Capacitive sensing elements

A common method of level measurement is to use a capacitance bridge. A typical arrangement is shown in Fig. 6.31a in which the sensor consists of two concentric metal cylinders. In the case of a circular tank, the wall of the tank can be employed as the outer cylinder of the sensor. The capacitance of the sensor is  

Capacitive sensing elements are incorporated into either electrical oscillator circuits or into a.c. deflection bridge circuits. An example of the latter is shown in Fig. 6.3117. In this the output voltage is  

The level in the tank can then be determined from equations 6.51 and 6.53. 

If the material is non-conducting then a bare capacitance probe can be employed—otherwise the probe must be insulated. Difficulties can arise due to unexpected variations in the dielectric constant of the material being measured (e.g. in the case of a powder which is prone to absorb water vapour). 

---

Fig. 10.12 Electric field distribution in the capacitive sensing element utilizing time transients as...

Fig. 10.13 Transient signals obtained from the capacitive sensing element (Fig. 10.12), used for multivariate dynamic analysis (Kummer et al., 2006). Data for (a) pure alcohols and (b) ethanol/toluene mixtures of different ratios...

Elastic elements such as diaphragms and capsules are used as sensing elements in combination with different displacement transducers (e.g. inductive, potentiometric, etc.) for low/medium vacuum measurements down to about 1 torr. Elastic elements have been employed in conjunction with capacitive transducers for the measurement of pressures as low as 10 5 torr. 

Complementary structures exhibit substantially lower offset and reduced susceptibility to errors by using two nominally equal sense elements that change in opposite direction. This can be achieved for example by adding a second fixed electrode above the movable plate [3], Most planar processes have no provision for stacking a third electrode above the movable structure. In this case, complementary parallel-plate sensing can still be achieved with the planar comb structure shown schematically in Fig. 6.1.4. The sense capacitance consists of many individual capacitances between movable fingers and fixed electrodes connected in parallel to detect in-plane displacements. 

The sensing element (1) with two acceleration sensors AO, AC2 feeds the Coriolis signals via two capacitance-to-voltage converters (3, 3 ) to a summing node. The different signs double the Coriolis signal and eliminate common mode acceleration signals. 

Figure 11A.2 Variable capacitance differential pressure transducer, (a) Differential pressure sensing element, (b) Pressure signal transmission system.

Impedimetric measurements are inherently nonselective, but the modification of electrode surfaces with selective recognition agents has resulted in selective sensing devices based on impedimetric transducers. Reviews of immunosensors and cell-based biosensors include sections on impedance transduction [177, 250, and references therein]. A review of bioelec-tronic noses, devices intended to mimic the human olfactory system in detecting and identifying odors, includes interdigital structures for complex impedance measurements and arrays of capacitive sensor elements [251]. 

In most cases the pressure is detected using diaphragm sensors classified in terms of sensing element as metal strain gauge, piezoelectric semiconductor, electric capacitance, reluctance, and LVDT (linear variable-differential transformers) sensors. The structure of a silicon semiconductor sensor, where the pressure is detected by a silicon diaphragm, is shown in Fig. 5. In most cases these sensors can be used at temperatures lower than 70°C. 

Table II. Characteristics of Pressure Transducer Sensing Element capacitive potentiometer Rise Time 2-3 milliseconds Hysteresis Less than 0.2%...

The momentum-capacitance mass flowmeter is shown schematically in Fig. 8.19. This flowmeter has two sensing elements located in the fluid stream. One element is a fluid sampling densitometer while the other element senses fluid momentum. Both elements are open mesh configurations offering small impedances to flow. The densitometer element employs honeycomb plates in... 

---