Environmental Engineering Reference
In-Depth Information
Frequency of Observations
Construction progress, data trends, and interpretation requirements influence the fre-
quency of the observations. Periodic monitoring is always required. The frequency of
readings increases when conditions are critical.
Data Processing and Interpretation
Data are recorded, plotted, reviewed, and interpreted on a programmed basis, either peri-
odic or continuous. Failures have occurred during monitoring programs when data were
recorded, collected, and filed, but not plotted and interpreted.
4.1.5
Transducers
General
Transducers, important elements of complex instruments, are devices actuated by energy
from one system to supply energy, in the same or some other form, to a second system. They
function generally on the membrane principle, the electrical resistance gage principle, or the
vibrating-wire principle. Occasionally they function on the linear-displacement principle.
Membrane Principle
Pressure against a membrane is measured either hydraulically (e.g., by hydraulic
piezometers) or pneumatically (e.g., by the Gloetzl pressure cell, the pneumatic piezome-
ter, or geophones).
Resistance Strain Gages
Applications
Resistance strain gages are cemented to structural members or used as sensors in load
cells, piezometers, extensometers, inclinometers, etc.
Principle
The straining of a wire changes its cross-sectional area and consequently its electrical
resistance. When a strain-gage wire is attached to a structural member, either externally or
as the component of an instrument, measurements of changes in electrical resistance are
used to determine strains in the structural element. There are several types of gages.
Bonded Gages
The most common form is the bonded gage, which consists of a thin wire filament or metal
foil formed into a pattern and bonded to a backing of paper or thin plastic
(Figure 4.1)
.
The
backing is then cemented to the surface where the measurements are to be made.
Gage lengths range from 1/16 to 6 in., and strain sensitivity is usually of the order of 2
to 4 microstrains, with ranges up to 20,000 to 50,000 microstrains (2-5% strain) for normal
gages. High elongation gages ranging up to strains of 10 to 20% are available.
Encapsulated Gages
Bonded gages mounted and sealed in the factory into a stainless steel or brass envelope
are termed as encapsulated gages. They are welded to the measurement surface or embed-
ded in concrete. Encapsulated gages provide better protection against moisture than
unbonded gages.
