Hey have you heard of the slurry test? It can be done with pH and ppm meters. You take 2ozs of your medium and 2ozs of neutral PhD water, let them sit until your medium is saturated. Then test. I got this from the Nectar for the gods feeding schedule. They call for pH between 6.3-6.8 and ppms 300-500.
Hope this helps. I thought it was pretty useful and an affordable way of checking medium pH and ppms. It won't tell you npk though.