PCB Piezotronics EX619A11 User manual

Model EX619A11
Charge Output Accelerometer
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 traceability to SI through
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
Warranty, Service, Repair, and
Return Policies and Instructions

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, NY14043 USA
Toll-free: [phone]
24-hour SensorLineSM: [phone]
Website: www.pcb.com
E-mail: [email protected]

PCB工业监视和测量设备 - 中国RoHS2公布表
PCB Industrial Monitoring and Measuring Equipment - China RoHS 2 Disclosure Table
部件名称
有害物质
铅(Pb)
汞
(Hg)
镉
(Cd)
六价铬(Cr(VI))
多溴联苯 (PBB)
多溴二苯醚(PBDE)
住房
O
O
O
O
O
O
PCB板
X
O
O
O
O
O
电气连接器
O
O
O
O
O
O
压电晶体
X
O
O
O
O
O
环氧
O
O
O
O
O
O
铁氟龙
O
O
O
O
O
O
电子
O
O
O
O
O
O
厚膜基板
O
O
X
O
O
O
电线
O
O
O
O
O
O
电缆
X
O
O
O
O
O
塑料
O
O
O
O
O
O
焊接
X
O
O
O
O
O
铜合金/黄铜
X
O
O
O
O
O
本表格依据 SJ/T 11364 的规定编制。
O：表示该有害物质在该部件所有均质材料中的含量均在 GB/T 26572 规定的限量要求以下。
X：表示该有害物质至少在该部件的某一均质材料中的含量超出 GB/T 26572 规定的限量要求。
铅是欧洲RoHS指令2011/65/ EU附件三和附件四目前由于允许的豁免。
CHINA RoHS COMPLIANCE

DOCUMENT NUMBER: 21354
DOCUMENT REVISION: C
ECN: 45605
Component Name
Hazardous Substances
Lead
(Pb)
Mercury
(Hg)
Cadmium
(Cd)
Chromium VI
Compounds
(Cr(VI))
Polybrominated
Biphenyls
(PBB)
Polybrominated
Diphenyl
Ethers (PBDE)
Housing
O
O
O
O
O
O
PCB Board
X
O
O
O
O
O
Electrical
Connectors
O
O
O
O
O
O
Piezoelectric
Crystals
X
O
O
O
O
O
Epoxy
O
O
O
O
O
O
Teflon
O
O
O
O
O
O
Electronics
O
O
O
O
O
O
Thick Film
Substrate
O
O
X
O
O
O
Wires
O
O
O
O
O
O
Cables
X
O
O
O
O
O
Plastic
O
O
O
O
O
O
Solder
X
O
O
O
O
O
Copper Alloy/Brass
X
O
O
O
O
O
This table is prepared in accordance with the provisions of SJ/T 11364.
O: Indicates that said hazardous substance contained in all of the homogeneous materials for this part is below the limit
requirement of GB/T 26572.
X: Indicates that said hazardous substance contained in at least one of the homogeneous materials for this part is above
the limit requirement of GB/T 26572.
Lead is present due to allowed exemption in Annex III or Annex IV of the European RoHS Directive 2011/65/EU.

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.
DOC: 55308
REV: NR
ECO:41036

2
TABLE OF CONTENTS
OPERATING GUIDE........................................................................................................................................................... 1
PIEZOELECTRIC CHARGE MODE ACCELEROMETERS ............................................................................................. 1
1.1 Cables in Explosive Atmospheres........................................................................................................................... 3
1.2 High Temperature Differential Charge Output Sensor............................................................................................ 3
2CABLING ..................................................................................................................................................................... 3
2.1 General Precautions and Considerations................................................................................................................. 3
2.1.1 Proper Cable Type and Care.......................................................................................................................... 3
2.2 Softline Cable.......................................................................................................................................................... 4
2.3 Integraral Hardline Cable........................................................................................................................................ 4
2.4 Hardline Connection Type ...................................................................................................................................... 4
2.4.1 PCB / Lemo type connector........................................................................................................................... 4
2.4.2 High Temperature 2 –pin 7/16-27 UNS connector........................................................................................ 4
2.5 In Line Differential Charge Amplifier..................................................................................................................... 4
3INSTALLATION OVERVIEW.................................................................................................................................... 5
3.1 Equipment Inspection.............................................................................................................................................. 5
3.2 Polarity Test............................................................................................................................................................ 5
3.3 Sensor Location....................................................................................................................................................... 5
3.4 Mounting Sensor ..................................................................................................................................................... 5
3.5 Route Mineral Insulated Hardline Cable................................................................................................................. 6
3.5.1 Care................................................................................................................................................................ 6
3.5.2 Bend Radius................................................................................................................................................... 7
3.5.3 Clamp Cable .................................................................................................................................................. 7
4POWERING.................................................................................................................................................................. 8
4.1 Installation............................................................................................................................................................... 8
4.2 Operation................................................................................................................................................................. 8
5HIGH-TEMPERATURE OPERATION ...................................................................................................................... 9
5.1 Introduction............................................................................................................................................................. 9
5.2 Reduced Resistance at Charge Amplifier Input....................................................................................................... 9
5.3 Solution to Reduced Resistance ............................................................................................................................ 10
5.4 Low-Frequency Response Limitations .................................................................................................................. 10
5.5 Other Precautions.................................................................................................................................................. 12
6ACCELEROMETER CALIBRATION....................................................................................................................... 12
6.1.1 SENSOR RECALIBRATION..................................................................................................................... 12
6.1.2 BACK-TO-BACK CALIBRATION THEORY .......................................................................................... 13
6.1.3 PCB CALIBRATION PROCEDURE ......................................................................................................... 14

