Do it again and watch the returns again.

BTW -- Many of us often use a little DE in our sand filters on a routine basis, not necessarily for testing to see if the filter is working properly but just for the little extra it can give in filtration. As you found, the pressure goes up quickly. You just have to learn how much to put in for your setup so that you only get that slight increase.

More about this here: http://pool9.net/de-test/

(That is not a speed meter; it is a pressure gauge.)