inverting the vial 3 times is the correct procedure but you have to make sure the tablet is fully dissolved. Also high water temp will give a lower reading since the precipitate will be more soluable so having the sample at the correct temperature is important. We get many customers who leave a water sample in their car all day! (I TRY to tell them that this will affect the test results but some of them just don't care.) The first thing I always do is to take the temp of the sample and if it's above 80 degrees I stick it in our freezer at work for a few minutes to bring it down before I test it.