Campbell Scientific ST350 User Manual
ST350 Strain Transducer
2/08
Copyright © 2008
Campbell Scientific, Inc.
Warranty and Assistance
The ST350 STRAIN TRANSDUCE R is wa rra nt ed by C AM PB ELL
SCIENTIFIC, INC. to be free from defects in materials and workmanship
under normal use and service for thirty-six (36) months from date of shipment
unless specified otherwise. Batteries have no warranty. CAMPBELL
SCIENTIFIC, INC.'s obligation under this warranty is limited to repairing or
replacing (at CAMPBELL SCIENTIFIC, INC.'s option) defective products.
The customer shall assume all costs of removing, reinstalling, and shipping
defective products to CAMPBELL SCIENTIFIC, INC. CAMPBELL
SCIENTIFIC, INC. will return such products by surface carrier prepaid. This
warranty shall not apply to any CAMPBELL SCIENTIFIC, INC. products
which have been subjected to modification, misuse, neglect, accidents of
nature, or shipping damage. This warranty is in lieu of all other warranties,
expressed or implied, including warranties of merchantability or fitness for a
particular purpose. CAMPBELL SCIENTIFIC, INC. is not liable for special,
indirect, incidental, or consequential damages.
Products may not be returned without prior authorization. The following
contact information is for US and International customers residing in countries
served by Campbell Scientific, Inc. directly. Affiliate companies handle
repairs for customers within their territories. Please visit
www.campbellsci.com to determine which Campbell Scientific company
serves your country. To obtain a Returned Materials Authorization (RMA),
contact CAMPBELL SCIENTIFIC, INC., phone (435) 753-2342. After an
applications engineer determines the nature of the problem, an RMA number
will be issued. Please write this number clearly on the outside of the shipping
container. CAMPBELL SCIENTIFIC's shipping address is:
CAMPBELL SCIENTIFIC, INC.
RMA#_____
815 West 1800 North
Logan, Utah 84321-1784
CAMPBELL SCIENTIFIC, INC. does not accept collect calls.
ST350 Table of Contents
PDF viewers note: These page numbers refer to the printed version of this document. Use
the Adobe Acrobat® bookmarks tab for links to specific sections.
1. Introduction ...............................................................1-1
1.1 Typical Application.............................................................................. 1-1
2. Specifications............................................................2-1
3. Sensor Alignment and Installation..........................3-1
3.1 Alignment............................................................................................. 3-1
3.2 Installation............................................................................................ 3-2
3.3 Adjusting Excessive Transducer Offset................................................ 3-3
4. Wiring.........................................................................4-1
4.1 Initial Check-Out.................................................................................. 4-1
4.2 Excitation Voltage.................................................................................4-1
5. Mounting of Sensor to Various Surfaces................5-1
5.1 General.................................................................................................5-1
5.2 Mounting Information for Different Types of Surfaces.......................5-2
5.2.1 Steel............................................................................................ 5-2
5.2.2 Reinforced Concrete................................................................... 5-3
5.2.3 Pre-stressed Concrete..................................................................5-6
5.2.4 Timber ........................................................................................ 5-7
6. Calibration and Validation........................................6-1
7. Maintenance, Replacement Parts, and Repairs......7-1
7.1 Maintenance .......................................................................................... 7-1
7.2 Replacement Parts................................................................................ 7-2
8. Datalogger Programming.........................................8-1
8.1 CR1000 Example ................................................................................. 8-1
8.2 CR5000 Example ................................................................................. 8-2
Appendices
A. Special Instructions for using ST350.................... A-1
A.1 Instructions for Using ST350 Strain Transducer Extensions on
Reinforced Concrete Structures.............................................................. A-1
A.2 Attaching the Concrete Extension to a Strain Transducer.................. A-2
i
ST350 Table of Contents
B. ST350 Accuracy Verification..................................B-1
C. Calibration Sheets...................................................C-1
Index.........................................................................Index-1
Figures
B.1 Verifying the Accuracy of ST350 Strain Transducers.........................B-1
