Vibration meters use PNP transistors instead of NPN because they are inexpensive.
The PNP transistors used by the circuits needed a negative bias voltage rather than a favorable bias voltage. Don opted for -18V. This was subsequently altered to -24V to give the transducer more linear range.
At the period when the vibration meters were being researched, the industrial instrumentation community had not yet standardized on+ 24
so many Bently Nevada electric current vibration sensors had been installed that consumers did not enthusiastically welcome the change to+ 24V instead of-24V. Therefore, to this day it has stayed-24V.
An eddy-current probe works by passing an alternating current through a coil of wire and measuring the coil’s impedance.
This impedance changes when the probe comes close to an electrically conductive material and the change in impedance is proportional to the physical gap between the coil and the conductive target.
In turn, the sensing electronics transform this impedance shift into a voltage, giving a directly proportional electrical output to the physical gap that is directly proportional to the vibration.
It was not a deliberate effort on Bently Nevada’s part to “be distinct” or “non-conformist.” It was based quite literally on which parts were the least costly when the technology was first implemented 50 years ago.