FC test seems to be meaningless, I'm about to drown myself!

[midijamm]

I have been having what seemed like testing inaccuracies for some time now. I figured maybe my methodology is wrong, maybe my measuring is inconsistent, maybe my reagents are bad……

So I got a speed stir and a sample sizer and new reagents from TFTest Kits and every test gives a different result.
I thought I must be doing it wrong so I read everything I can find on here and slowed down my drops as well as getting full drops that hang before dropping and being sure the DPD powder is in adequate amounts, etc.

I got 28 drops, 24 drops 32 drops……

This is crazy. My head is about to explode. Someone PLEASE tell me what stupid mistake I am making! I would love to be humiliated by my own stupidity, just let me get this testing right!

 

[jblizzle]

Are you using a 10ml water sample?
Are you holding the bottle vertically?

A variation of 8 drops sure does seem excessive.

 

[BuckeyeChris]

take one big measuring cup's worth of water out of the pool, mix that up real good in the measuring cup, then do a couple back to back tests with that. It's possible the chemicals in your pool aren't mixed well enough, and every time you dip the thing in the pool you're hitting a different patch of water.

 

[midijamm]

Are you using a 10ml water sample?
Are you holding the bottle vertically?

A variation of 8 drops sure does seem excessive.

That's Yes, and Yes

To begin with I was dropping fast. That's when I get 24 drops. Slowed down, measured DPD powder generously, got full hanging drops, and tried to be completely consistent…two tests in a row and got 28 and 32

- - - Updated - - -

take one big measuring cup's worth of water out of the pool, mix that up real good in the measuring cup, then do a couple back to back tests with that. It's possible the chemicals in your pool aren't mixed well enough, and every time you dip the thing in the pool you're hitting a different patch of water.

gonna try that now but the pump has been running all day

 

[kimkats]

The cup/container you are getting the water with...is it used ONLY for getting water out of the pool?

I was not thinking one day and used the cup I put my chlorine in the pool with to take water out of the pool to test. DUH I wonder why my FC was off the charts! LOL

Just throwing that out there.

Kim

 

[jblizzle]

I would have though dropping fast would have been more drops not less.

You should have a drop about every second.

 

[midijamm]

OK, here are 6 tests from the same cup. Seems to indicate that the minutest variation can skew the test wildly! As I have been going I am trying to be consistent in all aspects and letting each drop fall off of it's own weight. This seems to be the solution.

28
30
27
29
29
29

 

[jblizzle]

I would not call that a wild distribution. The test is good to +/- 1 drop and that is basically what you have.

 

[midijamm]

I would not call that a wild distribution. The test is good to +/- 1 drop and that is basically what you have.

OK, time to take a break and read my Buddhist book "Comfortable with Uncertainty"

Thanks

 

[Scuba_Steve]

In my experience with drop based tests, the higher the number of drops being used, the more inaccuracy.

Drops are not perfectly uniform. Close, but not perfect.

If something only requires a few drops to change colors, then this variation probably will not affect the test.

Your using around 30. If your bottle has a tiny bit of static electricity on the tip it will cause the drops to form slightly small. Over 30 drops you may be off by a couple drops because of this. Or air getting mixed in the drops, or inconsistant squeezing pressure, or etc etc etc...