I recently purchased a digital differential manometer (fieldpiece). I've used it once to verify airflow on a small (3-5 ton) package unit that had been iced up. I drilled holes into the plenums and screwed the threaded brass fitting in. I got a reading (.68 inWc). My question- is this an acceptable method or do I need to use the air pressure probes that go into the ducts? any other advice on taking air pressure measurements to verify air flow is appreciated.