Answer / steve sabin

As a 20-year Bently Nevada employee, and as someone
responsible for answering such questions over the years
within our company as well as externally, allow me to
explain the rather interesting circumstances behind the use
of negative voltages (-24 vdc) for powering eddy current
vibration oscillator/demodulator devices.

The short answer is that when Don Bently 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 transistors or P-N-P transistors. At the time,
transistors were quite expensive, so he chose the least
expensive of the two: P-N-P (apparently, PNP transistors
they were less expensive to manufacture 50 years ago than
their NPN counterparts).

Because the circuits used PNP transistors, a negative bias
voltage was required rather than a positive bias voltage.
Don chose -18V. This was later changed to -24V to allow
more linear range from the transducer.

At that time, the industrial instrumentation community had
not yet standardized on +24 vdc, and by the time they did,
there were so many Bently Nevada eddy current 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.

This was not a deliberate effort to "be different" or "non-
conformist" on the part of Bently Nevada. It was quite
literally based on which components were the least
expensive when the technology was originally introduced 50-
plus years ago.

Regards,

Steve Sabin
Marketing Communications Manager
Bently Nevada Asset Condition Monitoring
GE Energy Services

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