Measurement Technology - The Electronic Nose Artificial Olfaction Technology - page 3

signal is directly generated in the sensing element, as is the case of a ther-
mocouple that generates a voltage in response to a difference in temperature
between its two ends. Table 1.1 shows the examples of sensor elements.
3. The variable conversion element serves the function of altering the nature of the
signal generated by the sensor. The method of converting the nature of the
signal generated in the sensor to another suitable signal form (usually electrical)
depends essentially on the sensor. Individual signal conditioning modules are
characteristics of a group of sensing elements. Analog electronic circuits finally
produce electrical signals in the form of voltage or current in specific ranges.
Variable conversion elements are needed where the output variable of a pri-
mary transducer is in an inconvenient form and has to be converted into a more
convenient form. For instance, the displacement-measuring strain gage has an
output in the form of a varying resistance. The resistance change cannot be
easily measured and so it is converted to a change in voltage by a bridge circuit,
which is a typical example of a variable conversion element. In some cases, the
primary sensor and variable conversion element are combined, and the com-
bination is known as a transducer.
4. The signal processing/transmission element is used to process the signal gen-
erated by the first stage for a variety of purposes such as, filtering (to remove
noise), diagnostics (to assess the healthiness of the sensor), linearization (to
obtain an output which is linearly related with the physical measurand) etc.
Signal processing systems are therefore usually more general purpose in nature.
Signal transmission is needed when the observation or application point of the
output of a measurement system is some distance away from the site of the
primary transducer.
5. The target signal-handling element may perform a variety of functions
depending on the target application. It may therefore contain data/signal display
modules, recording or storage modules, or simply a feedback to a process
control system. The output of the primary sensing element may be electrical
signal of any form. It may be voltage, a frequency or some other electrical
parameters. When the elements of an instrument are actually physically sepa-
rated, it becomes necessary to transmit data from one to another.
Examples contain a temperature chart recorder, an instrumentation tape
recorder, a digital display or an analog to digital converter (ADC) tracked by an
interface to a process control computer.
1.4 Units of Measurement
Near the beginning systems of measurement were based on whatever was available
as a measuring unit. For purposes of measuring length, the human upper body was
a convenient tool, and gave us units of the hand, the foot, and the cubit. Although
generally adequate for barter trade systems, such measurement units are of course