Np is propeller RPM.
Such gauges are only found on turboprops, on piston engines the prop RPM is either the same as the engine RPM or geared down by a fixed ratio so we are only interested in engine RPM.
IOW - you are asking in the wrong forum, turboprop jockeys are found in the Business Aviation areas.
http://www.pprune.org/biz-jets-ag-flying-ga-etc-36/