Gas sensing principle

The gas response, AV, is a Eangmuir response, which saturates at higher gas pressures, P [22]  

Because chemical reactions on the catalytic metal surface are responsible for the detection of gases, the sensor response is highly temperature-dependent. In general, an increased temperature gives a faster gas response (see Section 2.5.1 and [2]). Thus, both the catalytic metal type and the temperature influence the gas response [23, 24]. 

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We recently published a chapter in the book Silicon Carbide Recent Major Advances by Choyke et al. [19] that describes SiC gas sensor applications in detail. In this book, we emphasize device properties applications are only briefly reviewed at the end. The device and gas sensing properties of various field-effect chemical gas sensing devices based on SiC are described, and other wide bandgap material devices are reviewed. The detection principle and gas response is explained, and the buried channel SiC-FET device is described in detail. Some special phenomena related to the high-temperature influence of hydrogen at high temperature are also reported. 

The Seebeck coefficients Qa and <2b are material constants of conductors A and B, respectively. They depend primarily on two parameters their work function (see Appendix C) and their thermal conductivity. There are many combinations of electronic conductors producing V of few mV °C 1. It is interesting to note that direct modulation of one or both Seebeck coefficients by chemical interaction with an electron acceptor or electron donor gas is possible. It has been demonstrated as a sensing principle for detection of gaseous NO2 with an ti Oj/Au thermocouple junction (Liess and Steffes, 2000). 

Furthermore, different derivations of the LAPS principle have been proposed either to allow a multiple detection of different ions on the same sensor chip or to extend the measurement capabilities of LAPS devices. Another focus for alternative applications is LAPS-based sensors for gas-sensing purposes by making use of catalytic effects of thin... 

When working with sensors, one of the most important issues is cross-sensitivity. Due to the sensing principle, this notably affects metal oxide gas sensors, especially in the case of measurements performed in real life conditions. To prove real life feasibility, it is necessary to keep as close as possible to the real life conditions of the application. In the present case, the real life conditions are mainly represented by the use of ambient air as a carrier gas, but also by the chosen experimental set up. 

For gas sensing devices the conductimetric measuring principle is used in most cases with good success (see section on gas sensor systems). However, for practical applications and commercialization much more investigations are still needed. 

Some studies were devoted to Tm3+-doped fluoride fibers for hydrocarbon gas sensing, especially CH4 with respect to safety issues. A special laser cavity was designed in order to take maximum advantage of the broad tunability of the 3H4 - 3H5 transition of Tm3+ around 2.3 /un. A flat laser output power could be achieved over the 2.3-2.4 pm range where methane shows two strong absorption peaks. The sensor, whose principle is based on the direct absorption of laser light... 

The first gas-sensing electrode based on a similar principle was the carbon dioxide electrode, developed to determine CO2 in blood. Later, sensors for other gases (e.g., SO2, NOx, and HCN, etc.) appeared on the market. 

The gas-sensing configuration described above forms a very useful basic unit for potentiometric measurements of biologically important species. In principle, the immobilized or insolubilized biocatalyst is placed on a conventional ion-selective electrode used to measure the decrease in the reactants or the increase in products of the biochemical reaction. The biocatalyst include... 

Lubbers DW, Koster T, Holst GA. 02 flux optode a new sensing principle to determine the oxygen flux and other gas diffusions. Adv Exp Med Biol 1996 388 59-68. 

In principle, this sensor is also applicable to CO measurement in the gas phase for it was possible to keep the enzyme stable in a wet medium behind the gas-permeable membrane. In comparison with other biocat-alytic gas-sensing devices, e.g. those for methane (Karube et al., 1982a) or NH3 and NO2 (Hikuma et al., 1980b) the sensor was more compact and its response was substantially faster. This enzyme electrode therefore represents a promising approach to novel gas sensors. 

To conclude this section, it is intriguing to observe how developments in instrumental analysis have led to an increase of sensitivity and a dramatic decrease of analysis time required by procedures based on isothermal distillation. The method of Conway, developed more than 35 years ago [4.11], still in use in clinical and pharmaceutical laboratories, requires many hours to perform an assay of a volatile species. The samples are kept in small, enclosed chambers containing the donor and the acceptor liquid, respectively, and after the diffusion process has reached equilibrium, the acceptor liquid is titrated. Gas-sensing probes, which operate on the principles of ion-selective electrodes, separated from the... 

The sensing principle of SAW-type devices is based on a frequency shift during wave transmission over the sensing area between the input IDT and output IDT. It is also well-known that semiconductor devices are based on conductivity modulation due to the adsorption of gas molecules on the sensing membrane. 

Electrochemical sensor fabrication has dominated the analytical application of polymers. In some sensors the polymer film acts as a membrane for the preconcentration of ions or elements before electrochemical detection. Polymers also serve as materials for electrode modification that lower the potential for detecting analytes. In addition, some polymer films function as electrocatalytic surfaces. Using a polymer in biosensors is a very rapidly developing area of electroanalytical chemistry. Polymeric matrix modifiers have been applied as diffusional barriers in constructing not only sensitive amperometric biosensors, but also electrochemical sensors that apply potentiometric, conductimetric, optical, and gas-sensing transducer systems. The principles, operations, and application of potentiometric, conductimetric, optical and gas sensors are described in Refs. 13, 39-41. In this chapter, we focus mainly on amperometric biosensors based on redox enzymes. 

See also Extraction Solvent Extraction Principles Solid-Phase Extraction Solid-Phase Microextraction. Flow Injection Analysis Principles Instrumentation. Ion Exchange Principles. Ion-Selective Electrodes Liquid Membrane Gas Sensing Probes Enzyme Electrodes. Membrane Techniques Dialysis and Reverse Osmosis Ultrafiltration Pervaporation. Solvents. 

In this overview, the basic principle of all optical remote gas-sensing methods is introduced and is used to explain how the accuracy and sensitivity with which a target gas can be measured are defined by the spectroscopic properties of the gas itself. These properties define the region of the spectrum in which it has significant absorption features together with their strength and structure. 

Although the principle of calibration of remote gas sensing monitors is straightforward - if inconvenient, a further difficulty lies with establishing the zero level for the measurement. This is readily achieved in point-sensing instruments by flushing them with... 

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