Electronic capacitance sensors

The Series 1151 differential pressure transmitter manufactured by Rosemount (MinneapoHs, Minnesota) uses a capacitance sensor in which capacitor plates are located on both sides of a stretched metal-sensing diaphragm. This diaphragm is displaced by an amount proportional to the differential process pressure, and the differential capacitance between the sensing diaphragm and the capacitor plates is converted electronically to a 4—20 m A d-c output. 

Gopel, W. Schierbaum, K.D. Electronic Conductance and Capacitance Sensors. In Sensors A comprehensive Survey. 

With the Ee301 EDISEN - electronic GmbH extent the application possibilities of it s digital capacitive sensors. 

Most capacitive evaluation circuits do not achieve the maximum possible resolution but are limited by the electromechanical interface, shortcomings in the electronic circuits, or stray signals coupling into the detector and corrupting the output. Section 6.1.2 below illustrates approaches to maximize the sensitivity of capacitive sensor interfaces, potential error sources, and approaches to minimize them. Electronic circuit options are discussed in Section 6.1.3. 

L. K. Baxter, Capacitive Sensors — Design and Applications, Electronics Technology, IEEE, New York, 1997. 

Fig. 7.1.5 Capacitive interface electronics for MEMS a) inertial sensors [65], and b) a differential position sense interface [65]...

The major part of the volumes consists of a careful description of basic sensors in Chapters 7-13. They include liquid electrolyte sensors, solid electrolyte sensors, electronic conductivity and capacitance sensors, field effect sensors, calorimetric sensors, optochemical sensors, and mass sensitive sensors. 

Functional polymers may be used for dihierent types of chemical sensors, including acoustic wave sensors (bulk acoustic wave, surface acoustic wave, and flexural plate wave sensors), electronic conductance sensors (semiconducting and capacitance sensors), and calorimetric sensors. ... 

The electronic circuits used with variable capacitance transducers, are essentially the same as any other circuit used to measure capacitance. As with the inductance transducers, these circuits can take the form of a bridge circuit or specific circuits that measure capacitive reactance. One concern with capacitive sensors is that the sensor should be electrically shielded if there are electrically conductive objects in the vicinity of the sensor. Otherwise stray capacitances may affect the measurement. 

If a negative potential is applied across the sensor, electrons are repelled from the semiconductor -insulator interface forming a layer depleted in majority carriers. The depletion layer can be modeled as an insulator with a width (and therefore capacitance, Cd) that is a function of the bias potential. As the bias potential becomes more negative, the depletion layer width increases, and Cd decreases, until the maximum depletion layer width is attained. 

Polliack et al. recently compared the accuracy, hysteresis, drift, and effect of curvature on sensor performance for these commercial prosthetic systems (force sensitive resistors F-Socket Measurement System, Tekscan, Inc., Boston, Mass. and Socket Fitting System, Rincoe, Golden, Colo prototype capacitive sensor. Novel Electronic, Minneapolis, Minn.). These authors concluded that the current systems were more appropriate for static use because the hysteresis, drift, and large standard deviations become more problematic during dynamic and long-term loading. 

Sensors for tough conditions conductometric gas sensors support for catalytic gas sensors electronic nose electrodes for high-temperature electrochemical sensors pH sensors humidity sensors optical sensors capacitance sensors Conductometric sensors FET sensors sensors for microenvironments... 

We need to say that insulating polymers, in which all four valence electrons of carbon are used up in covalent bonds, can also be applied in gas sensor design. Moreover, experiment has shown that, for several types of gas sensors such as capacitive sensors (see Chip. 16 [Vol. 1]) and resistive sensors based on composite materials (Chap. 13 [Vol. 2]), the application of insulating polymers is actually preferable. 

Equipment based on the change in electrical conductivity of immobilized salt solution is available from American Instrument Company, Beckman Instruments, Nova Sina AG, and Ro-tronic AG. Equipment based on change in electrical capacitance of polymer thin films is available from WeatherMeasure and General Eastern Corporation. The approximate price range for water activity measurement instruments based on electronic sensors is 6000 to 8000. 

Flg. 7.6. Electronic pressure transducer. This sensing unit is based on the measurement of the capacitance between the diaphragm D and sensors S. Px is the pressure being measured and PH is a reference pressure which may be a high vacuum. 

Potentiometric measurements are done under the condition of zero current. Therefore, the domain of this group of sensors lies at the zero-current axis (see Fig. 5.1). From the viewpoint of charge transfer, there are two types of electrochemical interfaces ideally polarized (purely capacitive) and nonpolarized. As the name implies, the ideally polarized interface is only hypothetical. Although possible in principle, there are no chemical sensors based on a polarized interface at present and we consider only the nonpolarized interface at which at least one charged species partitions between the two phases. The Thought Experiments constructed in Chapter 5, around Fig. 5.1, involved a redox couple, for the sake of simplicity. Thus, an electron was the charged species that communicated between the two phases. In this section and in the area of potentiometric sensors, we consider any charged species electrons, ions, or both. 

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