GAS SENSORS Prepared by: Jiturvi Chokshi ENPM-808B

Sensors are mostly electrical or electronic. Gas sensors Introduction: Sensor: A sensor is a technological device that detects / senses a signal, physical condition and chemical compounds. It is also defined as any device that converts a signal from one form to another. Sensors are mostly electrical or electronic.

Introduction: Sensor: Applications Automobiles Machines Robotics Gas sensors Introduction: Sensor: Applications Automobiles Machines Robotics Industry Medicine Aerospace etc.

Introduction: Sensor: Examples of sensors Thermometer Thermocouple Gas sensors Introduction: Sensor: Examples of sensors Thermometer Thermocouple Phototransistor Photo resistor Microphone Seismometer Hydrophone etc.

Gas sensor is a subclass of chemical sensors. Gas sensors Introduction: Gas Sensor: Gas sensor is a subclass of chemical sensors. Gas sensor measures the concentration of gas in its vicinity. Gas sensor interacts with a gas to measure its concentration. Each gas has a unique breakdown voltage i.e. the electric field at which it is ionized. Sensor identifies gases by measuring these voltages. The concentration of the gas can be determined by measuring the current discharge in the device.

Applications of Gas Sensor: Process control industries Gas sensors Applications of Gas Sensor: Process control industries Environmental monitoring Boiler control Fire detection Alcohol breath tests Detection of harmful gases in mines Home safety Grading of agro-products like coffee and spices

Operating parameters: Operating temperature Operating humidity Gas sensors Operating parameters: Operating temperature Operating humidity Disadvantages: Bulky Consume lots of power Require “risky” high voltage to operate.

Gas sensing technologies: Metal Oxide Based Gas Sensors Capacitance Based Gas Sensors Acoustic Wave Based Gas Sensors Calorimetric Gas Sensors Optical gas sensors Electrochemical gas sensors

Gas sensing technologies: Metal Oxide Based Gas Sensors Metal oxide sensors are also known as chemiresistors. The detection principle of resistive sensors is based on change of the resistance of a thin film upon adsorption of the gas molecules on the surface of a semiconductor. The gas-solid interactions affect the resistance of the film because of the density of electronic species in the film.

Gas sensing technologies: 2. Capacitance Based Gas Sensors They measure the change in dielectric constant of films between the electrodes as a function of the gas concentration. The capacitive sensor relies on inter-digitated electrode structures, which correspond to the two plates of a standard capacitor, to monitor changes of the dielectric coefficient of the film.

Gas sensing technologies: 2. Capacitance Based Gas Sensors (Contd.) The simple theory behind it is if the dielectric constant of the film is lower than that of the analyte, the capacitance will increase and vice versa.

Gas sensing technologies: 3. Acoustic Wave Based Gas Sensors Sound based gas sensors are known as acoustic wave based gas sensors. To launch the acoustic waves, this type of sensor use piezoelectric material either in the thin film form or in bulk form which has one or more transducers on its surface.

Gas sensing technologies: 3. Acoustic Wave Based Gas Sensors (Contd.) Then type of acoustic wave generated and device resonant frequency has been determined. Depending on that, it is possible to measure properties, processes, or chemical species in the gas phase, liquid phase, vacuum or thin solid films.

Gas sensing technologies: 4. Calorimetric Gas Sensors The principle of calorimetric gas sensors based on change in temperature at catalytic surfaces. It consists of a surface of a film of a catalytically active metal (e.g. Platinum, Palladium or Rhodium) . It burns combustible gases. Heat is generated due to the combustion. This heat is balanced by a reduction in the electrical heating power. Thus the power consumption indicates the concentration of gas.

Gas sensing technologies: 5. Optical gas sensors Following methods are used : Ellipsometry (Technique for the investigation of the dielectric properties) Spectroscopy (luminescence, phosphorescence, fluorescence, Raman) Interferometry (white light Interferometry, modal Interferometry in optical waveguide structures)

Gas sensing technologies: 5. Optical gas sensors (Contd.) In these sensors a desired quantity is determined by: Refractive index (Speed of the light) Absorbance and Fluorescence properties (of the analyte molecules or a chemo-optical transducing element.)

