PCB Piezotronics 340A75 User manual

Model 340A75
High-temp., miniature (2 gm), ceramic shear charge accel., 3 pC/g, 12k Hz,
Installation and Operating Manual
For assistance with the operation of this product,
contact PCB Piezotronics, Inc.
Toll-free: 800-828-8840
24-hour SensorLine: 716-684-0001
Fax: [phone]
E-mail: [email protected]
Web: www.pcb.com

The information contained in this document supersedes all similar information that
may be found elsewhere in this manual.
Total Customer Satisfaction – PCB
Piezotronics guarantees Total Customer
Satisfaction. If, at any time, for any
reason, you are not completely satisfied
with any PCB product, PCB will repair,
replace, or exchange it at no charge. You
may also choose to have your purchase
price refunded in lieu of the repair,
replacement, or exchange of the product.
Service – Due to the sophisticated nature
of the sensors and associated
instrumentation provided by PCB
Piezotronics, user servicing or repair is
not recommended and, if attempted, may
void the factory warranty. Routine
maintenance, such as the cleaning of
electrical connectors, housings, and
mounting surfaces with solutions and
techniques that will not harm the
physical material of construction, is
acceptable. Caution should be observed
to insure that liquids are not permitted to
migrate into devices that are not
hermetically sealed. Such devices should
only be wiped with a dampened cloth
and never submerged or have liquids
poured upon them.
Repair – In the event that equipment
becomes damaged or ceases to operate,
arrangements should be made to return
the equipment to PCB Piezotronics for
repair. User servicing or repair is not
recommended and, if attempted, may
void the factory warranty.
Calibration – Routine calibration of
sensors and associated instrumentation is
recommended as this helps build
confidence in measurement accuracy and
acquired data. Equipment calibration
cycles are typically established by the
users own quality regimen. When in
doubt about a calibration cycle, a good
“rule of thumb” is to recalibrate on an
annual basis. It is also good practice to
recalibrate after exposure to any severe
temperature extreme, shock, load, or
other environmental influence, or prior
to any critical test.
PCB Piezotronics maintains an ISO-
9001 certified metrology laboratory and
offers calibration services, which are
accredited by A2LA to ISO/IEC 17025,
with full traceablility to N.I.S.T. In
addition to the normally supplied
calibration, special testing is also
available, such as: sensitivity at elevated
or cryogenic temperatures, phase
response, extended high or low
frequency response, extended range, leak
testing, hydrostatic pressure testing, and
others. For information on standard
recalibration services or special testing,
contact your local PCB Piezotronics
distributor, sales representative, or
factory customer service representative.
Returning Equipment – Following
these procedures will insure that your
returned materials are handled in the
most expedient manner. Before returning
any equipment to PCB Piezotronics,
contact your local distributor, sales
representative, or factory customer
service representative to obtain a Return
Warranty, Service, Repair, and
Return Policies and Instructions

Materials Authorization (RMA)
Number. This RMA number should be
clearly marked on the outside of all
package(s) and on the packing list(s)
accompanying the shipment. A detailed
account of the nature of the problem(s)
being experienced with the equipment
should also be included inside the
package(s) containing any returned
materials.
A Purchase Order, included with the
returned materials, will expedite the
turn-around of serviced equipment. It is
recommended to include authorization
on the Purchase Order for PCB to
proceed with any repairs, as long as they
do not exceed 50% of the replacement
cost of the returned item(s). PCB will
provide a price quotation or replacement
recommendation for any item whose
repair costs would exceed 50% of
replacement cost, or any item that is not
economically feasible to repair. For
routine calibration services, the Purchase
Order should include authorization to
proceed and return at current pricing,
which can be obtained from a factory
customer service representative.
Warranty – All equipment and repair
services provided by PCB Piezotronics,
Inc. are covered by a limited warranty
against defective material and
workmanship for a period of one year
from date of original purchase. Contact
PCB for a complete statement of our
warranty. Expendable items, such as
batteries and mounting hardware, are not
covered by warranty. Mechanical
damage to equipment due to improper
use is not covered by warranty.
Electronic circuitry failure caused by the
introduction of unregulated or improper
excitation power or electrostatic
discharge is not covered by warranty.
Contact Information – International
customers should direct all inquiries to
their local distributor or sales office. A
complete list of distributors and offices
can be found at www.pcb.com.
Customers within the United States may
contact their local sales representative or
a factory customer service
representative. A complete list of sales
representatives can be found at
www.pcb.com. Toll-free telephone
numbers for a factory customer service
representative, in the division
responsible for this product, can be
found on the title page at the front of this
manual. Our ship to address and general
contact numbers are:
PCB Piezotronics, Inc.
3425 Walden Ave.
Depew, NY 14043 USA
Toll-free: [phone]
24-hour SensorLineSM: [phone]
Website: www.pcb.com
E-mail: [email protected]
DOCUMENT NUMBER: 21354
DOCUMENT REVISION: B
ECN: 17900

