BDI ST350 User manual

ST350 - STRAIN
TRANSDUCER

Version 3.0, 0718
OPERATIONS MANUAL: ST350 STRAIN TRANSDUCER
3
Document Revision History
Version
Date
Changes
1.0
5/15/2012
Initial release document
2.0
6/7/2012
Updated to reflect the change in hardware
Document formatting updated
2.1
1/2/2013
Interim version-minor changes to formatting
2.2
4/2/2013
Updated to reflect the recent CE compliance approval
Power ratings updated
3.0
7/20/18
Updated to include new corrosion protection
Minor changes to specifications
New template and styles were applied
No part of this operations manual may be reproduced, by any means, without the written consent of BDI.
The information contained within this manual is believed to be accurate and reliable. However, BDI assumes no responsibility
for errors, omissions or misinterpretations. The information herein is subject to change without notification.
Copyright © 1989 –2018
Bridge Diagnostics, Inc. (dba BDI)

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WARRANTY INFORMATION
BDI warrants its products to be free from defects in materials and workmanship under normal use and service for thirty-six (36)
months from date of shipment. This warranty shall be void if any products have been subjected to modification, misuse, neglect,
accidents of nature, or shipping damage. Batteries have no warranty.
All equipment manufactured by BDI is intended for use by a qualified professional only. Under this warranty BDI's obligation is
limited to repairing or replacing (at BDI's option) of defective products. In no event shall BDI be liable for punitive, exemplary,
special, indirect, incidental, or consequential damages and the customer shall assume all costs of removing, reinstalling, and
shipping of defective products. EXCEPT AS STATED HEREIN, BDI MAKES NO WARRANTIES, EXPRESSED OR IMPLIED, AND
SPECIFICALLY DISCLAIMS ANY IMPLIED WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY.
Before returning any product, BDI technical support must be contacted at [phone] or by visiting BDITEST.COM/contact
and submitting a request. A technician will help determine the nature of the problem and if it cannot be resolved, authorization
will be given to return the item. A return merchandise authorization (RMA) will be sent to the customer to be filled out and
shipped back with the equipment. BDI will not accept shipment of any product without prior authorization as provided herein.
Ship all equipment to:
BDI
ATTN: TECHNICAL SUPPORT
740 S PIERCE AVE UNIT 15
LOUISVILLE CO 80027
+1.303.494.3230

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TABLE OF CONTENTS
1. Introduction .......................................................................................................................................................7
1.1 About the ST350 Strain Transducer ..................................................................................................................7
1.2 About this manual..........................................................................................................................................7
2. ST350 Overview..................................................................................................................................................8
2.1 Technical Specifications ..................................................................................................................................8
2.2 Options and Accessories .................................................................................................................................9
2.3 Applications ................................................................................................................................................10
3. Operation.........................................................................................................................................................10
3.1 Theory of Operation.....................................................................................................................................10
3.2 Potential Strain Measurement Issues ..............................................................................................................11
3.2.1 Temperature Variation..........................................................................................................................11
3.2.2 Instrumentation Cable Issues ................................................................................................................ 12
3.3 Connecting to Data Acquisition Systems ..........................................................................................................12
3.3.1 Excitation Voltage ................................................................................................................................12
3.3.2 Electrical Connections...........................................................................................................................12
3.3.3 Applying Calibration Factors ..................................................................................................................13
3.4 Verifying ST350 Output ................................................................................................................................13
3.4.1 Resistance Test ...................................................................................................................................13
3.4.2 Resolution/Noise Test...........................................................................................................................14
3.4.3 Sensor Response Test ..........................................................................................................................14
3.4.4 Field Adjusting Excessive Offsets............................................................................................................ 15
3.5 Verifying Accuracy .......................................................................................................................................15
3.5.1 Factory Calibration ...............................................................................................................................15
3.5.2 Items for Consideration for User Verification Tests.................................................................................... 15
4. Installation....................................................................................................................................................... 17
4.1 General Installation Guide ............................................................................................................................. 17
4.1.1 Prepare Mounting Area .........................................................................................................................17
4.1.2 Installation..........................................................................................................................................18
4.2 Installation on Steel Members........................................................................................................................19
4.1 Installation on Concrete Members .................................................................................................................. 23
4.1.1 Concrete Mounting Studs ...................................................................................................................... 23
4.1.2 Attaching ST350 with Extension to R/C Members ......................................................................................24
4.1.3 Attaching Extension to the ST350...........................................................................................................25
4.2 Installation on Other Surfaces........................................................................................................................29
4.2.1 Timber Members..................................................................................................................................29
4.2.2 Composite Materials .............................................................................................................................29
5. Maintenance & Recalibration...............................................................................................................................29
5.1 Maintenance Considerations ..........................................................................................................................29
5.2 Recalibration...............................................................................................................................................29

