1. Why Bently Nevada use negative voltage for vibration probes?
Answer-- When Don Bentley worked on making solid-state 
versions of the eddy-current measurement system (it was 
actually originally designed in the 1930s by GE engineers 
using vacuum tubes), he had a choice between using N-P-N 
s 
were quite expensive, so he chose the least expensive of 
transistors or P-N-P transistors.  At the time, transisto
r
the two: P-N-P (apparently, PNP transistors they were less 
expensive to manufacture 50 years ago than their NPN 
ive bias volta
counterparts).

ecause the circuits used PNP transistors, a negative bias 


Bvoltage was required rather than a posi
tge.  
Don chose -18V.  This was later changed to -24V to allow 
more linear range from the transducer.

ime they did, 
there were so many Bently Nevada eddy curren
At that time, the industrial instrumentation community had 
not yet standardized on +24 vdc, and by the 
tt vibration 
sensors installed that changing to +24V rather than -24V 
was not greeting with enthusiasm by users.  Hence, it has 
remained -24V to this day.