Capacitive pressure sensors

Fig. 11. Capacitive pressure sensor in the differential measurement configuration. Courtesy of Rosemount, Inc.

Fig. 5.61 shows a section through a capacitive pressure sensor. It consists of a metallic housing that is divided into two chambers by the electrically conductive... 

Fig. 6.11. Capacitive pressure sensor with flexible diaphragm...

Kirstein, T., Meyer, U., Troster, G., (ETH Zurich), 2005. Textile Capacitive Pressure Sensor. World patent iqrpUcation PCT WO 2005/121729. 

Sergio, M., Manaresi, N., Tartagni, M., Guerrieri, R., Canegallo, R., 2002. A textile based capacitive pressure sensor. Proceedings of IEEE 2, 1625—1630. 

Chang SP, Allen MG (2004) Demonstrahon for integrating capacitive pressure sensors with read-out circuitry on stainless steel substrate. Sensors and Actuators A. 116 195-204. 

Many of the variations developed to make pressure sensors and accelerometers for a wide variety of appHcations have been reviewed (5). These sensors can be made in very large batches using photoHthographic techniques that keep unit manufacturing costs low and ensure part-to-part uniformity. A pressure differential across these thin diaphragms causes mechanical deformation that can be monitored in several ways piezoresistors implanted on the diaphragm are one way changes in electrical capacitance are another. 

It is also interesting to briefly consider online measurements of variables different from temperature [5], Since pressure is defined as the normal force per unit area exerted by a fluid on a surface, the relevant measurements are usually based on the effects deriving from deformation of a proper device. The most common pressure sensors are piezoresistive sensors or strain gages, which exploit the change in electric resistance of a stressed material, and the capacitive sensors, which exploit the deformation of an element of a capacitor. Both these sensors can guarantee an accuracy better than 0.1 percent of the full scale, even if strain gages are temperature sensitive. 

Different feedstocks and/or solvents require different solvent to feed ratios. Each packing has a specific flooding point These two effects can lead to a great difference in mass flow. Because of this, pressures and temperatures should be controlled automatically. The flow must be measured exactly, preferably with a mass flow meter, but automatic flow control is not suitable for a multi-purpose plant The valves which are used for pressure and temperature control should be installed so that they can be replaced easily or adapted to new conditions. A very important requirement for the continuous operation of the plant is liquid level indication and control in the extractor and regenerator. Therefore the plant is equipped with capacitive level sensors which are part of a control circuit. The suitability of these sensors for measuring the level of oily products, vitamins and some type of hydrocarbons in supercritical systems have been tested in the lab previously. 

The sections below describe three representative capacitive interfaces. The single-ended parallel-plate style is used in inertial and pressure sensors where fabrication constraints preclude a complementary design. Many microfluidic applications, such as particle sorting or fluid level detection, and proximity sensors fall into this category also. Section 6.1.2.3 describes the advantages of complementary interfaces. The last section is devoted to comb style sensors. 

Tab. 7.3.1 provides a summary of the principles and features of the pressure sensors. The pressure sensors are broadly divided into three groups according to which principle they employ piezoresistive sensors, capacitive sensors, and piezoelectric sensors. 

A capacitance sensor can be built using layers of mica insulators sandwiched between corrugated metal layers. Pressure flattens the corrugations, moving the metallic plates closer to each other, thus decreasing the plate separation which increases the capacitance. This sensor is not damaged by large overloads, since... 

All microfabricated pressure sensors derive their sensitivity from the elastic deformation of a membrane. Measuring the elastic deformation provides the desired signal information. In the broad class of microfabricated pressure sensors, the two most widely used phenomena are the change of resistance due to stress in boron-doped (p" ") resistors and the direct measurement of the membrane displacement using optical or capacitive methods. 

Fung C D and Ko W H 1982 Miniature capacitive pressure transducers Sensors... 

Beriani, A., Rebollo, I. and Berenguer, R. A Passive UHF RFID Pressure Sensor Tag with a 7.27 bit and 5.47pJ Capacitive Sensor Interface, IEEE MTT-S, p 1-3, 2012. 

Recently, Baoqing et al. introduced a low-cost microfluidic droplet capacitance-based pressure sensor using elastic and capacitive electrode-electrolyte interfaces to implement ultra-sensitivity reported at 1.59 pF/kPa and a resolution of 1.8 Pa [4], As Fig. 5 shows, the... 

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