PCB Piezotronics EX619A11, Installation And Operating Manual

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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.
Figure 1. 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 2. (Test set-ups may be automated and vary,
depending on the type and number of accelerometers being calibrated.)
Figure 2.
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 3.