3
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.
1.1 Cables in Explosive Atmospheres
The cable lengths (Integral and Cable and Cable Assemblies) INSTALLED IN AN EXPLOSIVE ATMOSPHERE IS
DEFINED IN THE ATEX, IECEx, CSA, ETC. APPROVAL CERTIFICATES.
1.2 High Temperature Differential Charge Output Sensor
Ceramic or Single crystal shear-structured accelerometers offer high performance for precision vibration
measurements in high-temperature environments. The use of ceramic or single crystal 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 piezoelectric
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 or up to 1300 °F (700 °C) for special applications.
These accelerometers are ideal for structural testing, machine monitoring, and vehicular shock, high
temperature machinery and power generation turbine and other 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.
2 CABLING
2.1 General Precautions and Considerations
2.1.1 Proper Cable Type and Care
Ascertain that you have ordered the correct cable type. Due to the high-impedance nature of the output signal
generated by charge mode accelerometers, several important precautionary measures must be followed.
When using soft-line cable always use special low-noise PCB Series 045 Low-Noise Cable (or equivalent) for
connecting to the input of the differential charge-output accelerometers. For extremely high temperature
charge mode applications use mineral insulated (MI) hard-line.
Care and attention to installation is essential, as the reliability and accuracy of your system is no better than
that of the output cable. Cables and connectors must be kept clean and dry to maintain high insulation
resistance and low frequency responce. In the event that the insulation resistance is compromised, inspect,
clean, and bake cables and connectors to restore insulation resistance.

4
2.2 Softline Cable
Special high temperature low-noise, shielded cable 2-wire cable assembly is required with charge mode
sensors for applications up to 500°F to connect the transducer to the charge amp. When additional
mechanical protection is required a stainless steel armor can be used.
The shield acts as a Faraday cage to reduce electrical noise from corrupting the signals, and minimizes
capacitively coupled noise from other electrical sources.
Standard, two-wire, or coaxial cable, when flexed, generates a charge between the conductors. This is referred
to as triboelectric noise and cannot be distinguished from the sensor's charge output. Low-noise cables have a
special graphite lubricant between the dielectric and the braided shield, which minimizes the triboelectric effect
and improves the quality of the sensor's charge output signal.
When using separate cables 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.
2.3 Integral Hardline Cable
For extremely high temperature (>500°F) charge mode applications use mineral insulated (MI) hardline. Keep
cable clean to maintain insulation resistance and good low-frequency response
2.4 Hardline Connection Type
2.4.1 PCB / Lemo type connector
The Lemo connector is PCA.0S.302.CLAC42 that is specially adapted for use with hardline cable. The
connector is a self-latching system that allows the connector to be mated by simply pushing the plug axially into
the socket. When required the connector is disengage by a single axial pull on the outer release sleeve.
2.4.2 High Temperature 2 –pin 7/16-27 UNS connector
This connector is a Model GP, 2 Pin Jack, 7/16-27 thread. The connector is welded to the hardline to provide a
high temperature, hermetic connection. The GP connector uses a threaded connction to mate with a GN or ET
2-Socket plug and torqued to 5 ft*lbs +/- 1 ft*lb
2.5 In Line Differential Charge Amplifier
The differential in line charge amplifier is purchased separately
A conventional method for conditioning the high-impedance signal generated by a charge output sensor is to
use a differential 422 series in-line charge amplifiers operate from an ICP ® signal conditioner. The unit
employs a high gain amplifier to perform the impedance transformation. The charge output of the transducers
may be scaled in terms of acceleration, pressure or force. The output is then mV/g, mV/psi or mV/lb,
respectively.

