1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN IN –
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH 0976
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
ENGINEERING AND TECHNOLOGY (IJARET)
ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
Volume 4, Issue 6, September – October 2013, pp. 175-184
© IAEME: www.iaeme.com/ijaret.asp
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IJARET
©IAEME
METAL OXIDE SEMICONDUCTOR BASED EXHALED BREATH PELLET
SENSOR
Pauroosh Kaushal1, Tapan Dhalsamanta2, Rohini Prashant Mudhalwadkar3
1
(Instrumentation & Control, College of Engineering Pune, India)
(Instrumentation & Control, College of Engineering Pune, India)
3
(Instrumentation & Control, College of Engineering Pune, India)
2
ABSTRACT
The analysis of the exhaled breath components allows clinicians and researchers to assess
different body functions in a convenient and flexible way. In this study, a proportion of zinc oxide
and tin oxide nanopowder was used to fabricate gas sensor in the form of pellet. The effect of
exhaled breath and exhaled breath components at room temperature on the electrical conductivity,
sensitivity, response time and recovery time of pellet sensor has been studied. The study revealed
that the electrical conductivity of pellet sensor decreases with exposure of exhaled breath. The
electrical conductivity of the pellet sensor gives response differently for different exhaled breath
components. Characterization of pellet sensor for response of exhaled breath is done and can be
useful for disease detection or quantification of exhaled breath components.
Keywords: exhaled breath, gas sensor, tin oxide, zinc oxide, sensitivity
1. INTRODUCTION
In last few years, breath analysis is emerged as one of the most effective technique for exhaled breath
analysis [1]. Comparing the traditional diagnosis tests for normal clinical practices, breath analysis is
non-invasive, real-time, and least harmful to not only the subjects but also the personnel who collect
the samples. Breath is a complex matrix where flow rate, temperature, humidity, and gas
concentration are multifarious, time-dependent variables. Certain gases in the breath are known to be
indicators of the presence of diseases and clinical conditions [2]. Furthermore, volatile organic
compounds (VOCs) found in human breath have been linked to various physiological conditions. For
example, nitric oxide is indicative of pulmonary dysfunction, acetone has been linked to diabetes,
ammonia is an indicative of renal disease etc [3] [4]. Table 1 shows typical composition from the
exhaled breath of the healthy persons. Hence, understanding the correlation between breath
compounds and the condition of the human body has attracted increasing interest in clinical
diagnostics.
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2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
Table 1: Typical composition from the exhaled breath of the healthy persons
Concentration (v/v)
molecules
Percentage
Oxygen, water , carbon dioxide
Parts-per-million
Acetone, carbon monoxide
Parts-per-billion
Nitric oxide, ammonia
In order to detect the breath compounds, many techniques have been proposed. There are
analytical methods and non conventional methods to detect compounds of exhaled breath. Laser
spectroscopy, conducting polymer nanojunction, Quantum Cascade Lasers are some of the
techniques used for real time analysis of exhaled breath. These techniques are quite accurate but are
expensive, time consuming, and non-portable [5][6][7].
One of the techniques for gas analysis is metal oxide semiconductor (MOS) gas sensors
which respond to specific gases [1][8][9]. When metal oxide crystals like SnO2 are heated at certain
high temperature, they exhibit sensitivity towards oxidizing and reducing gases by a variation of
their electrical properties. Different composition of metal oxide gas sensors has sensitivity towards
specific target gases [10]. This technique eliminates the disadvantages of other techniques by its low
cost and easy operation features. These sensors are one of the efficient and cheapest ways to analyze
gases and can be used for analysis of the exhaled breath components. Among many of the market
available gas sensors for different gases, these pellet sensors are one of the cheapest and effective
methods for analyzing the exhaled breath. The proposed system makes use of MOS sensor, made up
of tin oxide doped with zinc oxide that at are sensitive to composition of exhaled breath components.
Characterization of the sensor for response of exhaled breath and its components is done. The
usefulness of the system for disease detection and quantification of breath components are discussed.
2. MATERIAL AND METHODS
This section is focused on fabrication of sensor which are prepared in the laboratory in the
form of pellets. Tin oxide nanopowder and zinc oxide nanopowder are selected for pellet preparation
because of their availability, non-toxicity, suitability of doping and high chemical stability.
The resistance of pellet changes when gases pass through it and adsorbed on the surface. The
adsorption is maximum for the nanopowder as they have high surface to volume ratio which makes it
very sensitive upon gas injection. The size of the particle used is less than 100 nm. Formations of
Pellets involve the following steps:
2.1
Weighting
Weighting of the powder is done with micro balance machine available in the lab. The two
powder ratio is pre decided and the weighing is done, deciding the percentage composition of
mixture. The weight ratios decided for pellet design is a mixture of zinc oxide nanopowder (10%)
and tin oxide nanopowder (90%).
