Every sensor is designed to work over a specified range. The design ranges
are usually fixed, and if exceeded, result in permanent damage to or destruction
of a sensor. It is customary to use transducing elements over only the
part of their range where they provide predictable performance and often
When making a measurement it is necessary to start at a known datum, and
it is often convenient to adjust the output of the instrument to zero at
the datum. It, therefore, is a value ascribed to some defined point in
the measured range.
The signal level may vary from its set zero value when the sensor works.
This introduces an error into the measurement equal to the amount of variation,
or drift as it is usually termed. Zero drift may result from changes of
temperature, electronics stabilizing, or aging of the transducer or electronic
Sensitivity of a sensor is defined as the change in output of the sensor
per unit change in the parameter being measured. The factor may be constant
over the range of the sensor (linear), or it may vary (nonlinear).
Resolution is defined as the smallest change that can be detected by a
The time taken by a sensor to approach its true output when subjected to
a step input is sometimes referred to as its response time. It is more
usual, however, to quote a sensor as having a flat response between specified
limits of frequency. This is known as the frequency response, and it indicates
that if the sensor is subjected to sinusoidally oscillating input of constant
amplitude, the output will faithfully reproduce a signal proportional to
The most convenient sensor to use is one with a linear transfer function.
That is an output that is directly proportional to input over its entire
range, so that the slope of a graph of output versus input describes a
Hysteresis refers to the characteristic that a transducer has in being
unable to repeat faithfully, in the opposite direction of operation, the
data that have been recorded in one direction (Figure 2).
If a meaningful measurement is to be made, it is necessary to measure the
output of a sensor in response to an accurately known input. This process
is known as calibration, and the devices that produce the input are described
as calibration standards.
A dynamic range of stimuli which may be converted by a sensor id called
a span or an input full scale (FS). It represents the highest possible
input value which can be applied to the sensor without causing unacceptably
large inaccuracy (shown in Figure 3).
Full Scale Output
Full scale output (FSO) is the algebraic difference between the electrical
output signals measured with maximum input stimulus and the lowest input
stimulus applied. This must include all deviations from the ideal transfer
function. For instance, the FSO output in Figure 3 is represented by SFS.
A very important characteristic of a sensor is accuracy which really means
inaccuracy. Inaccuracy is measured as a ratio of the highest deviation
of a value represented by the sensor to the ideal value. It may be represented
in terms of measured value (Delta, shown in Figure 3)
Sensors in the Industrial Environment
In the laboratory the atmosphere is relatively free from contaminants,
the temperature is relatively stable, the area is free from vibration,
and personnel are specialized in handling the equipment they are using.
The industrial measure is often made under reverse conditions. The produced
signal must be capable of transmission to recording equipment which may
be a considerable distance away. The wiring between the two may be induced
much electrical noise.