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
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