Capacitive Instrument

A schematic diagram of the instrumentation for the capacitive flowmeter is presented in Fig. 6.31. The drive electrode was driven by 100-kHz sine waves 

FIGURE 6-30 Unrolled view cf electrode configuration ofANL solid/gas capacitive flowmeter all dimensions in cm electrodes are on outside surface of a ceramic tube electrode gap is 0.23 cm guard strip width, 0.34 cm and all electrodes are 0.036-mm copper foil. 

FIGURE 6-31 Schematic diagram of current-sensing instrumentation of the ANL solid/gas capacitive flowmeter. 

FIGURE 6-34 Typical cross-correlation functions measured by ANL solid/gas capacitive flowmeter at (left column) Ms = 3.2 Ib/s and VP = 25.1 m/s and (right column) Ms = 4.4 Ib/s and VP = 24.0 m/s for three electrode spacings. 

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The cross-correlation technique is used in the ANL capacitive instrument to measure particle velocity. The outputs from the velocity-sensing electrodes were amplitude-modulated capacitor currents. If one assumes that the output capacitance signals vary sinusoidally (Aasincoat), and the applied voltage to the... 

STM and SFM belong to an expanding family of instruments commonly termed Scanning Probe Microscopes (SPMs). Other common members include the magnetic force microscope, the scanning capacitance microscope, and the scanning acoustic microscope. ... 

An important point to be considered when the instrument is used for A.C. voltage measurement is the terminal connections. One terminal will be clearly designated as the high-potential connection, and this should be adhered to. The HT terminal will have a low value of capacitance to other bodies and to earth while the corresponding capacitance of the other is high. If the instrument is in a metallic case this should be connected to the mains earth as a safety precaution. In some cases, the low-voltage terminal is also connected to the metallic case. If this is so, the instrument will effectively earth the circuit under test, which may give rise to problems. 

Impedance Some of the errors arising from the use of linear polarisation resistance led to interest and development in a.c. systems.An early development used a fixed a.c. frequency and a commercial instrument was produced in the UK. Inaccuracies still occurred, however, and were due to the electrode impedance which is fequency dependent. Electrode reactions have a capacitance component, in addition to resistance, resulting in a requirement to measure the impedance. However, the total impedance comprises values for the reaction, solution, diffusion and capacitance. Measurements at different frequency are more reliable, particularly where high solution resistances occur. Simplifications for industrial monitoring have been developed consisting of two measurements, i.e. at a high (10 kHz) and low frequency (0-1 Hz). The high-frequency measurement can identify the... 

Nanoindentations were carried out by using a commercial AFM (AutoProbe CP Research, Park Scientific Instruments) equipped with a commercial capacitance transducer (TriboScope, Hysitron) with a three-sided pyramidal dia-... 

Capacitance and surface tension measurements have provided a wealth of data about the adsorption of ions and molecules at electrified liquid-liquid interfaces. In order to reach an understanding on the molecular level, suitable microscopic models have had to be considered. Interpretation of the capacitance measurements has been often complicated by various instrumental artifacts. Nevertheless, the results of both experimental approaches represent the reference basis for the application of other techniques of surface analysis. 

The pressure range for DR measurements is normally one decade below the above data, and this has to be considered in the specification of the plant. All measurements discussed above have to be carried out by capacitance vacuum gauge, because these instruments measure pressure independently of the type of gas. All vacuum gauges based on the change of heat conductivity as a function of pressure show a result which depends... 

FIGURE 2.45. Equivalent circuit for the cell and instrument. WE, RE, and CE, working, reference, and counter electrodes, respectively iph, photocurrent ij/, double-layer charging current Q, double-layer differential capacitance Rc, Ru, cell compensated (by the potentiostat) and uncompensated resistances, respectively Rs, sampling resistance RP, potentiostat resistance E, potential difference imposed by the potentiostat between the reference and working electrodes Vpu, photo-potential as measured across the sampling resistor. Adapted from Figure 1 of reference 51, with permission from Elsevier. 

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