These gages are intended for use on materials with relatively low stiffness (low Young's modulus), such as wood. To avoid unwanted localized reinforcement of the strain zone by the gage, they have an especially compliant backing material.  
More information on the PFLW / PLW series

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PLW / PFLW Series - Low Modulus Materials
Gage
Pattern
Sensing
Element
Length
(mm)
Sensing
Element
Width
(mm)
Backing
Length
(mm)
Backing Width
(mm)
Resistance Ω
PFLW-30 30 2.3 40 7.0 120
PLW-60 60 1.0 74 8.0 120
Single Axis
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