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LIST OF TABLES
Table 1: ST350 Specifications ................................................................................................................................... 8
Table 2: ST350 Options and Accessories .................................................................................................................... 9
Table 3: Typical Applications for ST350 strain transducers ...........................................................................................10
Table 4: ST350 Wiring Designations & Intelliducer Pinout ............................................................................................12
Table 5: ST350 Extension Limits...............................................................................................................................24
Table 6: Gage Extension Factors ..............................................................................................................................24
Table 7: Maximum Strain Ranges .............................................................................................................................25
LIST OF FIGURES
Figure 1: ST350 Drawing (inches) ............................................................................................................................. 8
Figure 2: ST350 Schematic......................................................................................................................................11
Figure 3: Noise levels of the ST350 on a 24-bit data acquisition system .........................................................................14
Figure 4: Typical Smooth Output Response and Zero Offset .........................................................................................14
Figure 5: Alignment Marking for ST350 Installation .....................................................................................................18
Figure 6: Mounting Position & Surface Preparation Marks.............................................................................................18
Figure 7: Typical ST350 Installation..........................................................................................................................19
Figure 8: Identifying Metric vs. Imperial Tabs.............................................................................................................19
Figure 9: Mounting Tabs on the ST350 using the Tab Jig.............................................................................................20
Figure 10: Marking ST350 Center Mounting Location...................................................................................................20
Figure 11: Tab Removal Tool (TRT) ..........................................................................................................................21
Figure 12: TRT with a Tab Inserted ..........................................................................................................................22
Figure 13: Removing a Tab with the TRT...................................................................................................................22
Figure 14: ST350 Concrete Drilling Jig.......................................................................................................................23
Figure 15: ST350 Extension Mounting Jig ..................................................................................................................26
Figure 16: ST350 Extension Mounting Diagram ..........................................................................................................26
Figure 17: Aligning ST350 and Extension in the Jig .....................................................................................................27
Figure 18: Extension Alignment Tab .........................................................................................................................27
Figure 19: Attaching Tab to Extension.......................................................................................................................27
Figure 20: ST350 With Extensions Attached to an R/C Structure ...................................................................................28

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1. INTRODUCTION
1.1 ABOUT THE ST350 STRAIN TRANSDUCER
The ST350 strain transducers have been designed for recording dynamic and other induced strains on all types of structural
members and to be compatible with most data acquisition systems. While mostly used in rugged field applications, they are also
suitable for many laboratory and manufacturing scenarios. The ST350 internal circuitry consists of a full Wheatstone Bridge with
four fully-active 350Ω foil gages optimized to provide a high electrical output for a given strain magnitude. Each transducer is
individually calibrated with a highly-accurate, N.I.S.T.-traceable calibration system.
Each unit is fully sealed, designed to exceed the IP67 rating, and equipped with two pre-drilled mounting holes to keep the gage
lengths consistent for all installations. Based on the structure’s material and length of time the ST350 is to be installed, various
mounting techniques can be used including adhesives, welding, expandable anchors, and screws.
1.2 ABOUT THIS MANUAL
This is a comprehensive document that explains the functions and features of the ST350. BDI also manufactures two types of
data acquisition systems, which will be referenced throughout the manual.
1. Structural Testing System (STS): Rugged, wireless, battery powered DAQ that includes an intelligent (Intelliducer)
connector design, which makes the system extremely easy to deploy on a variety of field projects.
2. Structural Monitoring System (SMS): Modular system with 4- or 16-channel nodes that can be used in laboratories or
on permanent large scales monitoring projects.
The following highlighted message blocks will periodically appear and contain important information that the user should be
aware of.
STOP: This symbol and corresponding message represents information regarding the device that if not
followed could lead to damaging the device! Pay close attention to this message.
WARNING: This symbol and corresponding message represents vital information and is critical for the
device operation and/or the operational settings/configuration.
INFORMATION: This symbol and corresponding message represents general information and/or tips on
successfully operating/configuring the device.