B.1.1 Introduction ............................................................................... B-1
B.1.2 Background................................................................................ B-1
B.1.3 Factory Calibrations................................................................... B-1
B.1.4 Temperature Effects................................................................... B-2
B.1.5 Specimen Type and Size............................................................ B-2
B.1.6 Items for Consideration............................................................. B-2
B.1.7 Other Considerations................................................................. B-4
C.1 Example of Calibration Sheet — BDI Supplied.................................. C-1
C.2 Example of Calibration Sheet — CSI Supplied................................... C-2
3-1. Measurement Axis...............................................................................3-1
3-2. Surface Preparation - Location ............................................................3-2
3-3. ST350 Mounting Example ................................................................... 3-2
4-1. ST350 Electrical Wiring Diagram.......................................................4-1
7-1. ST350 Strain Transducer Test Output .................................................7-1
7-2. Proper Connection to Data Acquisition System for Tension and
Compression............................................................................................7-2
A-1. Extension Jig...................................................................................... A-3
A-2. Drawing Extension Jig.......................................................................A-3
A-3. Picture Compressing ST350 for Mounting Purposes ........................ A-4
A-4. Extension Alignment Tab..................................................................A-4
A-5. Desired Gage Length.........................................................................A-4
A-6. Example of Ceiling Mounting............................................................A-5
C-1. Bridge Diagnostics Calibration Sheet................................................C-1
C-2. Campbell Scientific’s Calibration Sheet............................................ C-2
Tables
A-1. Recommended Lower and Upper Gage Limits................................. A-1
A-2. Maximum Strain Ranges....................................................................A-2
ii
Section 1. Introduction
This manual provides information for interfacing the ST350 Strain Transducer
to Campbell Scientific’s Dataloggers. Unless otherwise specified, all part
numbers are Campbell Scientific's.
This manual contains information on sensor specifications, operating
principles, installation, alignment, and calibration. The multiplier and offset
values given here are based on calibration data obtained from the Bridge
Diagnostic’s Calibration Sheet (see example Appendix C).
The most direct approach to quantifying live-load stresses in a structural
member is to record the induced strain. However, it can be tedious work
installing foil strain gages in the field since careful surface preparation and
soldering is often required. Now, most field strain gage installations can be
replaced with the highly accurate ST350 Strain Transducer. These units are
rugged and can be installed in any weather. Since they are pre-wired and easy
to mount, ST350 Strain Transducers will drastically reduce your field
installation time.
1.1 Typical Application
This transducer is typically used for dynamic or event driven stress in structural
members such as bridges or buildings. The ST350 Strain Transducers have
been designed for recording Live Load
there will be little to no temperature change during any short time-span testing
sequence.
When a transducer is attached to a structure, it is forced to have the same
deformation as the structure. However, if a temperature increase (or decrease)
occurs, and since the ends of the sensor are "anchored", the transducer will
expand between the end blocks and register compression. The same goes for a
drop in temperature which will register tension. If the sensor is to be mounted
on the structure for a long period of time, it will need to have its "zero" reset
periodically as it drifts around with temperature changes.
strains only. Hence it is assumed that
1-1
Section 1. Introduction
1-2
Section 2. Specifications
Effective gage
length:
Overall Size: 4.375 in x 1.25 in x 0.5 in (111 mm x 32 mm x 13 mm).
Cable Length: 10 ft (3 m) standard, any length available.
Material: Aluminum
Circuit:
Accuracy: ±2%, reading individually calibrated to NIST standards.
Strain Range:
Force req’d for
1000 με:
Sensitivity:
Weight: Approximately 3 oz. (85 g).
Environmental: Built-in protective cover, also water resistant.
3.0 in (76.2 mm). Extensions available for use on R/C
structures.
Full wheatstone bridge with four active 350 Ω foil
gages, 4-wire hookup.
Approximately ±2000 εμ.
Approximately 17 lbs. (76 N).
Approximately 500 εμ/mV/V.