2.2
Pulverizing
Pulverizing is done to mix properly both the powders to obtain a properly mixed ratio. This
process is carried out with isopropyl alcohol being added at each interval.
2.3
Pressing
Before forming the pellets using any hydraulic press machine, the pulverized powder is
mixed with a drop of Poly Vinyl alcohol (PVA) which acts as a binding agent. This process results in
good formation of pellets and enhances the press effect carried out further. The formation of pellets
is done using a hydraulic pellet press machine available in the laboratory. The pulverized powder is
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3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
taken in the required quantity, 0.3 grams, and put into the die set. Pellets are pressed for duration of 5
mins usually with a pressure range from 90- 100 Kg/cm2 resulting in pellets of size 8mm.
2.4
Sintering
Sintering is the process wherein the formed raw pellets undergo heat treatment under closed
conditions. This hardens the raw pellet and makes it quiet sturdy. Sintering is carried out at a
temperature from 800ºC for 6 hours of time.
2.5
Contacts
The contacts for pellets are made up of material having high conductivity. Silver paste is used
as a contact material. They are placed on the pellet ends to reduce contact resistance and so to get
better response of the pellet sensor.
The sensor was fabricated on a small piece from general purpose board. The output resistance
is measured from the two ends of silver wire. Since the output is of high resistance, it needs to be
converted in voltage form to get processed and displayed. To achieve this conversion, output of
pellet is fed to input of wheatstone bridge. Fig. 1 shows the pellet sensor with the contacts.
Figure 1: Pellet sensor with contacts attached
3. GAS SENSOR ASSEMBLY
Composition of nanopowder tin oxide (90%) and nanopowder zinc oxide (10%) is used for
pellet sensor fabrication. The pellet is placed in a small enclosure, where sample gas is injected. The
sample gases are either exhaled breath or individual exhaled breath components. To collect exhaled
breath sample, a mouthpiece is used for direct injection of sample on pellet. The responses of
components of exhaled breath on pellet sensor are known by injecting gases through canisters. The
enclosure of the system is done in such a way that there will not be any interference of external air
with the sample gas. The enclosure size is 4*4*2 inch. Electrical conductivity of pellet sensor is
recorded through contacts of pellet which measures the interaction between the sample gas and the
sensor. The resistance of pellet is recorded using multimeter and readings are plotted. The interval
time of recording was chosen based on the time taken for the pellet’s resistance to revert to the
original reading. All the measurements were taken at a constant temperature decided by the heater.
The incandescent bulb is used as heater which operates at 3V supply. This was done to guarantee that
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4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
the sensor is at stable temperature. After 20 minutes of switching on the bulb, stable resistance is
observed in the range of 200k to 350k . Resistance values are converted into voltage form using
wheatstone bridge ranges from 0 to 5V. It is connected to the analog input port of microcontroller.
Then it is displayed in 16 x 2 LCD display. In the final output display, real time voltage change of
the sensor is displayed. Fig. 2 shows the sensor assembly interfaced with signal processing circuitry.
The tests were repeated for samples in order to get precise results.
Figure 2: Gas sensor assembly interfaced with display
4. RESULT AND DISCUSSION
The composition of exhaled breath has a great impact on response of the gas sensor. The
exhaled breath contains percentage of O2, N2 and CO2. Experimentations are performed using
standard gas injection and exhaled breath on MOS pellet. The responses from each of the sensors are
recorded and characterization is carried out.
4.1
Exhaled Breath Response on Pellet Sensor
Subject is asked to inhale maximally and instructed to exhale the air in 2 blow through the
mouth piece. The change in resistance is observed with respect to time and recorded. The response
curve is plotted and characterization of sensor is done. Response curve for exhaled breath of three
normal subjects are shown. Fig. 4, fig. 5 and fig. 6 show responses of exhaled breath injection on
pellet sensor.
Figure 4: Exhaled breath response of pellet sensor for subject 1
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5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
Figure 5: Exhaled breath response of pellet sensor for subject 2
Figure 6: Exhaled breath response of pellet sensor for subject 3
From above figures, it is seen that the resistance of the pellet increases as soon as the exhale
breath is injected on the sensor, then it settled to its original value after few minute. The resistance of
pellet in air (Ra) is found to be 0.22 M . The ratio of peak resistance (Rp) and resistance in air is
computed. The resistance ratio value gives a reciprocal of sensitivity. The ratios of subjects are
compared. It is found to be repeatable. The collected data are analyzed and tabulated to characterize
the pellet sensor as shown in table 2.