1
General
OPERATING GUIDE
for use with
PIEZOELECTRIC CHARGE MODE ACCELEROMETERS
SPECIFICATION SHEET, INSTALLATION DRAWING
AND CALIBRATION INFORMATION ENCLOSED
PCB ASSUMES NO RESPONSIBILITY FOR DAMAGE CAUSED TO THIS PRODUCT AS A RESULT OF
PROCEDURES THAT ARE INCONSISTENT WITH THIS OPERATING GUIDE.
1.0 INTRODUCTION
Congratulations on the purchase of a quality PCB charge
mode accelerometer. In order to ensure the highest level
of performance for this product, it is imperative that you
properly familiarize yourself with the correct mounting
and installation techniques before attempting to operate
this device. If, after reading this manual, you have any
additional questions concerning this sensor or its
application, feel free to call an Application Engineer at
716-684-0001 or the closest PCB representative.
2.0 CHARGE MODE ACCELEROMETERS
Ceramic shear-structured accelerometers offer high
performance for precision vibration measurements in
high-temperature environments. The use of ceramic
sensing crystals, operating in the shear mode, reduces
erroneous output due to base strain, thermal transients,
and transverse motion.
Charge mode accelerometers output a strong, high-
impedance charge signal directly from their ceramic
sensing element. They do not contain built-in signal
conditioning electronics; the signal is conditioned
externally by either a laboratory-style charge amplifier
or in-line charge converter prior to being analyzed by a
readout or recording device. The absence of built-in
electronics permits operation to elevated temperatures of
500 °F (260 °C) for most models.
These accelerometers are ideal for structural testing,
machine monitoring, and vehicular shock and vibration
measurement tasks where high temperatures preclude
the use of accelerometers with built-in microelectronics.
Enclosed is a Specification Sheet, which lists the
complete performance characteristics of the particular
accelerometer.
3.0 OPTIONAL FEATURES
In addition to these standard features, a variety of
options are also available. When listed before the model
number, the following prefix letters indicate that the
sensor is manufactured or supplied with optional
features: Aoption- adhesive mount; Joption- electrical
ground isolation; Moption- metric mounting thread; P
option- positive polarity output (for use with non-
inverting, signal conditioning amplifiers); and Woption-
water-resistant cable connection.
4.0 INSTALLATION OVERVIEW
When choosing a mounting method, consider closely
both the advantages and disadvantages of each
technique. Characteristics like location, ruggedness,
amplitude range, accessibility, temperature, and
portability are extremely critical. However, the most
important and often overlooked consideration is the
effect the mounting technique has on the high-frequency
performance of the accelerometer.
Shown hereafter are six possible mounting techniques
and their effects on the performance of a typical
piezoelectric accelerometer. (Note that not all of the
mounting methods may apply to your particular sensor.)
The mounting configurations and corresponding graph
demonstrate how the high-frequency response of the
accelerometer may be compromised as mass is added to
the system and/or the mounting stiffness is reduced.
Note: The low-frequency response is unaffected by the
mounting technique. Its roll-off behavior is dependent

