8.1. LABORATORY MEASUREMENTS
467
• Instrument accuracy and reliability
• Negligible response to accelerations
• High resolution and low noise level
• Low nonlinearity over the measurement range
• Low hysteresis and small zero shift
Instrument Components
Practically all instruments are comprised of two essential components: a
transducer (or sensor) that converts the physical quantity being measured
into a signal that can be recorded, and a recording unit to capture the
measurement. Other instrument components can include mechanical or
electronic signal conditioners that relay the signal to the recording device
with minimum distortion, and control units that allow the instrument to
be configured properly and operated more easily. For example, a pressure
cell senses the combination of static and dynamic pressure and converts it
into an analog electrical signal. The electrical signal might be enhanced or
otherwise conditioned before being converted from analog form into digital
form for recording by a computer system. Conversion of voltages into pressure units is done by applying known conversion factors either before the
data are recorded or at some later time.
Most sensors measure some property that can be related to the physical
quantity we want to measure. For example, spring-type scales weigh an
object by relating the deflection of a spring to the force (weight) being
exerted on the scale. Likewise, some wave gauges relate changes in water
elevation to changes in the capacitance of a wire that pierces the sea surface.
In both cases, the relationship between what is actually measured and the
physical quantity that is sought must be determined from known conversion
constants or through a “calibration process.”
A strong physical relationship must exist between the physical quantity
that is actually measured by the sensor and the physical quantity we are
attempting to characterize with the measurement. In the case of a springtype scale, the physical relationship between the spring deflection and the
imposed weight is well known and reliable. Furthermore, we know the range
of spring deflection over which this relationship is valid.
The difficult quantities to measure are those that are not easily related
to physical quantities we know how to measure. Often we are forced to
make additional assumptions about the physical relationship, and conse
quently, our measurements are more in doubt unless extensive validation
has taken place. For example, sea surface elevations can be estimated from
measurements of wave velocities near the bottom using linear wave
ory. Studies have shown a reasonable correspondence exists until the waves
467
• Instrument accuracy and reliability
• Negligible response to accelerations
• High resolution and low noise level
• Low nonlinearity over the measurement range
• Low hysteresis and small zero shift
Instrument Components
Practically all instruments are comprised of two essential components: a
transducer (or sensor) that converts the physical quantity being measured
into a signal that can be recorded, and a recording unit to capture the
measurement. Other instrument components can include mechanical or
electronic signal conditioners that relay the signal to the recording device
with minimum distortion, and control units that allow the instrument to
be configured properly and operated more easily. For example, a pressure
cell senses the combination of static and dynamic pressure and converts it
into an analog electrical signal. The electrical signal might be enhanced or
otherwise conditioned before being converted from analog form into digital
form for recording by a computer system. Conversion of voltages into pressure units is done by applying known conversion factors either before the
data are recorded or at some later time.
Most sensors measure some property that can be related to the physical
quantity we want to measure. For example, spring-type scales weigh an
object by relating the deflection of a spring to the force (weight) being
exerted on the scale. Likewise, some wave gauges relate changes in water
elevation to changes in the capacitance of a wire that pierces the sea surface.
In both cases, the relationship between what is actually measured and the
physical quantity that is sought must be determined from known conversion
constants or through a “calibration process.”
A strong physical relationship must exist between the physical quantity
that is actually measured by the sensor and the physical quantity we are
attempting to characterize with the measurement. In the case of a springtype scale, the physical relationship between the spring deflection and the
imposed weight is well known and reliable. Furthermore, we know the range
of spring deflection over which this relationship is valid.
The difficult quantities to measure are those that are not easily related
to physical quantities we know how to measure. Often we are forced to
make additional assumptions about the physical relationship, and conse
quently, our measurements are more in doubt unless extensive validation
has taken place. For example, sea surface elevations can be estimated from
measurements of wave velocities near the bottom using linear wave
ory. Studies have shown a reasonable correspondence exists until the waves