I may have been testing wrong for 10 years...

joecoyle · Jun 23, 2018

Hi,

I would appreciate some clarity on procedure for the FAS/DPD test.

Normally I add one scoop of R0870 then add drops of R0871 until no pink remains. Then multiply by 0.5.

However, I just came across a TFTestKits video on Youtube - FAS/DPD Chlorine test for the TF-100 - YouTube

This one says to subtract the last drop and then multiply by 0.5.

So, I'm just wondering which is the correct procedure? I could have been overestimating my FC for 10 years

- as i'm now in the middle of a SLAM, so want to get it right.

Thanks
Joe

aussieta · Jun 23, 2018

you subtract the last drop that has no effect
when it changes to clear you count that drop, but the next drop will have no effect so is not counted

tomfrh · Jun 23, 2018

Count drops that have an effect.

the “subtract the last drop” assumes you’ve kept going until drops stop having an effect, in which case the last drop didn’t do anything and hence you should ignore it.

i suspect if you’ve been doing it for ten years you know when it goes clear.

Propbndr · Jun 23, 2018

I guess they are assuming you are adding an extra drop after tie sample turns clear. With as fast as they are adding r-0871 drops it would be easy to add to many drops. I add the drops slowly and really slow down once the color starts to clear. I only subtract one drop on any of the tests if I have a doubt of the final color change. With the fas test, even if you add one extra drop, your result is only only off by .5 ppm. That is a minuscule amount in the grand scheme of things.

ajw22 · Jun 23, 2018

+/- one drop will not affect your SLAM. Really doesn’t matter if you count it or not. You have many larger variables in play.

cfherrman · Jun 23, 2018

Your testing fine, it is more important to be testing the same over and over.

I personally don't do the extra drop even though you should as it keeps away testing error

tomfrh · Jun 23, 2018

I stop when it goes clear. I know what clear looks like.

Leebo · Jun 23, 2018

The addition of a drop isn’t as important with the Chlorine test but is vital for the other color changing tests such as TA or CH. The instructions apply for all, but I too tend to skip the extra drop on the chlorine test.