Rapitest NPK soil tester - Anyone use this for hydro?

http://www.discount-hydro.com/productdisp.php?pid=538&navid=13

It says it's for soil and a google search didn't pull any posts for it's use in a hydro application. Before I spend my money, has anyone tried using this for hydro?

Thanks!

Hard to believe nobody tried.

Here's are some other testers out there:

Quantofix teststrips

LaMotte Soil Tester