Campbell Scientific ST350 Strain Transducer for Structural Testing - Manual (Page 39)

Appendix B. ST350 Accuracy
Verification
B.1 Verifying the Accuracy of ST350 Strain
Transducers
B.1.1 Introduction
Often, our customers like to verify the accuracy of their new ST350 Strain
Transducers, something that we encourage them to do. However, there are
several pitfalls that can be made while trying to check these sensors out in the
laboratory. Having fielded similar questions from several customers, we have
assembled the following explanations to help avoid some of these problems. In
almost all of the cases we have seen, the measurements have been proven to be
correct, and the assumptions made in the "strain application system" or
structural system are either incomplete or incorrect.
Remember that these accurate sensors have been designed to help obtain the
structure's overall behavior, rather than at possible stress concentrations like at
connections and rivet points. This is because most bridge ratings are controlled
by the flexural or shear stresses, rather than localized stresses at a connection.
Therefore, it is best to keep the transducers away from 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.
B.1.2 Background
These full-Wheatstone bridge strain transducers were originally developed in
about 1970 for use in the driven pile industry. They were designed for
recording strains on the side of a pile (steel or concrete) as it was being driven
with a pile hammer. This operation applies very high accelerations and
requires a very rugged sensor to survive. Over the ensuing years, the
transducers have been tested extensively to determine their limitations, often
leading to design refinements. Based on the latest design, the Strain
Transducers have been modified slightly through the use of a different type of
internal strain gage that is better suited for static or "semi-static" structural load
testing.
B.1.3 Factory Calibrations
These sensors are calibrated by inputting a known excitation voltage and
applying a known strain and then recording the output over approximately a
1000
με range. The manufacturer’s calibration that we supply is performed
with a NIST-traceable system that consists of a small precision slide table and
an optical displacement sensor. The entire calibration process is always
verified by a calibrated precision micrometer. Reproducibility of this system is
typically better than one percent and in no case worse than two percent.
In field test applications with linear-elastic structures, we have found
repeatedly that we can expect reproducibility of the measurements of
approximately
± 2.0 microstrain. The errors contained in this result included
B-1