90225 Rev E 08/17 Page 3
1.0 Introduction
The 731A seismic accelerometer and P31 Power Unit Amplifier
are a unique state-of-the-art system for the measurement of
extremely low level vibrations. This system has the capability to
measure to the sub-micro g level by combining a high sensitivity
piezoelectric sensing element, an integral ultra-low noise
amplifier and a matching low-noise amplifier/filter/power supply.
The amplifier also contains an integration circuit to yield an
output proportional to velocity.
This system is particularly suited for the measurement of low level vibrations in bridge, building,
tower and dam motion studies, optical and photographic stability monitoring, electron microscope
and microtome monitoring, as well as vibration-isolation table monitoring.
2.0 Description
2.1 731A seismic accelerometer
The 731A seismic accelerometer incorporates an isolated piezoelectric
sensing element which minimizes the effects of transverse motion, base strain,
and thermal transients. The accelerometer also contains a proprietary solid
state charge amplifier with an extremely low noise floor. This enables the
sensing of low level vibration with a device which has a sensitivity of 10 V/g.
The 731A accelerometer is relatively small and comes in a case for safe
transport. Due to its extremely high sensitivity, the 731A is fragile. Do not drop
it! Protect the unit at all times from accidental impacts. Handling vibration and
shock must not exceed 15 g peak.
2.2 P31 power unit/amplifier
The P31 power unit/amplifier is specifically designed to further enhance the capabilities of the
731A seismic accelerometer by providing signal amplification, selective filtering, and signal
processing to yield user-selectable acceleration or velocity output signals. It also provides the
constant current needed to power the accelerometer’s internal amplifier.
The P31 is battery powered, using two standard 9 volt alkaline transistor batteries with an
operating life of approximately 75 hours. The green LED lamp glows for good battery condition
when the power switch is moved to the TEST position. All switches and the indicator light are on
the front panel, input and output connectors are on the rear panel. The power unit provides 2.4
mA constant current to the 731A accelerometer. A selectable-gain amplifier provides gains of 1,
10, and 100.
Acceleration or velocity output is switch selectable. The two acceleration settings provide integral
filtering with nominal system band-widths (minus 3 dB) of 0.05 to 450 Hz, and 0.05 to 100 Hz,
respectively. The velocity setting yields a nominal system response of 1.5 to 150 Hz (± 1 dB),