2
upon the coupling type or time constant of the external
charge amplifying device. This characteristic may be
fixed or adjustable, depending on the charge amplifier.
Please refer to the appropriate charge amplifier
specification sheet for information on low-frequency
characteristics.
Figure 1. Assorted Mounting Configurations and Their
Effects on High Frequency
4.1 STUD MOUNT
This mounting technique requires smooth, flat contact
surfaces for proper operation and is recommended for
permanent and/or secure installations. Stud mounting is
also recommended when testing at high frequencies.
Note: Do NOT attempt mounting on curved, rough, or
uneven surfaces, as the potential for misalignment and
limited contact surface may significantly reduce the
sensor’s upper operating frequency range.
Figure 2. Mounting Surface Preparation
STEP 1: First, prepare a smooth, flat mounting surface,
then drill and tap a mounting hole in the center of this
area as shown in Figure 2 and in accordance with the
Installation Drawing for the specific sensor that is
being mounted.
A precision-machined mounting surface with a
minimum finish of 63 µin (0.00016 mm) is
recommended. (If it is not possible to properly prepare
the test structure mounting surface, consider adhesive
mounting as a possible alternative.) Inspect the area,
checking that there are no burrs or other foreign
particles interfering with the contact surface.
STEP 2: Wipe clean the mounting surface and spread
on a light film of grease, oil, or similar coupling fluid
prior to installation.
Figure 3. Mounting Surface Lubrication
Adding a coupling fluid improves vibration
transmissibility by filling small voids in the mounting
surface and increasing the mounting stiffness. For semi-
permanent mounting, substitute epoxy or another type of
adhesive.
STEP 3: Screw the mounting stud into the base of the
accelerometer and hand-tighten. Then, screw the
sensor/stud assembly into the prepared tapped hole and
tighten to the recommended mounting torque as
indicated on the Installation Drawing.
Note: It is important to use a torque wrench during this
step. Under-torquing the sensor may not adequately
couple the device; over-torquing may result in stud
failure.
4.2 ADHESIVE MOUNT
Adhesive mounting is often used for temporary
installation or when the test object surface cannot be
adequately prepared for stud mounting. Adhesives like
hot glue and wax work well for temporary mounts; two-
part epoxies and quick-bonding gels provide a more
permanent mount.
Note: Adhesively mounted sensors often exhibit a
reduction in high-frequency range. Generally, smooth
surfaces and stiff adhesives provide the best frequency
response.

3
METHOD 1 - Adhesive Mounting Base
This method involves attaching a base to the test
structure, then securing the sensor to the base. This
allows for easy removal of the accelerometer. Also,
since the bases are of a “hard-coated” aluminum design,
they provide electrical isolation to eliminate ground
loops and reduce electrical interference that may be
present on the surface of the test object.
STEP 1: Prepare a smooth, flat mounting surface. A
minimum surface finish of 63 µin (0.00016 mm)
generally works best.
STEP 2: Stud-mount the sensor to the appropriate
adhesive mounting base according to the guidelines set
forth in STEPS 2 and 3of the Stud Mount Procedure.
STEP 3: Place a small portion of adhesive on the
underside of the mounting base. Firmly press down on
the assembly to displace any extra adhesive remaining
under the base.
Figure 4. Mounting Base: Adhesive Installation
METHOD 2 - Direct Adhesive Mount
For restrictions of space or for convenience, most
sensors (with the exception of integral stud models) can
be adhesive-mounted directly to the test structure.
STEP 1: Prepare a smooth, flat mounting surface. A
minimum surface finish of 63 µin (0.00016 mm)
generally works best.
STEP 2: Place a small portion of adhesive on the
underside of the sensor. Firmly press down on the top
of the assembly to displace any adhesive. Be aware that
excessive amounts of adhesive can make sensor removal
difficult.
Figure 5. Direct Adhesive Mounting
4.3 HANDHELD OR PROBE TIP MOUNT
This method is NOT recommended for most
applications. It is generally used only for machinery
monitoring and other portable trending applications.
Both the accuracy and repeatability at low (<5 Hz) and
high frequency (>1 kHz) ranges are questionable.
5.0 CABLING
Care and attention to installation is essential, as the
reliability and accuracy of your system is no better than
that of the output cable. Charge mode sensors require
the use of low noise cables. Cables and connectors must
be kept clean and dry to maintain high insulation
resistance. In the event that the insulation resistance is
compromised, inspect, clean, and bake cables and
connectors to restore insulation resistance.
STEP 1: Ascertain that you have ordered the correct
cable type. Use only PCB Series 003 Low-Noise Cable
(or equivalent) with charge-output accelerometers.
Some integral-cable units use Series 030 Cable, which is
also low noise. Keep connections clean to maintain
insulation resistance and good low-frequency response.
STEP 2: Connect the cable to the accelerometer. A
small amount of thread-locking compound placed on the
connector prior to attachment helps secure the cable
during testing. In harsh environments, the connection
can be sealed with silicone rubber, O-rings, and flexible
heat-shrink tubing.
TYPE 1: Coaxial Connectors
First, plug the male connector of the cable into the
mating female sensor connector. Then, holding the
sensor stationary, secure the connector in place by
tightening down the attached cable sleeve.
TYPE 2: Pigtail Connections
“Pigtail” connections are convenient for easy field repair
of cable connections. Simply solder the stripped cable
to the exposed pins on the sensor. (Check the
Installation Drawing to determine signal and ground