8
2. ST350 OVERVIEW
2.1 TECHNICAL SPECIFICATIONS
Figure 1: ST350 Drawing (inches)
Table 1: ST350 Specifications
MODEL
ST350
TYPE
350Ω
CIRCUIT
Full Wheatstone bridge with 4 active 350Ω strain gages
EXCITATION VOLTAGE
+1.0 to +10.0 Vdc
OUTPUT
mV level, ratiometric to Excitation Voltage
OFFSET
< 1.5 mV at time and temperature of calibration
POWER RATING:
MAX
TYPICAL
INTELLIDUCER1
300 mW
72 mW @ +5.0 Vdc
13 mW @ +5.0 Vdc
STRAIN RANGE
±4,000 µε (Calibrated to ±2,000 µε)
FORCE REQUIRED FOR 1,000µε
~17 lb (~76N)
TYPICAL SENSITIVITY
~500 με/mVout/Vin
ACCURACY2
< ±1%
CALIBRATION
Individually calibrated using N.I.S.T.-traceable automated system.
Calibration curve & factor provided
THERMISTOR (OPTIONAL)
3 kΩ - NTC
EFFECTIVE GAGE LENGTH
3.0 in (76.2 mm) [Gage Extensions available for R/C structures]
CABLE
Custom lead cable length made to order:
IC-02-187 [22 AWG, 2 shielded pair, drain wire, red PVC jacket]
IC-02-250 [22 AWG, 2 shielded pair, drain wire, blue PVC jacket]
IC-03-250 [24 AWG, 3 shielded pair, drain wire, black PVC jacket]
HOUSING
Machined 6061 Aluminum Alloy

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CORROSION PROTECTION
Hard Anodized Clear (MIL-A-8625 Type III)
WEATHER PROTECTION
Designed to exceed IP67
Optional 100 ft (30 m), waterproofing available
TEMPERATURE RATING3
-58° to +176 °F (-50° to +80 °C)
SIZE
4.38 in x 1.25 in x 0.50 in (111 mm x 32 mm x 12.7 mm)
WEIGHT
0.19 lb (85 g)
MOUNTING HOLES
Through holes for ¼ in (M6) bolts or anchors
Reusable mounting tabs (gluing/welding)
1Intelliducer connector required with STS Intelliducer data acquisition nodes.
2Accuracy defined at the calibrated ±2,000 µε range.
3Temperature limit based on instrumentation cable operating temperatures, call BDI for wide temperature range cable options.
2.2 OPTIONS AND ACCESSORIES
The ST350 is supported by several available options and accessories depending on the application and use. See Table 2 for a
list of options and accessories that we supply.
Table 2: ST350 Options and Accessories
OPTIONS AND ACCESSORIES
Intelliducer Connector –Required for use with STS data acquisition nodes,
cable is connected and potted for a weatherproof seal
Integrated Thermistor –Temperature range of -55 °C to +220 °C, ±0.5 °C
accuracy
Reusable Mounting Tabs –¼-20 or M6, zinc plated steel mounting tab
Tab Jig –Machined aluminum jig for safely attaching mounting tabs to the
strain transducer; Includes either 7/16 in or M10 end wrench
Gage Extension –Machined aluminum 24 in (610 mm) gage length extension
with 3.0 in (76 mm) increments

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Extension Jig –Aluminum mounting jig for safely accurately attaching Gage
Extensions
Installation Jigs –Welding or concrete drilling jigs
Protective Covers –Insulated aluminum protective covers
2.3 APPLICATIONS
Table 3: Typical Applications for ST350 strain transducers
HIGHWAY AND RAILWAY BRIDGES
Static and Dynamic Live Load Testing
Moveable Bridge Operation Forces
Overload Detection
RIVER CONTROL STRUCTURES
Navigation Lock Gate Load Responses (Lift, Miter, Radial)
Tainter Gates (Trunnion Friction)
Mechanical Drive Components (Torque, Force)
CABLE FORCES
Individual force & group balancing –static
LABORATORY TESTING
Classroom teaching tool
Measure responses for structural members and components
Integrate with existing A/D systems
BUILDINGS
ASTM Standard Floor Load Tests
Construction Vibration Monitoring
Concrete Curing Temperature Tracking
Earthquake Response Monitoring
3. OPERATION
3.1 THEORY OF OPERATION
The internal components of the ST350 consist of a custom-manufactured 350-Ohm Wheatstone bridge foil transducer-class
strain gage mounted inside a flexible proving ring. The four active arms are arranged inside the ring in such a manner that the
total output provides approximately 3.5 times the output compared to a typical ¼-bridge foil gage under the same induced
strain level.

