Information Technology Reference
In-Depth Information
Metal Oxide sensitive layers are typically made of SnO 2 doped with Pt or Pd.
These sensors can operate at high temperatures (300-5,000°C), which makes them
especially suitable for combustion gases.
Conductive Polymer sensitive layers are usually based on pyrrole, aniline, or thio-
phene. These sensors operate best at room temperatures. Compared to Metal Oxide
sensors these sensors have lower power consumption, and faster response and recov-
ery times. However, they are have lower sensitivity and are sensitive to humidity.
2.7.3
Optical Chemical Sensors
In optical sensors, an optical waveguide is used as the sensitive layer. Chemical reac-
tions between the waveguide and the target chemical substance cause a change in the
optical properties of the waveguide (e.g., the index of reflection). As a result the
amount (or the wavelength) of the light striking the sensor on the end of the wave-
guide varies.
These sensors are highly sensitive, can handle small quantities, are inexpensive,
and easy to sterilize.
Majority (about 60%) of chemical sensors are gas sensors. Most commonly used
chemical sensors include O 2 , pH, CO, CO 2 , NO X , Methane, etc. Table 2.1 gives an
overview of the range of characteristics for some of these sensors available on the
market.
2.7.4
Ion-Sensitive FET Sensor
An ion-sensitive field effect transistor (ISFET) is an ion-sensitive field effect tran-
sistor used to measure ion concentrations in solution; when the ion concentration
(such as pH) changes, the current through the transistor will change accordingly.
Here, the solution is used as the gate electrode. A voltage between substrate and
oxide surfaces arises due to an ions sheath.
An ISFET's source and drain are constructed as for a MOSFET. The gate
electrode is separated from the channel by a barrier which is sensitive to hydrogen
ions and a gap to allow the substance under test to come in contact with the sensi-
tive barrier. An ISFET's threshold voltage depends on the pH of the substance in
contact with its ion-sensitive barrier.
Table 2.1 Overview of important characteristics for some chemical sensors
Sensor type
Response time
Range
Accuracy
Temperature range
Oxygen
4 s
0-150 mm-Hg
CO
20 s
0-5,000 ppvmol
±5%
NO x
<40 s
0-100 ppm
−20°C to 50°C
H 2 S
<60 s
0-200 ppm
±2%
Search WWH ::




Custom Search