4
pins.) In many cases, it is also helpful to protect the
solder joint with heat-shrink tubing or epoxy.
Note: If you do not have the experience or resources to
attach pigtail leads, consult PCB to discuss factory
attachment. Damage caused by excessive heat is not
covered by warranty.
STEP 3: Route the cable to the signal conditioner,
making certain to stress-relieve the sensor/cable
connection and minimize motion by clamping the cable
at regular intervals.
Certain precautions should be used to avoid physical
damage and minimize electrical noise. For instance,
avoid routing cables near high-voltage wires. Do not
route cables along floors or walkways where they may
be stepped on or become contaminated. Shielded cables
should have the shield grounded at one end only.
STEP 4: Finally, connect the remaining cable end to the
signal conditioner. To dissipate charge that may have
accumulated in the cable, short the signal lead to the
ground lead prior to attachment.
6.0 POWERING
A conventional method for conditioning the high-
impedance signal generated by a charge output sensor is
to use a laboratory-style charge amplifier. The charge
amplifier converts the high-impedance charge signal
generated by the sensor to a low-impedance voltage
signal that can then be transmitted to a readout or a
recording device for analysis. Before connecting the
low-noise cable from the accelerometer to the charge
amplifier, be sure to ground both the charge amplifier
and the cable. This ensures that an excessive static
charge that may have accumulated across the
accelerometer or cable is harmlessly discharged. Failure
to observe this precaution can result in the destruction of
the input FET of certain amplifiers.
Once each element is connected, allow a few minutes for
the system to thermally stabilize. Place the switch on
the charge amplifier in the OPERATE position and
proceed with the measurement.
It is often convenient to normalize the accelerometer and
charge amplifier system to a precise sensitivity, such as
10.0 or 100.0 mV/g for ease of data analysis. This is
accomplished with most PCB laboratory charge
amplifiers and some miniature in-line units as well.
For fixed sensitivity in-line charge converters, such as
the PCB Series 422, the system sensitivity (mV/g) is
determined as the product of the charge amplifier
sensitivity (mV/pC) and the charge sensitivity of the
accelerometer (pC/g).
Note: When using charge-amplified systems, the noise
floor of the system is dependent on the input
capacitance to the charge amplifier. Since the cable
adds to the capacitance and to minimize the noise
threshold, keep the cable length between the
accelerometer and the charge amplifier to a minimum.
Cable length does not affect the system sensitivity of
charge-amplified systems.
Since charge amplifier operation varies, please contact
the respective signal conditioner manufacturer or check
the product manual for additional information.
7.0 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.

5
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.
7.1 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.
7.2 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 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 (VT/VR) must also be the ratio
of their sensitivities. With the sensitivity of the
reference standard (SR) known, the exact sensitivity of
the test sensor (ST) is easily calculated by using the
following equation:
ST=S
R(VT/VR)
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.