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The ST350 produces a voltage potential across opposing Wheatstone Bridge corners (Figure 2) which varies with tension and
compression. The sensitive (longitudinal) axis is parallel to the face of the serial plate on the top of the sensor housing (direction
of the cable exit).
Figure 2: ST350 Schematic
3.2 POTENTIAL STRAIN MEASUREMENT ISSUES
3.2.1 Temperature Variation
When the ST350 is attached to a structure subjected to temperature variations, the sensor is forced to undergo the same
temperature-induced deformations as the structural member. In the case of an increase in ambient temperature, the member
will expand. However, since the transducer ends are anchored to the structural member and the “temperature inertia” of the
ST350 is much less, (heats up and cools much faster than the large member) the gage will attempt to expand between the
anchored end blocks, which creates a compression (opposite of expected). The same goes for a temperature drop, which, since
STOP: It is important to note that the portion of the ST350 housing that the label is affixed to (the lid) is
actually “floating” and is only held in place through the weatherproof potting material that is used. Be
cautious while handling and while installing the ST350 not to put excessive pressure on the lid.
STOP: It is important to note that the ST350 was designed for short-term testing applications where changes
in temperature are generally small enough to not affect the output of the strain measurement. We have not
developed temperature compensation curves for the ST350 and therefore recommend that the sensors be
periodically zeroed during any long-term monitoring project and the focus on of the measurement be geared
toward the short-term live-load responses of the structure.

12
the gage cools off faster than the member, will register tension. If the sensor is to be mounted on the structure for an extended
period of time, it will need to have its zero-offset reset periodically as it drifts around with temperature changes.
If the ST350 is exposed to direct sunlight during live-load tests, such as on truss members or on top of a concrete slab, significant
temperature drift can be experienced during short periods of time due to changing cloud cover and/or breezes. Covering the
ST350 with foam and aluminum tape can usually reduce or eliminate this problem. BDI also supplied insulated aluminum covers
that help reduce these effects greatly.
3.2.2 Instrumentation Cable Issues
The output responses for the ST350 will vary slightly depending on the length of the instrumentation cable due to resistance of
the lead wires. To address this, ST350s are calibrated with the instrumentation cable installed, meaning that this resistance has
automatically been taken into account through the calibration process. If, however, the user splices a significantly long cable to
the cable that has been supplied with the unit, a small error that is approximately the ratio of the additional resistance to the
gage resistance will be introduced. Therefore, BDI always recommends calibrating the transducer with the cable that is going
to be used in the field.
3.3 CONNECTING TO DATA ACQUISITION SYSTEMS
This section outlines how to connect and test the ST350 for most standard data acquisition systems that are designed to handle
a differential voltage output.
3.3.1 Excitation Voltage
It is recommended that the Wheatstone bridge excitation voltage be at or below +10 Vdc since higher voltages can cause drift
and stability issues due to gage heating. For example, the BDI STS/SMS data acquisition systems uses a regulated +5 Vdc
excitation voltage which provides excellent results. Allowing the electronics and the gages to warm up for several minutes is
also recommended. A small amount of drift will be detected during the warming process, but should stabilize within several
minutes.
3.3.2 Electrical Connections
The ST350 uses a standard Wheatstone Bridge 4-wire hookup. Table 4 outlines the wiring color/signal connections and includes
the pinout for Intelliducer connectors used with the STS data acquisition nodes.
Table 4: ST350 Wiring Designations & Intelliducer Pinout
INSTRUMENTATION CABLE TYPE
SIGNAL
INTELLIDUCER PINOUT
IC-02-187
& IC-02-250
IC-03-250
RED
RED
+ Excitation
G
BLACK
BLACK (PAIRED W/ RED)
- Excitation (GND)
K
GREEN
GREEN
+ Signal
C
WHITE
BLACK (PAIRED W/ GREEN)
- Signal
J
BARE
ALL BARE WIRES
Shield/Drain (Earth GND)
Integrated into pin K
N/A
WHITE
+ Temp
B
N/A
BLACK (PAIRED W/ WHITE)
- Temp
Integrated into pin K
INFORMATION: When using the ST350 with any STS data acquisition node, the connection has already
been pre-wired to the twist-lock plug and no further action is necessary.