Temperature
Range:
Cable: BDI RC-187: 22 gage, two individually-shielded pairs
Options: Fully waterproofed, Heavy-duty cable, Special quick-
Attachment
Methods:
-58°F to 185°F (-50°C to 85°C) operation range.
w/drain.
lock connector (available upon request).
C-clamps, threaded mounting tabs & quick-setting
adhesive, wood screws, or concrete anchors.
2-1
Section 2. Specifications
2-2
Section 3. Sensor Alignment and Installation
3.1 Alignment
The BDI ST350 will only measure 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 a transducer is to mark a “grid” type pattern
for both the proper foot placem e nt 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 inches long and the transverse mark about
4 inches long. This will allow the marks to be seen while the transducer is
being positioned. This can be seen in the picture below.
MEASUREMENT AXIS
FIGURE 3-1. Measurement Axis
From the transverse mark, make two additional marks at 1.5 inches on either
side of the centering mark (see below photo). The areas circled below are the
portions of the cross-section that the necessary surface preparations must be
performed. Surface preparation techniques are explained in Section 5:
Mounting of sensor to various surfaces.
3-1
Section 3. Sensor Alignment and Installation
FIGURE 3-2. Surface Preparation - Location
3.2 Installation
Once surface preparation is complete, the transducer can be installed using the
selected mounting technique (see Sections 5- Mounting of Sensors to Various
Surfaces). The two marks 1.5 inches from the center-line are used to locate the
transducer longitudinally; align these marks with the center of the transducer
feet. Notice that the front of the transducer (end opposite of the cable) as been
machined to a slight point. This poi nt , along with the cable exit on the rear of
the transducer, should be aligned with the measurement axis line to ensure that
strain is being measured parallel to the measurement axis. An installed
transducer can be seen in the picture below. Note that if an R/C extension is
used, the longitudinal mark will need to be 30 inches long in order to be seen
behind the transducer/ extension combination. 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.
3”
SURFACE PREP. LOCATIONS
FIGURE 3-3. ST350 Mounting Example
3-2
Section 3. Sensor Alignment and Installation
3.3 Adjusting Excessive Transducer Offset
If it is determined that zeroing cannot be accomplished with the Wheatstone
Bridge circuit, then it is possible that the transducer has either been damaged or
deformed slightly. In many cases the deformation is caused by a thermal
change in the gage due to weather changes, such as location of the sun. In this
case, the offset can be adjusted by simply loosening one nut and allowing it to
return to a “zero-stress” state. Once the nut is loose, rebalance the bridge and
ensure the gage can be zeroed. Retighten the nut and again, rebalance and
ensure the gage is zeroed. If the transducer still cannot be zeroed, ensure that
the mounting surface is flat. In many mounting situations, especially on timber
and aged concrete, additional surface preparation will need to be performed to
obtain a flat mounting surface. If it has been determined neither of the above
are causing the excessive offset proceed with the following steps:
1. Determine which direction the offset is in.
2. If the gage is too far in compression, loosen the free end of the gage (the
end opposite of where the cable exits).
NOTE
Sensor is in compliance if the offset is within ±2.1 mV/V
excitation (approximately 1000 microstrain).
3. Pull on this end of the transducer gently and re-tighten the nut or C-clamp.
4. If enough force cannot be applied with the gage attached to the structure,
remove the gage and pull it from both ends. Hopefully, while watching
the gage in “Monitor” mode, the gage will come closer to zero.
5. If the offset is in the opposite direction (i.e. too far offset in tension)
perform steps two through four, except push on the transducer rather than
pull.
If this initial offset cannot be removed, please return the transducer to BDI for
evaluation.
Remember! The transducers are high-quality, precision sensors and are
therefore quite sensitive, so be very careful while handling them!
3-3
Section 3. Sensor Alignment and Installation
3-4
Section 4. Wiring
4.1 Initial Check-Out
Upon receiving new transducers, it is important to check that they are in proper
working order. Using an ohmmeter, read the resistances between the black and
red wires and then the green and white wires, both read ings should be very
close to 350Ω. If they are not, the unit may be unusable and should be returned
to BDI either for repair or replacement. This test should be performed on a
periodic basis, especially if the transducer has been dropped or otherwise
mishandled.
