Sensor Used in E-nose - The Electronic Nose Artificial Olfaction Technology - page 143

Traditionally, expert human panels are employed for this purpose. However,
reliance on human nose for VOC detection is difficult. If something is hazardous,
odor itself is an indication of same. Many times hazardous chemicals have no odor
or some would have pleasant odor while some substances have offensive odor. A
specific substance smell concentration level can only be smelled by human nose.
Human nose can easily come into a state of ''olfactory exhaustion.''
Human nose can only smell a substance beyond a certain concentration level.
Below this odor threshold, VOC can no longer be perceived. Even for a concen-
tration beyond the odor threshold, human nose can easily come into a state of
''olfactory fatigue'': human nose comes to a point that it is no longer able to smell
the odor while the odor still maintains a high concentration level. Due to these
limitations, a lot of studies have been performed trying to use chemical sensors to
replace human nose for odor detection.
A different way to independently analyze an odor is to design an instrument to
mimic the human sense of smell. This alternative technology will complement or
in some cases will be used as a replacement of the currently used techniques. This
process development is useful for the recognition system to produce unique
classification system of each chemical or smell, so that an automated identification
of that chemical or smell can be applied. The method to record or mimic elec-
tronically the human olfaction sense is characterized by inadequate and very
preliminary approaches.

6.3 Analysis and Classification of Sensor

All chemical sensors comprise appropriate, chemically sensitive materials that are
interfaced to a transducer. Interaction of the analyzed molecules with the chemi-
cally sensitive material generates some physical changes that are sensed by the
transducer and converted to an output signal. The range of gas-sensing materials is
potentially very broad and can be divided into a number of ways, either by
material type or by the nature of the interaction with the analyzer [ 2 ]. Table 6.1
summarizes the biological and engineered sensors for respective functions.

Table 6.1 Examples of engineered and biological sensors [ 30 ]
Engineered sensors Biological sensors

Sensor Sensed quantity Sensor Sensed quantity

Photodiode Light intensity Retina Light intensity

Psychrometer Humidity Chochlea Sound

Barometer Pressure Ear canal Level, rotation

Thermometer Temperature Taste bud Chemical composition

Phenolphthalein pH Skin Temperature

Timer Time Skin and hair Air flow

Odometer Distance(inferred) Olfactory cells Gas composition