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3.3.3 Applying Calibration Factors
Each ST350 is supplied with a N.I.S.T.-traceable calibration factor. Since this sensor is a ratiometric sensor and can be supplied
a range of excitation voltage, the supplied calibration factor is normalized for excitation voltage. To calculate the proper
calibration factor for the data acquisition system, the excitation voltage that is used must be multiplied by the General Gage
Factor (GGF). The following is an example of the supplied calibration factor:
GGF = ### με/mVout/Vexc
Where:
GGF = General Gage Factor
### = Numeric Calibration Factor
με = microstrain (strain x 10-6)
mVout = Output Voltage in Millivolts DC
Vexc = Excitation Voltage supplied to sensor in Volts DC
Example of applying the GGF:
This example is using a ST350 with a supplied GGF = 504.32 με/mVout/Vexc. The data acquisition system supplies a +5.0 Vdc
excitation and reads the output in volts so the GGF must be adjusted to με/Vout before applying the GGF in the results. The
current reading on the data acquisition system is 3.2312 x 10-3 Vdc.
Step 1: Convert GGF to με/mVout
 
     

 

Step 2: Apply GGF to output voltage from data acquisition system
         

  
3.4 VERIFYING ST350 OUTPUT
It is important to periodically verify the integrity and output of each ST350. Below is a list of tests that we recommend to help
verify that the ST350 is working correctly:
3.4.1 Resistance Test
Using a multimeter, read the Wheatstone Bridge opposing node resistances (black and red leads and then the green and white
leads). Both readings should be very close to 350Ω (<±2Ω). If the resistances vary significantly from the 350Ω value, the unit
may have been deformed by being dropped or mishandled and should be returned to BDI for evaluation.
STOP: Insure that the measured output is in terms of millivolts (mV), or the calculated strain values can be
off by orders of magnitudes.

14
The ST350 has been designed to minimize the amount of maintenance required to keep it operational. Before each use it is
recommended that every sensor be visually inspected for damage and powered on to ensure it is working properly. This should
be done two to three weeks before any scheduled testing in case any repairs are required.
3.4.2 Resolution/Noise Test
Ensure the ST350 resolution and electronic “noise”levels are as low as possible for the DAQs analog-to-digital convertor (A/D).
Run a short test (approximately 15-20 seconds) at approximately 30 Hz or higher and collect data from the sensors while not
being loaded or handled in any way. An example of an output seen for this test can be seen in Figure 3 which illustrates the
typical “noise level”with a 24-bit Analog to Digital (A/D) data acquisition system to be around ±0.2με. Note that the noise level
of the sensor will vary depending on the resolution of the user’s DAQ.
Figure 3: Noise levels of the ST350 on a 24-bit data acquisition system
3.4.3 Sensor Response Test
This test is to ensure the ST350 is producing a smooth output and is returning to its original position. Also ensures that the
responses correspond to typical structural testing (positive for tension and negative for compression). Run a test at a sample
frequency higher than 30 Hz and apply a smooth tension force (gently pulling on each end) followed by a smooth compression
force (gently pushing each end). The output returned should be a tension spike followed by a compression spike and should
not appear “stair-steppy”. An alternative test would be to attach the ST350 to a piece of steel or aluminum and then clamp that
to a rigid desktop so there is a cantilevered portion where the gage is attached. Run the data acquisition system as described
above and then slowly pull up on the “beam” followed by slowly pushing down on the beam. A typical response output for this
type of test is shown in Figure 4.
Figure 4: Typical Smooth Output Response and Zero Offset
Using this test data, also ensure the sensor returned to very near zero. In some cases, it may not return exactly to zero due to
the sensor being heated up from being handled and/or not being placed on the work surface in the same position as it was
sitting before being handled. If a significant offset remains after such a test, this can be an indication of possible damage and
the unit should be returned to BDI.
TYPICAL NOISE LEVELS
WITH 24-BIT A/D