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6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
Table 2: Characteristic of pellet sensor for exhaled breath
No of
subject
Peak
resistance(M )
Response
time (sec)
Settling
time(sec)
Preheating
time(min)
Resistance
ratio(1/sensitivity)
1
0.37
15
280
20
1.68
2
0.45
10
210
20
1.66
3
0.40
12
230
20
1.63
4.2
Pellet response for CO2 injection
Standard gases are introduced on pellet sensor using gas canisters. Since exhaled breath is
rich in carbon dioxide, the response of the gas is recorded on the sensor. Fig. 7 shows response of
pure CO2 injection on the sensor. The gas was injected for time of 60 seconds with flow rate of 0.25
litres/min.
Figure 7: Pellet sensor response for CO2 injection
It is observed that the response remains still for some time then it increases slowly to 3.8 volt.
It gets saturated at that point which indicated the maximum adsorption of gas on the sensor. The
pellet output voltage (Va) for CO2 in air is found to be 3.46 V and upon exposure to pure CO2 the
output voltage (Vp) rises to 3.8 V. The ratio of Vp and Va is calculated. The table 4.10 shows
voltage response of pellet for CO2 injection. Since there is a small change in response to CO2
injection, the voltage ratio is almost unity.
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7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
Table 4.10: Voltage response of pellet for CO2 injection
Concentration
Peak voltage
Voltage in air
Voltage ratio
99.9%
3.8
3.5
1.08
4.3
Pellet response for O2 injection
The response of pellet sensor for O2 injection remains constant throughout the experiment,
plotted in fig. 8. The voltage response to 99.9% of O2 injection is 4.37 V. So it can be conclude that
pellet sensor has no response for O2.
Figure 8: Pellet response for O2 injection
4.4
Humidity and Temperature Measurement of Exhaled Breath
Exhaled breath is very rich in humidity or moisture content. To have a quantitative moisture
content parameter, humidity sensor SY-HS-220 is used. Temperature change is also monitored
during the experiment. This is done using temperature IC LM35 which has a linear response over
wide temperature range.
We place temperature IC and humidity sensor in the system enclosure. Subject is asked to
inhale maximal and forcefully exhale on the sensors. Response of both the sensors is recorded and
analyzed. The humidity sensor output the voltage which is then compared with its characteristic
curve to know relative humidity of the sample. The characteristic curve of humidity sensor is shown
in fig. 9. By comparing the sensor output with relative humidity of exhaled breath of different
subjects, it is approximated that exhaled breath contains around 80% RH.
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8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
Figure 9: Characteristic curve of SY-HS-220 humidity sensor
The conclusion matches with the theory. Fig. 10, fig. 11 and fig. 12 show sensor output,
indicating the sensor output voltage increases and then decays with time. The humidity sensor
voltage output is compared with its standard chart to know relative humidity of the sample. During
exhalation, temperature increases by just 1o degree.
Figure 10: Subject 1 exhaled breath response 88% RH
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9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
Figure 11: Subject 2 exhaled breath response 81% RH
Figure 12: Subject 3 exhaled breath response 84% RH
The humidity sensor is characterized for exhaled breath response and tabulated, shown in
table 3. It can be observed that the value is quite repeated for exhaled breath response of all normal
subjects.
No of subject
Table 3: Characteristic of humidity sensor for exhaled breath
Peak voltage (V)
Relative
Response time
Humidity(%)
(sec)
Settling time
(min)
1
2.74
88
5
12
2
2.51
81
4
10
3
2.57
84
4
11
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10. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
5. CONCLUSION
The proposed system is noninvasive and low cost system model for exhaled breath analysis.
Pellet sensor has shown good stability for exhaled breath injection. The resistance of the pellet sensor
increases as exhaled breath is injected on the sensor and is found to be stable. The pellet sensor is
characterized by sharp change in resistance with constant resistance ratio and rapid settling time. The
sensitivity of sensor remains constant for normal exhaled breath exposure. The sensor gives a small
rise in output voltage when exposed to CO2 injection and unresponsive for injection of O2 gas. The
humidity level of normal exhaled breath is calculated and results are accurate. Hence, the study
revealed that the electrical resistivity increases when exposed to normal exhaled breath in the
surrounding air of the sensor. In future, the sensor performance for normal exhaled breath and
exhaled breath of diseased subjects can be compared. Disease detection can be possible based on the
comparison as sensor performance would be different for diseased exhaled breath injection.
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