Gas sensing technologies: 5. Optical gas sensors (Contd.)

Gas sensing technologies: 6. Electrochemical gas sensors It consists of: Chemical reactants (electrolytes or gels) Two terminals (an anode and a cathode) Anode is responsible for oxidization process and cathode is responsible for reduction process. As a result, current is created. Positive ions flow to the cathode and the negative ions flow to the anode.

Gas sensing technologies: 6. Electrochemical gas sensors (Contd.) We can find reducible gases (such as oxygen, nitrogen oxides and chlorine) at the cathode and oxidizable gases (carbon monoxide, nitrogen dioxide, and hydrogen sulfide) at the anode. The output is directly proportional to the concentration or partial pressure of the gaseous species.

Discussion on some gas sensors: Carbon monoxide (CO) gas sensor Carbon dioxide (CO2) gas sensor Hydrogen gas sensor

1. Carbon monoxide gas sensor Gas sensors 1. Carbon monoxide gas sensor It can either be battery-operated or AC powered. Mostly the sensor will not sound an alarm at lower concentrations (e.g. 100 ppm). The alarm will sound within a few minutes at 400 ppm. So the function is specific to concentration-time. Figure shows simple carbon monoxide sensor.

1. Carbon monoxide gas sensor (Contd.) Gas sensors 1. Carbon monoxide gas sensor (Contd.) Carbon monoxide sensor can be of different types such as: Semiconductor sensor Electrochemical sensor Digital sensor Biomimetic sensor (chem-optical or gel cell sensor)

2. Carbon dioxide (CO2) gas sensor Gas sensors 2. Carbon dioxide (CO2) gas sensor CO2 absorbs infrared light therefore CO2 sensor consists of a tube containing an infrared source at one end and an infrared detector at the other end. The infrared detector detects the infrared light which is not absorbed by CO2 between source and detector.

2. Carbon dioxide (CO2) gas sensor (Contd.) Gas sensors 2. Carbon dioxide (CO2) gas sensor (Contd.) Infrared radiation which is not being absorbed by CO2 produces heat so the temperature will increase. The infrared detector measures the temperature. A voltage is produced due to the temperature increase in the infrared sensor. We can read amplified voltage into the data logger.

Types of hydrogen gas sensor: Optical fiber hydrogen sensors Gas sensors 3. Hydrogen gas sensor Mostly palladium is used to detect hydrogen because palladium selectively absorbs hydrogen gas and forms the chemical palladium hydride. Types of hydrogen gas sensor: Optical fiber hydrogen sensors Nanoparticle-based hydrogen microsensors Diode based sensor

3. Hydrogen gas sensor (Contd.) Development: Gas sensors 3. Hydrogen gas sensor (Contd.) Development: There is a recent progress in developing MEMS (Micro-Electro-Mechanical Systems) based H2 gas sensors. These sensors couple novel thin films as the active layer with a MEMS structure known as a Micro-Hotplate. This coupling results in a H2 gas sensor that has several unique advantages in terms of speed, sensitivity, stability and amenability to large scale manufacture. Preliminary results are extremely encouraging and suggest that this technology has substantial potential for meeting the sensing requirements of a hydrogen based energy economy.

Proceeding of the 2000 Hydrogen Program Review, NREL/CP-570-28890 Gas sensors LITERATURE CITED: http://scholar.google.com/scholar?hl=en&lr=&q=gas+sensor http://en.wikipedia.org/wiki/Gas_sensor http://en.wikipedia.org/wiki/Hydrogen_microsensor http://vernier.com/probes/co2-bta.html http://sensors-transducers.globalspec.com/LearnMore/Sensors_Transducers_Detectors/Gas_Sensing/Gas_Sensors http://etd.ncsi.iisc.ernet.in/handle/2005/281 Proceeding of the 2000 Hydrogen Program Review, NREL/CP-570-28890 http://www.gasdetection.com/TECH/sensorprinciple2.htm http://physicsworld.com/cws/article/news/17869