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3.4.4 Field Adjusting Excessive Offsets
If it is determined that zeroing the Wheatstone bridge circuit cannot be accomplished with the DAQs circuitry, then it is possible
that the ST350 has either been deformed to a point where it is providing too much offset. Note that the sensor is in compliance
if offset is within ±2.5 mV. In cases where there is not time to send the sensor to BDI for evaluation and adjustment, the offset
can often be managed in the field after the gage has been installed through the following steps:
1. Loosen the free end of the gage (end opposite of where the cable exits).
2. Put the DAQ in real time monitoring mode and gently push or pull the transducer’s free end as required to bring the it into
the DAQs balancing range.
3. While holding the free end in place, tighten the mounting nut to specification.
4. Double check that balance can be achieved with the DAQ.
If enough force cannot be applied with the gage attached to the structure to allow it to be balanced, remove the gage and
gently apply a tensile of compressive force at both ends by hand. Hopefully, while watching the gage in a real-time output
mode, the gage can be brought into the DAQs balancing range.
If excessive offset cannot be removed, please return the ST350 to BDI for evaluation.
3.5 VERIFYING ACCURACY
Often, our customers like to verify the accuracy of their new ST350, something that we encourage them to do. In general, the
best way to verify most sensor responses is to use them in a system with well-known responses under load. For example,
pressure transducer outputs can be verified through the use of a known height of a column of water.
However, measuring strain involves such extremely small deformations that there are several pitfalls that can be made while
trying to evaluate accuracy. In almost all cases we have seen, the measurements have been proven to be correct, but the
assumptions made in the "strain application system" have been either incomplete or incorrect.
Remember that these accurate sensors have been designed to help obtain the structure's overall behavior since most evaluations
are controlled by flexural, compressive, or shear forces rather than localized stresses at a connection. Therefore, it is best
practice to avoid mounting transducers at possible stress concentrations or structural non-uniformities. For measuring local
strains in tight areas, either a small foil strain gage or an alternative method such as photo-elasticity is required.
3.5.1 Factory Calibration
BDI manufactures an Automated Strain Transducer Calibration System (ASTCS), which has been designed to accurately calibrate
the ST350 strain transducers using a precision linear stage coupled with a pair of extremely accurate LVDT transducers. A NIST-
traceable calibration kit is used to perform annual accuracy verification procedures. The result is that each ST350 is supplied
with a calibration certificate generated by the ASTCS.
3.5.2 Items for Consideration for User Verification Tests
Under no circumstances should loads be applied directly to the ST350. The ST350 is designed with a very flexible geometry,
which enables large strains to be measured with little axial load being transmitted through the ST350. Therefore, when testing
STOP: It is important to understand that the stated resolution and accuracy of a sensor does not guarantee
that a measurement can be taken in the field at the stated resolution or accuracy. The physical and
mechanical conditions of a test will often impart a practical limit of resolution and accuracy that can be
measured. It is the duty of the person responsible for the test to determine where such practical limits apply
when using any sensor from BDI or other manufacturers.

