ROLE OF SUBSTANTIAL CHARACTERISTICS IN ELECTRONIC NOSE SENSOR SELECTION FOR DIVERSE APPLICATIONS

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International Journal of Advanced Research in Engineering and Technology
(IJARET)
Volume 7, Issue 2, March-April 2016, pp. 177–185, Article ID: IJARET_07_02_017
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ROLE OF SUBSTANTIAL
CHARACTERISTICS IN ELECTRONIC
NOSE SENSOR SELECTION FOR DIVERSE
APPLICATIONS
Himanshu K. Patel
Instrumentation and Control Engineering,
Nirma University, Ahmedabad, India
Bhakturaj H. Shukla
Nirma University, Ahmedabad, India
Manhar D. Desai
KSV University, Gandhinagar, India
ABSTRACT
Electronic Nose has widen up the possibilities of detection and analysis of
certain factors for environmental, medical, narcotic, military and many more
application areas. A systematic approach for the parameter selection and
choice of proper sensor arrays determines the quality of the expected output.
The main motto behind the approach is to obtain certain outcomes as the final
result of analysis carried out on the basis of data acquired. Certain
characteristics and properties play a vital role in the entire process of odor
recognition, ranging from sensor selection to odor measurement and analysis.
This paper presents the discussion of prime properties and its importance with
examples, which are always been attached with real world challenges as well
as physical systems. Sensitivity and Selectivity are the key features of any E-
Nose systems. Reproducibility and Reversibility have opened up the door of
endless researches. Speed of response and Portability are the parameters
which have covered the entire periphery of E-Nose system in terms of
simplicity, time saving and compactness. In short, E-Nose sensors must match
the critical promises other than that of normal sensors do, and have to provide
the expected outcome in terms of suitable quality.

Himanshu K. Patel, Bhakturaj H. Shukla and Manhar D. Desai
Key words: Electronic Nose Sensors, Reproducibility, Sensitivity, Selectivity,
Speed of Response
Cite this Article: Himanshu K. Patel, Bhakturaj H. Shukla and Manhar D.
Desai. Role of Substantial Characteristics In Electronic Nose Sensor Selection
For Diverse Applications. International Journal of Advanced Research in
Engineering and Technology, 7(2), 2016, pp. 177–185.
http://www.iaeme.com/IJARET/issues.asp?JType=IJARET&VType=7&IType=2
1. INTRODUCTION
Electronic Nose is a system consisting sensor arrays. E-Nose sensors are essential
components to detect physical phenomena like presence of any biomarker and they
are mostly self-contained with the necessary circuitry for specific target gases. Each
sensor works on different principle and exhibit specific characteristics to identify
target gas by the chemical composition and reaction [1]. E-Nose system enables a
program in such a way that it’s functionality towards the parameter remains same.
The parameters which affect the sensor response or sensor output are heating time,
bias voltage, run time and delay time, resistance of sensor itself etc.
The distinctive sensing of lively compounds by the human nose is being important
in evaluating the quality of foods or even in other relevant applications. Hence, it is
not surprising that repeated efforts have been applied over the years to introduce
instruments operating on a similar principle as the human nose. The electronic nose or
simply the E-nose is an instrument that surrounds the human sensitivity to the
objectivity of the instrumental response and generates results similar to the human
nose and in less time.
An electronic nose is an appliance which constitutes an array of electro-chemical
sensors with partial distinctive and an appropriate pattern recognition system that is
capable of recognizing simple or complex odors. It can be regarded as a modular
system consisting a set of active materials which detects the odor, associated sensors
which transforms the chemical quantity or signal into electrical signals, followed by
appropriate signal conditioning and processing to distribute known odors or/and
recognize unknown odors [2].
The electronic nose is a quite neoteric tool that may be used for quality control,
safety and process monitoring, achieving in a few minutes procedures that may
presently require days to complete. Hence the major advantage of this instrument is
that in a matter of seconds, it generates objective, identical and reproducible aroma
discretionary with sensitivity comparable to the human nose for most of applications.
The term electronic nose was first used in a comical sense with sensor arrays in the
1980's [3]. As the technology developed, it became manifest that the human and
animal olfactory systems initiate on the same principle: A generally small number of
nonselective receptors usher the discretionary of thousands of different odors.
A good amount of Research and Development has been carried out into the field
of utilizing thick and thin film semiconducting materials for odor sensing. Research
efforts are pointed upon the use of arrays of metal oxide and conducting polymer odor
sensors.