5
3 INSTALLATION OVERVIEW
3.1 Equipment Inspection
Before installing the accelerometer, verify the insulation resistance (I/R) of the sensor is per specification. I/R
can be out of specification due to mishandling and/or damage.
3.2 Polarity Test
Use this test to verify the proper polarity response. Improper polarity will adversely affect the use of the sensor
for machinery diagnostics such as balancing.
Step 1 –Connect the sensor to a 422 style charge amp and any ICP ® signal conditioner. Using standard
cable, connect the powered sensor to an oscilloscope.
Step 2 –Set the time scale to 20 milliseconds/division
Step 3 –Hold the transducer in hand and tap the bottom. The wavefrom on the oscilloscope first goes positive
as shown in Figure 1. If the waveform goes negative the wiring is reversed, contact PCB for technical support.
FIGURE 1
3.3 Sensor Location
Characteristics like location, ruggedness, amplitude range, accessibility, temperature, and portability
are extremely critical.
For optimum performance and measurement find a rigid location on the machine casing that most accurately
represents the vibration of the rotor, bearing, fan, etc. to be measured.).
3.4 Mounting Sensor
Bolt mounting requires smooth, flat contact surfaces for proper operation and is recommended for permanent
and/or secure installations. Stud or bolt 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.
STEP 1: Verify that the ambient and surface temperature of the mounting location are within the
temperature range of the sensor.

6
Step 2: Prepare a smooth, flat mounting surface on the machine casing. Then drill and tap a mounting hole in
the center or on the corresponding bolt circle of the sensor 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 in (0.00016 mm) and flatness of at least
.001” (25.4m) is recommended. Inspect the area, checking that there are no burrs or other foreign particles
interfering with the contact surface.
Figure 2. Mounting Surface Preparation
STEP 3: Wipe the mounting surface clean, prior to installation.
STEP 4: Place the sensor on the mounting surface and attach with mounting bolts and tighten to the
recommended torque as indicated on the specification.
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 bolt failure.
3.5 Route Mineral Insulated Hardline Cable
3.5.1 Care
Certain precautions should be used to avoid physical damage and minimize electrical noise. For instance, route the cables
away from points that may exceed its operating temperature, avoid routing cables near high-voltage wires. Do not route

7
cables along floors or walkways where they may be stepped on or become damaged or contaminated. Avoid twisting,
kinking, or straining the cable. Shielded cables should have the shield grounded at one end only.
3.5.2 Bend Radius
The minimum bend radius (r) for both soft-line and hardline cable is determined by the cable diameter as
shown below:
Bends
Allowed
Cable Diameter
Minimum Bending Radius
Total
d
r
1
0.125” (3.2 mm)
0.60” (16 mm)
20
0.125” (3.2 mm)
2.0” (50 mm)
3.5.3 Clamp Cable
To minimize triboelectric (motion-induced) noise from the cable interfering with the sensors high impedance
charge output cable clamps must be used. Clamp the cable as close to the transducer as possible and should
be attached to the same surface that the head is mounted (See Figure 3) taking care not to induce stress into
the cable and possibly leading to intermittent or broken connections. Continue to clamp the cable at regular
intervals of approximately 1.5 ft (0.5 m)
Figure 3

8
4 POWERING
4.1 Installation
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.
Connect the transducer to the input of a PCB differential 422 series or equivalent charge amp using low noise
cable. Note: For optimum noise performance, the cable length between the sensor and the 422 should be
minimized.
Connect the output of the 422 to any ICP ® signal conditioner using standard cable. Finally, the output of the
signal conditioner may then be connected to an oscilloscope or other monitoring device. This output will be an
AC signal (see specification for actual frequency response) with a DC bias. Many PCB signal conditioners
remove the bias via an AC coupling circuit.
4.2 Operation
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.

9
5 HIGH-TEMPERATURE OPERATION
5.1 Introduction
When subjected to elevated temperature, all piezoelectric sensors/hardline cable systems exhibit decreased insulation
resistance, due in part to the piezoelectric element, but due mostly to the hardline cable necessary to withstand the high
temperatures. This situation can cause serious voltage offset problems in direct-coupled charge amplifiers. To solve
this problem, the user must AC couple (capacitor) the charge amplifier to the sensor/cable system. See 5.3 Solution to
Reduced Resistance , for complete details, or use different amplifiers.
5.2 Reduced Resistance at Charge Amplifier Input
Figure 5.1 illustrates a simplified schematic of a typical direct-coupled charge amplifier where:
Rf= Feedback resistor (ohms)
Ri= Input leakage resistance (ohms)
Eo= Steady-state output voltage (volts)
ei= Offset voltage: FET leakage (volts)
Cf= Feedback capacitor (farads)
Figure 5.1 Typical Charge Amplifier Schematic
The feedback capacitor Cf comes into play only in the dynamic situation and its influence does not affect the
steady-state situation. The voltage eiis a DC offset voltage, usually very tiny (microvolts), that exists at the
input gate of the MOSFET circuit. This minute leakage current exists in all real devices.
As demonstrated in Equation 1, the steady-state (DC) output voltage Eois:
Equation 1