16
typical structural members, the stiffness of the ST350 is inconsequential. The ST350 is intended to provide a measure of strain;
it is not a load cell.
Often, the first verification test to be performed is either on a bending beam or compression/tension specimen in a laboratory
testing machine, with the results compared to the output of a foil strain gage or the theoretical strain value. Remember that
these sensors are designed to measure "axial strain", flexural bending on structural members can be determined via axial strain
measurements as long as the applied curvature is relatively small such that the small angle theory is applicable (SIN θ = θ).
This means that if bending stresses are to be measured, it is best to use a beam with a minimum depth of approximately 12 in
(305 mm) or more, since the ST350 will actually be offset from the beam surface slightly due to the thickness of the mounting
tabs. However, with the beam depth of 12 in (305 mm) or more, this difference is minimal. Another thing to watch out for
during a beam bending test; is that it is very difficult to apply the load to the beam without inducing torsion or lateral bending.
This occurs because the beam was not perfectly "straight" or because the end conditions are not perfectly level with one another.
To minimize this, the ST350 should be mounted with the tab/adhesive technique to the center of the flanges, rather than with
C-Clamps on the edge of the flanges.
Attempting to measure strain on a thin strip of metal mounted as a cantilever beam is not a good verification test for these
sensors. The primary problem with a thin bending specimen is that a large degree of curvature is required to obtain a small
level of surface strain. In other the words, the ST350 will simply be bent rather than elongated. Furthermore, the actual location
of the ST350 will be relatively far from the neutral axis compared to the surface (exaggerated again by the thickness of the tabs
if they are used). Therefore, significant errors are induced when comparing surface strains obtained by a foil strain gage and
the ST350 reading.
For calibration purposes, it is highly recommended that strains be compared at constant moment regions rather than at locations
with significant moment gradients. For the "bending beam" type of test, we recommend a beam at least 10 to 12 feet (3 to 4
m) long, with a shorter beam 4 to 6 feet (1 to 2 m) set on top (with "pins" under each end), and the load cell above that. This
"4-point" type of setup will supply a constant moment region at mid-span. Remember, the strain measured from the ST350 is
averaged over the 3 in (76.2 mm) gage length. Therefore, any error in gage placement or in the assumed strain gradient will
cause errors in subsequent data comparisons.
In almost every case we have seen, a specimen that is supposedly undergoing tension only is actually bending as well. A popular
test is to use a "dog bone" with the ST350 mounted on one side and then the whole assembly put into tension. It is almost
impossible to get pure tension in this setup since the specimen may be slightly bent to begin with and "straightens out" slightly.
Also, since the ST350 themselves have a small amount of stiffness, they will cause a non-symmetrical system. Another
consideration is the distance of the centroid of the ST350 to the specimen's neutral axis. Since bending will most likely occur,
the output from the ST350 may be reduced or amplified since its centroid is about ¼ in (6.4 mm) away from the foil gage
(further from the neutral axis), and this might be the "compression" or "tension" side of the specimen. This phenomenon is very
critical on small laboratory specimens, but insignificant on larger structures where the depths of the sections are usually much
bigger.
For the tension test to be successful, the ST350 should be mounted on both sides of the specimen (on all four sides if the
stiffnesses are similar in two directions) and the output averaged to determine the tension strain. In addition, the specimen
should be relatively stiff compared to the ST350.
If a compression test is being attempted, then the ST350s need to be at least two-member depths away from the ends, (a
criterion for plane strain), with ST350’s mounted on both sides of the specimen and the data averaged. For compression
specimens, it may be necessary to place ST350’s on all four sides since it can often be difficult to know the exact orientation of
the neutral axis if the stiffness is approximately the same in both directions.
Using reinforced concrete as a test specimen material is a poor choice since inaccuracies in the reinforcement locations and
variations in the concrete's elastic modulus (often up to 20%) can cause larger errors than the accuracy range of the ST350.
For example, more aggregate near the surface of one gage will affect the modulus in that area. BDI addresses these issues with
reinforced concrete by using BDI gage extensions (which effectively multiply the strain over anywhere from two to eight gage
lengths). This approach amplifies the signal, thus also improving the signal to noise ratio. With a gage length that is too short,
stress concentrations, micro-cracking, or local effects might have an unusually large effect on the measurements.
For reinforced concrete structures (non-pre-stressed or post tensioned), because of the margins of unknowns in concrete
modulus, load magnitudes, placement of reinforcement, etc., in general, we prefer not to use measurements where the
maximum strain is less than about 30με if we are making conclusions based on the magnitude of strain. (Note that 2με is almost
10% of a 30με peak). This translates into only about 100 psi in concrete and 1ksi in steel, which is quite accurate for analytical