Role of Substantial Characteristics In Electronic Nose Sensor Selection For Diverse
Applications
2. GENERAL ODOR MEASUREMENT SYSTEM
2.1. Working of Odor Delivery System
The schematic of a generalized odor measurement system is shown in fig. 1. From the
same, one can easily depict the logical sequence of operations that takes place in E-
nose while used for an odor measurement.
The system functioning starts with the odor delivery from odor source to sensor
chamber. There are two main paths by which the odor can be delivered to the sensor
chamber, one is head space sampling and second is flow injection. In head space
sampling, the head space of an odorant material is physically shifted from a sample
vessel and entered into sensor chamber by means of automated procedure or manual
procedure. Same way carrier gas is used to carry the odorant from the sample vessel
into the sensor by the method called flow injection.
Figure 1 working of odor delivery
The sensor chamber consists of an array of selected odor sensors, which may be of
one or more types from semi-conducting polymer chemo-resistor, electrochemical
sensor, optical sensor and many more. The sensor electronic is not only used to
convert a chemical signal into electrical signal but also used to amplify it and
condition it. This function can be obtained using an analog electronic circuitry and
output can then be set to analog output such as 0 to 5 volt dc and/or 4 to 20 mA dc
current. The signal is then converted in to digital equivalent by an analog to digital
converter which includes the use of multiplexers as well and in turn enables the digital
signal for interfacing with microprocessor either through a serial cable (e.g. USB to
RS232) or via digital bus (e.g. GPIB). The microprocessor has been programmed to
carry out multiple tasks.
2.2. Signal Processing and Pattern recognition
The function of an electronic nose is to check and identify odorant sample and to
estimate its concentration. The major task of an electronic nose system comprises of
signal processing and pattern recognition. These two steps can be further divided into
four sequential stages named pre-processing, feature extraction, classification and
decision making; as clearly indicated in fig.2. The only thing which has to be sure that
is data or odorant must be complied and has to be provided to the sensory array for
further extraction.

After providing suitable odorant to the sensor array, sensory function start the
processing on that raw measurement depending on the characteristics of particular
sensor and then pre-processing and feature extraction takes place in order to justify
the sensor result. At classification stage there are so many algorithms which can affect
as well as take place depending on the odor fundamental and sensor response.
Classification contains several results and their groups i.e. odorant type, class and its
quantity. At final stage which comprises the feature extraction as well as classification
and generates the expected outcome as a result. Which will finally be used for
identification of odor class or any other relevant decision making process.
Figure 2 signal processing and pattern recognition
3. SIGNIFICANT PROPERTIES OF INTEGRATED SENSING
SYSTEM
A least appreciated sense amongst the five senses is sense of smell, while many
animals and insects solely rely on sense of smell. E-nose system has several
parameters or in other word it can be said properties, which serve as the figure of
merits. A tract to parameter choice is the most important phenomena in order to
maintain the qualitative output. Properties of sensory system depend on so many
surrounding factors and operating conditions. E-nose sensor arrays must contain some
of the logistic characteristics like sensitivity towards the specific target gas or likely to
the other input. Selectivity is one of the most superior aspects of gathering fruitful
results. P. M. Faia et. Al. have done the testing of FE2O3 sample on respective sensor
and obtained the results, the detailed discussion has been carried out by them in terms
of selectivity and sensitivity issues [4]. Sensor which they have used has certain size
and shape and they have made appropriate changes in it to obtain their needful results.
This is to justify the fact that selection of sensors does depend on various parameters.
3.1. Point of Selectivity
A sensor must respond to target gas, without being much affected by other gas
reaction [5]. MOS sensors and catalytic type Electro-chemical sensors are the best
type of gas sensor because of their inbuilt circuitry function. Selection of the sensor in
fig.3 has been made because of their physical appearance as well as sensing area and
flow range of target gas [6]. Some sensor has the sensing element inside of that core
material like MOS sensor has and some sensors have thin film coated and oxide
material on the top of the sensor area which is also a remarkable property of choosing
appropriate sensor.
Saumya and others worked out for choosing a proper sensor to detect the
biomarker for disease diagnosis like asthma [7]. Nitric oxide is a bio marker for
asthma detection. The detection of Nitric Oxide was done using NOB4 sensor which

Applications
is an electro-chemical type gas sensor, having some cross sensitivity to other gases in
percentage level. However, the detection of nitric oxide may also be observed by
some other sensors which are primarily meant for target gases other than nitric oxide
[8]. This detection is due to cross sensitivity of nitric oxide but the range of detection
is merely significant as compared to the target gas concentration.
Figure 3 sensor selection
3.2. Point of Sensitivity
The sensitivity is the another important parameter to be considered while selecting E-
nose sensors. Sensors are the device that choose up the chemical, physical,
dimensional variables and translate them into an electrical signal with suitable fidelity
to provide feedback to control unit. Sensors become most important as engineering
outturn is optimized by electronic signal [9]. The ceramic material derives much
importance than that of non ceramic type because of their properties of stability to
target compound. Electro-chemical sensors must respond up to certain level by which
they can sense target odor vapor range in ppb and ppm levels – as per requirements of
specific applications - with reliable accuracy. Sensitivity point is sometimes affected
by cross sensitivity which should be negligible in order to maintain linearity of sensor.
Some sensors exhibit non-linear characteristics e.g. MOS type sensors [10]. That non
linearity generates because of environmental condition in most of the case and in

