NBM 550 NBM-550 is part of the NBM-500 instrument family. It delivers extremely accurate results for electromagnetic field strength measurements. NBM-550 handles applications from a few Hertz through to long wave and on up to high frequency microwave radiation using a selection of interchangeable probes for electric and magnetic fields. The field strengths are captured with flat frequency response probes or probes with shaped frequency response, allowing the measurement values to be assessed against an environmental or occupational safety standard. The calibration factors for each probe are stored in the probe itself and are read out and taken into account automatically by the measuring set.