Applications conductance sensors

Low-temperature thermometers are obtained by cutting a metallized wafer of NTD Ge into chips of small size (typically few mm3) and bonding the electrical contacts onto the metallized sides of the chip. These chips are seldom used as calibrated thermometers for temperatures below 30 mK, but find precious application as sensors for low-temperature bolometers [42,56], When the chips are used as thermometers, i.e. in quasi-steady applications, their heat capacity does not represent a problem. In dynamic applications and at very low temperatures T < 30 mK, the chip heat capacity, together with the carrier-to-phonon thermal conductance gc d, (Section 15.2.1.3), determines the rise time of the pulses of the bolometer. 

Conductivity sensors are most commonly used for safety purposes in household appliances. Presence and absence of washing liquor, detergency, and water softener can be easily measured and proper operation ensured [71]. The various applications mainly differ by their design of electrode geometry and methods for electrical measurement. Due to the close relation between ionic conductivity and water hardness, the automatic water softener in an automatic dishwasher can be controlled by a conductivity sensor [72]. To isolate the transmission of the measured value from the process controller, the conductivity sensor could incorporate an opto-electronical coupling [73]. Thus, protective insulation of the electrodes in a washer-dryer could be ensured. 

I. Simon and M. Arndt. Thermal and gas-sensingproperties of a micromachined thermal conductivity sensor for the detection of hydrogen in automotive applications . Sensors and Actuators A97-98 (2002), 104-108. 

Important advantages of the bulk conductivity sensor are, besides selectivity, its simplicity (no reference needed) and its selectivity an important drawback is the T-dependence which can be quite significant (see above). Improvement via doping or by using a T-reference is straightforward, but partly at the cost of sensitivity, simplicity, or range of application. 

Membranes applications in sensors and microelectromechanical systems (MEMS) are increasing in importance in our society. The development of new device able to give rapid detection of chemical and biological species is central to many areas of life science and industrial production. In particular, conducting polymeric materials show major potentiality in this field, and are replacing classical inorganic semiconductor materials because of their better selectivity and rapid measurements, low cost, and easy manufacture for their preparation as films [39]. Moreover, appropriate molecular design of polymer properties can increase the efficiency of the system. 

Conductive polymers have been investigated for applications as sensors, with sensing accomplished through the following modes ... 

Pioneering work in the incorporation of functional proteins into polymer bilayers was performed by Meier et al., who integrated membrane proteins into black block copolymer membranes [250], This work proved that proteins could be incorporated into hyperthick triblock copolymer membranes while maintaining their functionality as measured by membrane conductance. Incorporation of proteins in black block copolymer films has been expanded for applications in sensors [251] and protein driven energy transduction [252] across polymeric biomembranes. 

The use of zeolite-hosted semiconductor oxides as chemicai sensors towards oxidizing or reducing gases might be attractive. Since the alteration of the conductivity depends on changes of the oxide stoichiometry [93,94], shorter diffusion distances in smaller clusters should result in shorter response times of the sensors. Fast response is a prerequisite for the application of sensors based on changes of the bulk composition, e.g. in air/fuel ratio control devices. 

The electrical conductivity detector measures the conductivity of the mobile phase. Conductivity detectors are universal and nondestructive and can be used in either direct or indirect modes. The conductivity sensor is the simplest of all the detectors, consisting of only two electrodes situated in a suitable flow cell. The basis of conductivity is the forcing of ions in solution to move toward the electrode of opposite charge on the application of a potential. To prevent polarisation of the sensing electrodes, AC voltages must be used and so it is the impedance (not the resistance) of the electrode system that is actually... 

There are many applications for conductimetric measurements, some of which are very specific. For example, in a medical application, a conductivity sensor has been used to determine levels of ammonia in human breath In the same article, the authors also demonstrated a second sensor for breath analysis of carbon dioxide. In environmental analysis, a samarium iron oxide (SmFeOs) gas sensor has been developed for the detection of ozone based on conductance at sub-ppm levels ". ... 

Conjugated polymers, discovered in the late 1970s, have attracted a variety of attentions because of their unique properties, such as electrical conductivity and color versatility. The conjugated polymers with different colors can be used as ideal electrochromic materials, which have potential applications in sensors, mirrors, displayers, and textiles. Most of their reversible electrochromic behaviors are caused by the electro-induced oxidation-reduction, that is, the reversible change of a chemical species between two redox states under a certain voltage (Niklasson and Granqvist, 2007 Beaujuge et al., 2010). 

The four sensor interrogation steps of pulse application, conductivity measurement, float period, and open circuit potential measurement are repeated for a user-defined number of cycles to produce a sensor response curve. Each set of four such steps produces a single-datum point of conductivity and open circuit potential data. Several such points obtained over a period of time produces a response curve. The response curve captures the change in conductivity of the transducer as a function of time following initialization. 

Some proton conductors have relatively high conductivities at room temperature. Introduction of these materials into electrochemical cells brings about attractive chemical sensors workable at room temperature. Potentiometric or amperometric detection of chemical components at room temperature would create new fields of application for sensors especially in bioprocess control and medical diagnosis. With an all-solid-state structure, the sensors would be compatible with micro-fabrication and mass production, and small power consumption associated with their ambient-temperature operation would be intrinsically suited for cordless or portable sensors. 

Numerous applications of electrodeless conductivity sensors have been published for the chemical, pulp and paper, aluminum, mining, and food industries. Similar instrumentation has been used for in situ measurements of the salinity of seawater. An instrument for continuous analysis of oleum in the range of 100-102% equivalent sulfuric acid, with an accuracy of 0.01%, is illustrated in Figure 11. A historical review has been published by Light (see Further Reading). 

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