6
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 suggest 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 piezoresistive
accelerometers.
NOTE: It is good measurement practice to verify the
performance of each accelerometer with a calibration
device before and after each measurement.
8.1 RECALIBRATION SERVICE
PCB offers recalibration services for our piezoresistive
accelerometers, as well as units produced by other
manufacturers. Our internal metrology laboratory is
certified to ISO 9001, accredited by A2LA to ANSI/IEC
17025 and ANSI/NCSL Z540-1, complies with ISO
10012-1 (and former MIL-STD-45662A), and uses
equipment directly traceable to NIST. Our investment in
equipment, traceability and conformance to industry
standards ensures accurate calibration against relevant
specifications, in a timely fashion.
8.2 BACK-TO-BACK CALIBRATION THEORY
Many companies choose to purchase the equipment
necessary to perform the recalibration procedure in house.
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.
Back-to-Back Calibration is perhaps the most common
method for determining the sensitivity of piezoresistive
accelerometers. This method relies on a simple
comparison to a previously calibrated accelerometer,
typically referred to as a reference standard.
Figure 9. Reference Standard Accelerometer
These high-accuracy devices, which are directly traceable to
a recognized standards laboratory, are designed for stability,
as well as configured to accept a test accelerometer. By
mounting a test accelerometer to the reference standard and
then connecting this combination to a suitable vibration
source, it is possible to vibrate both devices and compare
the data as shown in Figure 10. (Test set-ups may be
automated and vary, depending on the type and number of
accelerometers being calibrated).
Figure 10. Typical Back-to-Back Calibration System
Because the acceleration is the same on both sensors, the
ratio of their outputs (V
T
/V
R
) must also be the ratio of
their sensitivities. With the sensitivity of the reference
standard (S
R
) known, the exact sensitivity of the test
sensor (S
T
) is easily calculated by using the following
equation:
S
T
= S
R
(V
T
/V
R
)
By varying the frequency of the vibration, the sensor may
be calibrated over its entire operating frequency range.
The typical response of an unfiltered accelerometer is
shown in Figure 11.
Figure 11. Typical Test Accelerometer Response
8.3 PCB CALIBRATION PROCEDURE
Numerous precautions are taken at PCB to insure accurate
and repeatable results. This section provides a brief
overview of the primary areas of concern.
