FAS-DPD 10ml vs 25ml - Big Discrepancy

[stlmuelk]

Hi All,
I searched the forum before posting and I saw a few others had the same issue I did, but I did not find a post with a resolution.
The issue is a big discrepancy in the FAS-DPD FC numbers when I use 10ml vs 25ml sample. For example this morning I got 3.5ppm FC using 10ml (7 drops to clear), but 5.6ppm (28 drops) using 25ml sample.
In both tests today I used two scoops of powder. I've done this several times in the past with the same result - the FC ppm is always significantly higher when using a 25ml sample.
I'm using the Taylor K2006 test kit with a magnetic stirrer.
I've followed the instructions here Extended Test Kit Directions as best I can (as well as the card in the case).
I've tried reversing the order (e.g. 25 first then 10ml and vice-versa), using more or less powder (1 "heaping" vs 2 scoops on the 10ml), etc but the results are consistent. And testing the *same* sample size back-to-back is also consistent, e.g. I get the same answer in 2 consecutive 25ml tests.
So, there may not be a decent answer as to why this is happening, but for giggles let's assume I'm not doing something bone-headed...which result should I go with for dosing?
Thanks, Ken

 

[PoolStored]

Drop size, likely. Are you allowing the drop to form and drop under its own weight, or are you squeezing them out fairly rapidly?

 

[stlmuelk]

I think you got it. I videoed myself doing both tests and sure enough on the 25ml I'm going just a wee bit faster. I'm surprised how consistently I can be off, but nevertheless the tape don't lie. Whew! I'll switch to 10ml as it seems I go slower on that one. Thank you!

 

[Newdude]

Go about 1 second per drop and let gravity make each drop. Nobody. And I mean n o b o d y, will miss those 20 seconds of their life.

Also, take care that the bottle stays vertical.

@mgtfp suggested a while back for a gentle pre-squeeze of the 0871 before inverting so that the first couple of drops don't squirt out. I've been doing it since and love it.

 

[stlmuelk]

Go about 1 second per drop and let gravity make each drop. Nobody. And I mean n o b o d y, will miss those 20 seconds of their life.

Also, take care that the bottle stays vertical.

@mgtfp suggested a while back for a gentle pre-squeeze of the 0871 before inverting so that the first couple of drops don't squirt out. I've been doing it since and love it.

Thanks newdude!