6
Figure 11. Typical Test Accelerometer Response
7.3 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.
Since the Back-to-Back Calibration technique relies on
each sensor experiencing an identical acceleration level,
proper mounting of the test sensor to the reference
standard is imperative. Sensors with mounting holes are
attached directly to the reference standard with a stud
tightened to the recommended mounting torque. A
shouldered mounting stud is typically used to prevent
the stud from “bottoming out” in the hole.
Both mounting surfaces are precision-machined and
lapped to provide a smooth, flat interface according to
the manufacturer’s specification. A thin layer of
silicone grease is placed between the mating surfaces to
fill any imperfections and increase the mounting
stiffness. The cables are stress-relieved by first routing
them to the shaker head, securing them with tape or
cable ties, then routing them to a nearby stationary
location. This reduces cable motion, which is especially
important when testing charge output sensors and helps
to prevent extraneous noise or stresses from being
imparted into the system. A typical set-up is shown in
Figure 12.
Figure 12. Typical Calibration Set-Up
Adhesively mounted sensors use similar practices.
However, in this case, a small portion of quick-bonding
gel or similar temporary adhesive is used to attach the
test sensor to a reference standard designed with a
smooth, flat mounting surface.
In addition to mounting, the selection of the proper
equipment is critical. Some of the more important
considerations include: 1) the reference standard must
be specified and previously calibrated over the
frequency and/or amplitude range of interest; 2) the
shaker should be selected to provide minimal transverse
(lateral) motion and minimal distortion; and 3) the
quality of the meters, signal generator, and other devices
should be selected so as to operate within the limits of
permissible error.
7.4 COMMON MISTAKES
Most calibration errors are caused by simply
overlooking some of the fundamental principals of
dynamics. This section attempts to address some of the
more common concerns.
For stud-mount sensors, always mount the accelerometer
directly to the reference standard. Ensure that the
mounting surfaces are smooth, flat, and free of any
burrs. Always use a coupling fluid, such as silicone
grease, in the mounting interface to maintain a high
mounting stiffness. Mount the sensor according to the
manufacturer’s recommended mounting torque. DO
NOT use any intermediate mounting adaptors, as the
mounted resonant frequency may be reduced and
thereby compromise the high-frequency performance. If
necessary, use adaptor studs.

7
Figure 13. Stud Mounting
For adhesive mount sensors, use a thin, stiff layer of
temporary adhesive such as quick-bonding gel or
superglue. DO NOT use excessive amounts of glue or
epoxy, as the mounting stiffness may be reduced and
compromise high-frequency performance. It may also
damage the sensor during removal.
Figure 14. Incorrect Adhesive Mounting
Triaxial accelerometers should always be mounted
directly to the reference standard. Unless absolutely
required, DO NOT use adaptors to re-orient the sensor
along the axis of motion, as the mounting stiffness may
be altered. The vibration at the test sensor sensing
element may differ from the vibration at the reference
standard due to a “cantilever” effect, seen in Figure 15.
Figure 15. Mounting Triaxial Sensors (Incorrect)
Understand Back-to-Back Calibration limitations. Do
not expect the uncertainty of calibration to be any better
than ±2%. (In fact, the uncertainty may be as high as
±3% or ±4% for frequencies <10 Hz or >2 kHz.) Since
large sensors may affect high-frequency accuracy, verify
that the test sensor does not mass load the reference
standard. Validate your calibration system with another
accelerometer prior to each calibration session. Check
with the manufacturer for exact system specifications.
7.5 CONCLUSIONS
Without an adequate understanding of dynamics,
determining what, when, and how to test a sensor is a
difficult task. Therefore, each user must weigh the cost,
time, and risk associated with self-calibration versus the
services of an accredited laboratory.
3425 Walden Avenue, Depew, NY 14043
Toll Free: 888-684-0013 •24-hour SensorLineSM: 716-684-0001 •FAX: [phone]
E-mail: vibratio[email protected] •Website: www.pcb.com
File name: VIB-CHGOPGUIDE-0702 Doc. No. 18293 Rev. NR