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modeling and load rating reinforced concrete structures. For these types of structures, numbers that are claimed to be more
accurate are probably suspect. Using the ST350 on pre-stressed concrete will usually provide excellent measurements, not only
because there shouldn't be any cracking, but also the concrete modulus usually tends to be more uniform.
We understand that concrete strains are not as accurate as those taken on steel structures and therefore attempt to maximize
the accuracy with the gage extensions.
MEASURING THE APPLIED STRAIN OR LOAD:
Often, the output of a strain gage-based load cell is used in a testing machine as the basis for comparisons in
tension/compression tests. However, we have found that many of these units may not have been calibrated with N.I.S.T.
traceable equipment for years and may be producing inaccurate results. If a gage is manually read for hydraulic pressure, then
the result will be sensitive to jacking friction. Also, if stress and strain are being calculated (σ = E·ε, σ = My/I, etc.), then very
accurate measurements of the cross-sectional areas are required.
MAGNITUDE OF APPLIED LOADS:
Calibration tests should always be run up near the maximum safe linear range of the system. This will give the required
confidence that the output from the ST350 is indeed linear over the range of stresses interest.
We are confident that if the above precautions are taken, the ST350 will provide very accurate and reproducible results. If you
have any questions on the above discussion or have a lab testing "pitfall" experience that you would like to have us investigate
or think it may help other users, please contact us.
4. INSTALLATION
There are several alternative mounting methods that can be used depending on the orientation, location, material being
mounted to (steel, concrete, timber), and the length of test (hours, weeks, months, years). Due to the large number of
variables associated with adhesive use (thermal cycles, UV exposure, vibration, impact, moisture, corrosion of base steel,
etc.,) adhesive is recommended for temporary testing and monitoring applications only. Please contact BDI for further
mounting alternatives.
4.1 GENERAL INSTALLATION GUIDE
The ST350 can be installed on many structure types and in all types of applications, so it is impossible to outline all the details
for each installation. However, with practice and experience, the user can select from a combination of the mounting techniques
that BDI has developed over the years depending on the application. There are several alternative mounting methods that can
be used depending on the orientation, location, material being mounted to (steel, concrete, timber), and the length of test
(hours, weeks, months, years). Please contact BDI for further information.
4.1.1 Prepare Mounting Area
The ST350 only measures strain in the axis in which it is aligned with, therefore the more accurate the alignment, the more
accurate the measurements will be. The easiest way to align the ST350 is to mark a “grid” type pattern for both the proper foot
placement and measurement axis. First, locate the center-line of the gaging area in both the longitudinal and transverse
directions. For example, if measurements are to be obtained at the mid-span of a joist, locate the midpoint between the supports
and the center-line of the joist. The longitudinal mark should be about 8 in (203 mm) long and the transverse mark about 4 in
(102 mm) long. This will allow the marks to be seen while the ST350 is being positioned. This can be seen in Figure 5.

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Figure 5: Alignment Marking for ST350 Installation
From the transverse mark, make two additional marks at 1.5 in (38.1 mm) on either side of the centering mark in Figure 6. The
rectangular areas below are the portions of the member that the necessary surface preparations must be performed.
Figure 6: Mounting Position & Surface Preparation Marks
4.1.2 Installation
Once surface preparation is complete, the ST350 can be installed using the selected mounting technique (see Sections 4.2 and
4.3). The two marks 1.5 in (38.1 mm) from the center-line are used to locate the ST350 longitudinally; align these marks with
the center of the ST350 feet. Notice that the front of the ST350 (end opposite of the cable) has been machined to a slight point.
This point, along with the cable exit on the rear of the ST350, should be aligned with the measurement axis line to ensure that
strain is being measured parallel to the measurement axis. An installed ST350 can be seen in Figure 7. Note that if a ST350
extension is used, the longitudinal mark will need to be 30 in (762 mm) long in order to be seen behind the ST350 with an
extension installed (see Section 4.1.2 ). It is important that this line is drawn carefully as the strains are inherently more
susceptible to error due to misalignment as the gage length increases.
MEASUREMENT AXIS

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Figure 7: Typical ST350 Installation
4.2 INSTALLATION ON STEEL MEMBERS
In most situations, the most efficient method of mounting an ST350 is using the tab/glue method as it is the least invasive and
is truly a “non-destructive testing” technique. The following section outlines an installation for a steel surface.
Figure 8: Identifying Metric vs. Imperial Tabs
Place tabs in the machined slots of the ST350 Tab Jig and then place the ST350 on the positioned tabs as seen in Figure 9.
Thread on the 1/4-20 (or M6) nut and tighten to approximately 40 in-lb.
INFORMATION: BDI manufactures both Imperial (1/4-20) and metric (M6) tabs. To easily distinguish them
from each other, BDI has scribed all metric items as seen in Figure 8.
SCRIBE LINE INDICATES
METRIC THREADS

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Figure 9: Mounting Tabs on the ST350 using the Tab Jig
1. Locate the centerline of the gaging area in both the longitudinal and transverse directions. First, locate the midpoint and
draw two centerlines as shown in Figure 10. The longitudinal centerline should be approximately 4 in (100 mm) long and
the transverse centerline should be approximately 2 in (50 mm) long. This will allow the marks to be seen while the strain
gage is being positioned.
Figure 10: Marking ST350 Center Mounting Location
2. Remove paint or scale from the area where the two lines intersect using a power grinder until a clean metal surface is
obtained.
WARNING: Be careful not to over tighten the nuts on the tabs, over tightening may result in bending the
ST350. If the ST350 has been bent out of alignment, please send the ST350 back to BDI for realignment
and recalibration.