DPD Powder - 1 or 2 scoops?

[DanaSC]

The other day I became suspicious that I wasn't using enough DPD powder for my OCLT testing. I tested my water 3 times the other night and got different results each time (frustrating). I decided last night to use 2 heaping scoops of the DPD powder (goal FC was 20) and came up with result of FC 22. I decided only to test once so as not to drive myself crazy. This morning I tested with one heaping scoop (forgot the 2) and came up with 20.5, then did the test again with a slightly more heaping single scoop and came up with 21.5, then gave in and did 2 scoops and came up with 22.5. I've been doing the OCLT for 3 nights now and am pretty much at the point of just stopping because I'm a little frustrated with these results. I think we're fine - we've been .5 away from passing the past couple of nights, and I think it might just be testing errors.

How many scoops should I "really" be using when I'm shocking at 20?

 

[JasonLion]

There are a number of things that can affect the repeatability of the FAS-DPD test. The most important is that you hold the dropper bottle vertically, and squeeze gently so the drops seem to drop gently off the tip as if by their own weight. You also need to do the test fairly quickly, especially for high FC/CC levels. If you take too long doing the test you will get higher than actual results.

The amount of powder held by the scoop is not completely repeatable. The powder tends to be a bit granular, so there will be some variation in how much the spoon holds no matter what you do. Two level scoops or one heaping scoop should be enough for levels into the upper 20s, but if there is any doubt add more. Adding more will not hurt, while adding to little might cause a problem.

 

[DanaSC]

Thanks Jason! I've been holding the bottle vertically. I need to make sure next time that I'm letting the drops fall by themselves...but at the same time doing the test fast enough. I guess I'm still perfecting my skills. Maybe also a little frustrated because my clumsy self spilled my first jar of DPD powder so I had to order a second. Seem to be going through this stuff fast!