Biomedical Engineering Reference
In-Depth Information
Table 8.8 Compounds detected in human breath and sensors Required
Molecules in human breath
Requisite sensors
Disease condition
Acetone, isoprene, pentene, benzene, etc.
VOC sensor
Diabetes
Ammonia
NH 3
Renal function
Carbonyl sulphide, carbon disulphide
Sulphide sensor
Organ rejection,
Liver function, schizophrenia
Carbon monoxide, carbon dioxide
CO and CO 2 sensor
Ulcers
Nitric oxide
NO sensor
Asthma
Hydrogen
H 2 sensor
amperometric electrochemical gas sensors. Table 8.4 contains specifications of
sensor (Alpha sense Ltd.) which used four electrode sensors as sensor array used
for developing miniature sensor used in specific
analysis of E-nose system
(Table 8.9 ).
NOB4 sensor complies with the required characteristics when the sensor is
tested in standard conditions as specified as follows:
Sensitivity
nA/ppm in 5 ppm NO
700-1300
Response time
t 90 (s) from zero to 5 ppm NO
\80
Zero current
ppb equivalent in zero air at 20 C
\55
Noise a
RMS noise (ppb equivalent)
\5
Limit of detection a
ppb equivalent
\3
Range
ppm no limit of performance warranty
20
Linearity
ppm error at full scale, linear at zero and 5 ppm NO
\±1
Over gas limit
Maximum ppm for stable response to gas pulse
50
Alpha sense toxic gas sensors are prepared using three electrochemical cells
that operate in the amperometric mode. The output of this sensor current is linearly
proportional to the partial volume of the toxic gas such as CO or H 2 S. Figure 8.4
shows the schematic of sensor and Fig. 8.5 shows the structure of a toxic gas
sensor [ 46 ]. Figure 8.6 shows the potentiometric circuit which will minimize the
current transients by keeping the working electrode at a stable potential. While
electrochemical toxic gas sensors are planned to operate in the stability area region
to reduce sensitivity to small reference electrode-potential changes, one cannot
conclude that small changes in the reference electrode potential can be ignored.
The Figaro sensors (TGS) are available for detection of exhaled breath for
diagnosis of asthma in human. Deposition of a metal oxide semiconductor (MOX),
i.e., SnO 2 and WO 3 , as a thin film on inter digit electrodes and catalytic reactions
of the metal oxide surface with the target gas molecules, temperature between 250
and 350 C, the resistance between the electrodes is changed, and measured in the
TGS sensors development. A simple circuit is required for performance is the
advantage of MOX sensors (resistive) over the other types of gas sensors. Design
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