certain case it is heating time. To maintain that sensitivity level, the proper way is to
maintain the fix location for operation and use and by creating artificial box in which
sensor and circuitry can be fixed up. Proper calibration unit and algorithm can provide
sufficient and better outcomes.
3.3. Speed of Response
Response time of any sensor is the key factor of its characteristics. By providing
suitable range of target gas, the sensor start performing its internal functioning
process and gives output in terms of electrical signal in some time. The faster the
response of sensor, better is the possibility of the sensor to get selected for a specified
application.
Speed of response can vary with sensor to sensor and with characteristics of
sensor type. For example, MOS sensors have faster response than electro-chemical
sensor because of its thin film oxide layer. Target gas directly goes through the
sensing element and provides the output in millisecond or seconds whereas in electro-
chemical sensor, oxidation and reduction process takes place and hence some more
time is consumed and provides outputs after few seconds only.
Figure 4 response of sensor
Figure 4 displays the response of a typical sensor, whose resistance value changes
while subjected to the target gas or odorant. In figure 4 it is clearly illustrated that
baseline is obtained with the help of a reference gas, which may be even standard
atmospheric condition. By applying odorant the sensor resistance gets changed and
hence the curve is obtained as shown. The response time is the time during which the
curve attains the peak position in proportion of the odorant concentration, whereas
recovery time is the time during which the sensor resistance reduces to its original
value from the peak value. With reference to fig.4, the odorant off time is the recovery
time where the target gas no longer exists and hence sensor response is no longer
exists after some amount of time limit.
3.4. Reproducibility
The reproducibility of the sensor is the phenomena in which the sensor response
characteristics must be reproducible. In other words, it is the ability of the sensor to
respond in identical manner when the odorant concentration is kept constant over
same operation conditions.

Applications
Figure 5 sensor reproducibility
According to the Sami Elhag’s test on 10 µM concentration of glu [11], sensor to
sensor reproducibility test has been performed by five sensor electrodes using the
same set of conditions and same functionalized membrane. (Fig.5)
It has been noted that utilization of more number of electrodes does not produce
any significant effect on reproducibility parameter. MOSFET sensors have the strong
transient due to which the reproducibility is much better than the normal
potentiometric sensors [12].
3.5. Reversibility and Portability
It is the most obvious fact that the sensor response attains its peak value after
exposure to the gas, but at the same time, it should be able to return back to its initial
state after some time or in other words, it can be said that sensor response should be
able to recover immediately after the exposure to gas [13]. From the research it has
been found that the electro-chemical sensors provide much higher accuracy but
reversibility parameter reflects more time to recover immediately because of its
internal property [14]. On the other hand, QCM and catalytic type of sensors offers
much higher accuracy with reference to the reversibility.
Figure 6 mq7 sensor response
The graph in fig. 6 represents the MQ-7 sensor output after being exposed to
carbon monoxide gas and peak is visible at duration on 145 and 225 millisecond. In
normal condition sensor response is at 9 ppm scale. When subjected to the target gas
the peak of 10 ppm is obtained and after that it tends to return back to its original
value. Meanwhile, if the same gas sample is subjected again at say, 10 ppm level, the

corresponding rise in magnitude is identical. Hence, it is most visibly proven that
MOS sensors possess very good reversibility.
E-Nose sensor should be small and compact in size so that less sample value may
be used. Portability is also a desirable characteristic, particularly when source input or
target point is at distant and remote location [15]. Arduino based MQ series sensors
are most preferable in terms of size and shape, which provides compactness and
simplicity with high accuracy and portable enough to make it useful from anywhere.
4. CONCLUSION
In this paper E-nose system has been discussed more accurately and with a different
perspective relevant to its significant properties and characteristics. In this rapid tour
from molecule to smell, an E-nose can be analyzed and described in several
physiological steps covering signal processing, pattern analysis, sampling and
applications comprised with neural network. Momentous characteristics affecting the
sensor performance are identified and overviewed to facilitate the sensor selection
process for a specific application. At the same time, it is also evident from the
discussion that no sensor can assure best results with reference to all important
characteristics. However, the essence of the electronic nose can be attributed to its
objectivity, rapidity, versatility, non requirement of the sample to be pre-treatment,
easy to use and many more. By the means of parametric evaluation, E-nose system
will open up many practical opportunities in the world of sensory interfaces.
ACKNOWLEDGEMENT
Support of Nirma University, H.K. Automation and Vadilal Gas Pvt. Ltd are
acknowledged.
REFERENCES
[1] Himanshu Patel, Electronic Nose: Introduction, Sensor and Application, Lap-
Lambert, 2012.
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Applications
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