Environmental Engineering Reference
In-Depth Information
Instrumentation Characteristics
Precision
refers to the degree of measurement agreement required in relation to true or
accepted reference values of the quantity measured.
Sensitivity
refers to the smallest unit detectable on the instrument scale, which generally
decreases with variations in the instrument measurement range.
Repeatability
refers to obtaining sequential readings with similar precision. It can be the
most significant feature, since it indicates the trend in change of the measurement quan-
tity and is often more important than sensitivity or precision.
Drift
refers to the instrument error progression with time and affects precision.
Reliability
refers to the resistance to large deformations beyond the desired range, high
pressures, corrosive elements, temperature extremes, construction activities, dirty envi-
ronment, humidity and water, erratic power supply, accessibility for maintenance, etc. For
a particular application, the less complicated the instrument the greater is its reliability.
Thus, mechanical instruments are preferred to electrical; stationary parts are preferred to
moving parts. Electrical instruments should be battery-powered for greater reliability.
Service life required
can vary from a few weeks, to months, to a year, or even several years
and is influenced by the reliability factors.
Calibration
Instrument Precision
Precision is verified upon the receipt of an instrument from the manufacturer by checking
it against a standard and corrections are considered for temperature and drift. After instal-
lation, precision is verified, then checked periodically during monitoring and upon proj-
ect completion.
Bench Marks or Other Reference Data
Reference points such as bench marks are installed or established and protected to ensure
reliability against all possible changes that may occur during the project life. Care should
be taken to ensure that the bench mark does not move. The best bench marks are sup-
ported on sound rock; Borros points
(Section 4.3.2)
set in hard soil are an option where the
rock is very deep. “Temporary” reference points that require transfer to permanent refer-
ences should be avoided.
Installation and Maintenance
Some systems are relatively simple to install and maintain, since they are portable and
require only surface reference points; others require boreholes or excavations and are
therefore difficult to install and maintain. These factors substantially affect costs.
Operations
Data Collection and Recording
Data collection should take place on a planned basis. Data may be collected and recorded
manually or automatically, directly or remotely, and on a periodic or continuous basis.
Automatic collection and remote recording can be connected to alarm systems and are
useful in dangerous situations, or in situations where long periods of inactivity may be fol-
lowed by some significant occurrence such as an earthquake or heavy rainfall. Readout
systems measure and display the measured quantity.