i
f
io R
R
eE 1
This equation shows that if the input (leakage) resistance at the charge amplifier is extremely high
(approaching infinity), the output DC voltage approaches ei, usually a very tiny voltage. However, as Ri
decreases, the term

10
i
f
R
R
1
increases, such that the output voltage can, with large ratios of Rf / Ri, become large enough to result in a large
Eo, perhaps large enough to be outside the normal output voltage range of the charge amplifier.
Because of the feedback capacitor Cf, this output voltage change usually does not occur rapidly but rather, it
manifests itself as a slow drift in the output voltage level. If Riis low enough with respect to Rf, the voltage drift
may continue until saturation of the charge amplifier occurs.
5.3 Solution to Reduced Resistance
Since the drift or offset problem is caused by a static or steady-state imbalance at the input of the charge
amplifier, the solution involves blocking this steady-state effect while allowing the desired dynamic phenomena
to pass. This may be accomplished by installing a series capacitor at the input of the charge amplifier,
between the offending sensor (or low-impedance hardline) and the input.
Figure 5.2 Piezoelectric System Block Diagram
Figure 5.2 illustrates a block diagram of the piezo-electric system where:
Ct= Shunt capacitor
Cs= Series blocking capacitor
With the series blocking capacitor Csin place as shown, the dynamic charge (Q) generated by the sensor
element is distributed across the two capacitors, Ctand Cs, in proportion to the size (capacitance) of each. If
Cs, for example, is equal to 100 times Ct, 99% of the charge appears at the input of the charge amplifier, while
1% is across the shunt capacitor Ct. This results in a 1% decrease in apparent sensitivity of the system.
This therefore demonstrates the importance of selecting the series blocking capacitor at least two orders of
magnitude higher than the total shunt capacitance Ctacross the input of the charge amplifier.
It is also important that this capacitor be of high quality, with a leakage resistance of greater that 1012 ohms, to
avoid the DC offset discussed previously in 5.1, Introduction.
5.4 Low-Frequency Response Limitations

11
In a normal charge amplifier, the low-frequency response is set by the RC time constant, as established by the
product of Cfand Rf. The system acts like a high-pass first order RC filter with a -3 dB frequency established
by the relationship:
Equation 2
ff
oCR
.16
f
where:
fo= -3 dB Frequency (Hz)
Rf= Feedback resistor (ohms)
Ct= Feedback capacitor (farads)
However, after the addition of the series blocking capacitor Cs, the system becomes the equivalent of two high-
pass filters in series, one as previously mentioned and one comprised of series capacitor Csand total
equivalent shunt resistance Ri. This new cutoff frequency is:
Equation 3
si
oCR
.16
f
To avoid compromise of the low-frequency response established by the charge amplifier parameters and
illustrated by Equation 2, the product of RiCsshould be several orders of magnitude higher than RfCf.
The approximate final system discharge time constant becomes:
Equation 4a
seconds
CR1 CR11
TC
ffsi


If the input coupling time constant (RiCs) is very much greater than the discharge time constant of the charge
amplifier (RfCf), Equation 4a then becomes:
Equation 4b
Seconds0
CR1
si

Equation 5
TC = RfCf
With the product RiCschosen to be much greater than RfCf, the system discharge time constant is simply RfCf
(seconds). The feedback parameters of the charge amplifier establish the low frequency characteristics of the
system, unaffected by the degraded input resistance parameters of the test sensor and/or cable.

12
5.5 Other Precautions
Always remember to keep the OPR-GND switch on the charge amplifier in the GND position while connecting
or disconnecting sensors, cable, or capacitor to the input connector. Stray or accumulated electrostatic
charges may build to the point that they may saturate or even damage the input circuitry of the charge
amplifier.
Operate the charge amplifier in the SHORT time constant while the sensor is subject to elevated or changing
temperatures.
If it is not necessary to procure data during the transition from room temperature to operating temperature,
place the OPR-GND switch in the 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

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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 (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 = SR(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 3.

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Figure 3. Typical Test Accelerometer Response
6.1.3 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 4.
Figure 4. 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.

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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.
Figure 5. Stud Mounting
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.
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