn't something I found for a while either. I'm a sponge and I ran across it one day and remembered.

 

[ecowolf]

Perhaps adding a link in Pool School to the "test kit extended directions" article would be helpful to many.

 

[jblizzle]

Perhaps adding a link in Pool School to the "test kit extended directions" article would be helpful to many.

Earlier I actually bought that up to the other moderators after this thread. So maybe that will get done.

Posted from my Droid with Tapatalk ... sorry if my response is short

 

[ecowolf]

Well, here are the results from last nights OCLT, making sure that I used plenty of powder so there was a small amount of undissolved powder on the bottom:

Last night: 2 tests. Both counted 31 drops. FC=15.5 ppm

This morning: 4 tests. Test one: 28 drops FC=14ppm // Test 2: 30 drops FC=15ppm // Test 3: 28 drops FC=14ppm // Test 4: 30 drops FC=15.

I'm not sure what to make of this anymore. Maybe this much variation at high FC is normal? And why does the inconsistency seem to show up in the morning tests only?

I'm really baffled.

 

[ecowolf]

Hmm...I just remembered this quote from the extended directions article:
"The precision of the measurement is plus or minus one drop when up to 10 drops of titrant are used, or plus or minus 10% of the final reading, when more than 10 drops of titrant are used."

If this is true, then my expected variation would be around 1.5 ppm. That's almost exactly what it was! 14 - 15 ppm or a variation of 1.
Now, at yesterday's levels of 20-24, my variation would have been expected to be 2 ppm to almost 2.5 ppm. That one doesn't match as much. I was getting everywhere from 0-4 variation. Wait, though: If my "true" reading were 20, variation would read it anywhere from 18-22. If my true reading were 24, variation would read it anywhere from 22-26. Those numbers do cover the range of readings I was getting! Hmm...

And it is interesting that the variation seems to have become smaller as I have dropped down in FC

I am developing two possible theories here:
1. Is all this just from inherent non-precision in the test due to the higher FC I have been at?

2. Could the imprecision possibly be increased by a drop in water temperature? I was thinking about how the variation seems to be so much worse in the morning, and I thought: what is changing overnight in our pool? Temperature was the only thing I can think of. Our temperatures here in this part of CA can drop almost 40 degrees every night. The pool usually loses anywhere from 3-5 degrees overnight.

Just some theories.