An eddy current sensor, type NCDT series DZ130 from Micro Epsilon, is
demonstrating considerable success in automotive turbocharger
applications, where it is capable of counting revolutions accurately at
up to 400,000rpm. The tiny 3mm diameter sensor simply detects the
presence of a metallic turbine blade (0.5mm maximum gap), and no magnet
attachments or rotor modifications are required. It can operate at
temperatures between -40°C and +150°C, and its compact transmitter
provides both analogue and digital outputs.