Leadwire Selection

Leadwire Selection
Introduction: Leadwire Selection
Leadwire Selection
Leadwires, properly selected and installed, should play a passive, yet critical, role
in strain measurement. Their primary function is to deliver power to those portions
of the strain gage circuit remote from the measurement instrument and to retrieve
the strain-induced measurement signal from the gage installations. Ideally,
leadwires take nothing from the measurement signal and add nothing to it.
While all leadwires do influence the signal to some extent in actual practice,
Micro-Measurements offers a complete line of wires and cables which can be used
to minimize leadwire errors. Following is a brief outline of the more important
selection considerations:
Temperature Capabilities
Number of Conductors
Diameter of Conductor
Solid vs. Stranded Wire
Twisted/Shielded Cables
Adhesion to Environmental Protection
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Introduction: Leadwire Selection
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Temperature Capabilities: Leadwire Selection
Leadwire Selection
Temperature Capabilities
Copper conductors are used extensively in strain gage applications because they
represent a good compromise between electrical conductivity and cost. Their "use
temperature" range is influenced by both the electrical insulation and conductor
plating, if any, provided with them.
Multi-stranded wires with lead-tin coatings and PVC insulation are the most
economical choice and are widely used in strain gage applications. They are,
however, restricted to an upper temperature limit of about +180° F (+80° C).
Because PVC cracks at low temperatures, these leadwires should only be used
above -60° F (-50° C). Silver plating extends the use temperature of multi-stranded
wire. With Teflon coatings, the range is -452° to +500° F (-269° to +260° C), and
with polyimide, -452° to +600° F (-269° to +315° C). Teflon is often used for
room-temperature applications when fire codes specify that the insulation must not
support combustion.
Polyurethane enamels on unplated single conductor wires are usable between -100°
and +300° F (-75° to +150° C), while polyimide coatings extend the limit from
-452° to +600° F (-269° to +315° C). Nickel plating and fiberglass braid insulation
on single conductor wires provide the widest use temperature range, -452° to +900°
F (-269° to +480° C).
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Number of Conductors: Leadwire Selection
Leadwire Selection
Number of Conductors
Micro-Measurements offers 1-, 3-, and 4 conductor wires and cables.
Single-conductor wires are used almost exclusively for making intrabridge
connections between gages, or between gage and terminal strip. Three-conductor
cables are used for quarter-bridge circuits and half-bridge applications. Four-wire
cables, of course, are for connecting full bridges to the strain gage instrumentation.
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Diameter of Conductor: Leadwire Selection
Leadwire Selection
Diameter of Conductor
From the standpoint of error reduction, the largest possible wire diameter should be
used to minimize leadwire desensitization of gage factor and leadwire-related
thermal output. From a practical standpoint, however, smaller wires are often
necessary because of their influence on measurements (reinforcement, mass effects,
etc.), or physical size of wire (ability to solder to gage tabs). As a good rule of
thumb, the longer the leadwire, the larger the wire diameter should be.
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Solid vs. Stranded Wire: Leadwire Selection
Leadwire Selection
Solid vs. Stranded Wire
In general, stranded wire is more flexible and less susceptible to fatigue-failure.
Therefore, solid wires are recommended for use primarily in making intrabridge
connections where they can be readily formed to the shape of the test part and
bonded to the surface to minimize fatigue. An exception is nickel-plated wires,
used for elevated temperatures and available only as solid conductors.
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Twisted/Shielded Cables: Leadwire Selection
Leadwire Selection
Twisted/Shielded Cables
Special twisted and/or shielded cables are generally needed when leadwires pass
through electric and/or magnetic fields which can superimpose electrical noise on
the measurement signal. A detailed discussion of these noise sources, and the
recommended cable selections to minimize their effects, is given in Tech Note
TN-501, Noise Control in Strain Gage Measurements.
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Adhesion to Environmental Protection: Leadwire Selection
Leadwire Selection
Adhesion to Environmental Protection
A leadwire is the only portion of a gage installation which should extend beyond
the bounds of the protective coating. As such, it always represents a possible
avenue for moisture invasion into the installation. A good moisture-proof bond
between leadwire coating and protective coating is essential for long-term stability.
To ensure the best possible bonds, cables with PVC insulation should be precoated
with thinned M-Coat B before environmental protections are applied. Teflon cable
coatings should be etched with M-LINE TEC-1 Tetra-Etch Compound to improve
the bond.
Micro-Measurements Catalog A-110, Strain Gage Accessories, presents an
extensive selection of wires, cables, and accessories to handle a broad range of
applications. Each has been thoroughly field tested and found to give excellent
sensor performance when properly used in the specified environments. If you have
any questions about the right leadwire for your specific application, our
Applications Engineering Department will be pleased to make specific
recommendations.
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