MANUAL: 55308
REV: A
ECO: 49582
11
GND position to keep spurious, thermally generated
charges grounded.
It is prudent to momentarily switch to the GND
position even during the measurement period to
ensure that excess charges do not accumulate at
the input of the charge amplifier.
6
ACCELEROMETER
CALIBRATION
Accelerometer calibration provides, with a definable
degree of accuracy, the necessary link between the
physical quantity being measured and the electrical
signal generated by the sensor. In addition, other
useful information concerning operational limits,
physical parameters, electrical characteristics, or
environmental influences may also be determined.
Without this link, analyzing data becomes a nearly
impossible task. PCB provides a calibration record
that documents the exact characteristics of each
sensor. (The type and amount of data varies
depending on the sensor type, contractual
regulations, and other special requirements.)
Under normal operating conditions, piezoelectric
sensors are extremely stable, and their calibrated
performance characteristics do not change over
time. However, harsh environments or other
unusual conditions that cause the sensor to
experience dynamic phenomena outside of its
specified operating range may temporarily or
permanently affect the sensor. This change
manifests itself in a variety of ways, including a shift
of the sensor resonance due to a cracked crystal, or
a temporary loss of low-frequency measuring
capability due to a drop in insulation resistance.
For these reasons, it is recommended that a
recalibration cycle be established for each
accelerometer. This schedule is unique and is
based on a variety of factors, such as extent of use,
environmental conditions, accuracy requirements,
trend information obtained from previous calibration
records, contractual regulations, frequency of
“cross-checking”
against
other
equipment,
manufacturer recommendation, and any risk
associated with incorrect readings. International
standards, such as ISO 10012-1, provide insight
and
suggested
methods
for
determining
recalibration
intervals
for
most
measuring
equipment. With the above information in mind and
under “normal” circumstances, PCB conservatively
suggests a 12- to 24-month recalibration cycle for
most piezoelectric accelerometers.
Note:
It is good measurement practice to verify the
performance of each accelerometer with a
Handheld Shaker or other calibration device before
and after each measurement. The PCB Model
394C06 Handheld Shaker operates at a fixed
frequency and known amplitude (1.0 g) to provide a
quick check of sensor sensitivity.
6.1.1 SENSOR RECALIBRATION
Accelerometer recalibration services are typically
performed by PCB’s internal metrology laboratory.
(Other international and private laboratories are
also available.) The PCB laboratory is certified to
ISO 9001, accredited by A2LA to ISO 17025,
complies with ISO 10012-1 (and former MIL-STD-
45662A), and uses equipment directly traceable to
N.I.S.T. This assures an accurate calibration of
relevant specifications.
In addition, many companies choose to purchase
the
equipment
necessary
to
perform
the
recalibration procedure themselves. While this may
result in both a savings of time and money, it has
also been attributed to incorrect readings and costly
errors. Therefore, in an effort to prevent the
common mistakes associated with customer-
performed calibration, this document includes a
broad overview of the Back-to-Back Calibration
technique. This technique provides a quick and
easy method for determining the sensitivity of a test
accelerometer over a wide frequency range.
6.1.2 BACK-TO-BACK CALIBRATION
THEORY
Back-to-Back Calibration is perhaps the most
common method for determining the sensitivity of
piezoelectric accelerometers. This method relies on
a simple comparison to a previously calibrated
accelerometer, typically referred to as a reference
standard.