Campbell Scientific, Inc. data acquisition systems support the use of a full
Wheatstone bridge sensor. The ST350 strain transducer has four active arms
consisting of 350 Ω strain gages. This configuration provides approximately 3
to 3-1/2 times the output of a standard 1/4-arm foil gage installation for a given
strain level. The connection sequence is shown in the following figure.
FIGURE 4-1. ST350 Electrical Wiring Diagram
NOTE
Output = V
Therefore, [+ Output
Compression]
is defined as ((+ Sig) – (- Sig))
diff
4.2 Excitation Voltage
The recommended excitation voltage is generally between 2.5 and 5 volts DC.
NOTE
When programming this transducer use Reverse Excitation to
cancel effects of Lead Resistance. CSI recommends performing
a reverse measurement to eliminate any hardware offsets. See
datalogger programming example for further information.
Once the transducer has been connected to the data acquisition system, the user
should verify output by monitoring the signal in real time while gently placing
the transducer in tension and compression by hand. This will ensure that
= Member in Tension] and [- Output = Member in
4-1
Section 4. Wiring
tension provides a positive output signal and compression a negative signal. If
a tension force provides a negative signal (and vice-versa), the user should
either switch the signal leads or make appropriate adjustments to the signal
conditioning.
NOTE
Before going to the field, Campbell Scientific highly
recommends that a simple validation be performed by the user to
ensure that signal conditioning, gains, and calibration factors are
being properly applied.
Please see informational write-up entitled “Verifying the Accuracy of ST350
Strain Transducers – Appendix B” on some of the things to look out for while
running your own calibration verification.
4-2
Section 5. Mounting of Sensor to Various Surfaces
5.1 General
In most situations, other than reinforced concrete, the most efficient method of
mounting a transducer is using the tab/glue method. This method is the least
invasive and is truly a “non-destructive testing” technique. Below is an outline
for implementing the glue/tab technique. Tips and alternative mounting
techniques for different mounting surfaces can be found in the following
sections.
1. Place two tabs in mounting jig (if available, if not simply hold with vice
grips). Place transducer over mounts and tighten the 1/4-20 nuts until
tight. Be sure that the transducer calibration number is facing up. This
procedure allows the tabs to be mounted without putting stress on the
transducer itself.
2. Mark the centerline of the transducer location on the structure. Place
marks 1-1/2 inch on both sides of the centerline and using a grinder,
remove paint or scale from these areas. For steel structures, a power
grinder is recommended for the initial cleaning. If available, use a
portable grinder (a Makita Model 9500D battery-powered grinder with a
46-grit wheel works very well) to “touch up” the newly-cleaned surface.
If attaching to concrete, lightly grind the surface with the portable grinder
to remove any scale and remove dust with a shop rag or paint brush.
3. Using the portable grinder, very lightly grind the bottom of the transducer
tabs to remove any oxidation and/or other contaminants. Before
mounting, set the transducer in the location it is to be attached, and ensure
that the tabs seat uniformly on the member and that the transducer doesn’t
“rock”. This is important for a good bond.
4. Apply a thin line of adhesive to the bottom of each transducer tab (Loctite
410 Black Toughened Adhesive, Part # 41045 in 0.7oz containers)
about 1/4” wide. If bonding to concrete, slightly more adhesive is
necessary to allow some to flow out and around the tabs. Mount the
transducer in the marked location, and then pull it away. This action will
apply adhesive to the structural member at the tab locations.
5. Spray each adhesive contact area on the structural member (just one “light
shot”) with the adhesive accelerator (Loctite Tak Pak 7452, Part # 18637
in 0.7oz aerosol spray container).
6. Very quickly, mount transducer in its proper location and apply a light
force to the top of the tabs (not the center of the transducer) for
approximately 15-20 seconds.
If the above steps are followed, it should be possible to mount each
transducer in approximately five minutes.
5-1