Capacitive Flow Instrument

FIGURE 6-16 Typical transformer-ratio-arm bridge transducer. (Source Huang, 1988) 

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The combination of concentration and velocity measurements to form a mass flow instrument is implemented in a variety of ways. Some units employ the same sensing principle for both of these quantities. Examples include the use of capacitance change for volumetric concentration measurement and cross correlation of the fluctuations in capacitance for velocity measurement [20]. 

The choice of a filler depends on the number of bottle sizes being considered for use on it. For example, if 3 1 PET bottles are to be filled, a 126 mm pitch between filling valves is required. If only small bottles up to 500 ml are to be filled, then a 70 mm pitch will suffice. This has a direct effect on the size of the filler and its footprint on the factory floor. Unless the filler bowl level is kept within tightly controlled limits, pressure head variations will affect the rate of flow into containers. Systems such as that shown in Figure 7.15 need to be employed. Wherever possible there should be minimal contact between any instrumentation and the product. Conventional float valves should be avoided and simple capacitance probes, which are easily cleanable but small and very effective, should be used. It is not uncommon in older fillers to only have one float valve. This often gives rise to filler bowl flooding, which may lead to inconsistent fill level and poor counter-pressurisation of the container pre-filling. 

On the air side calibrated thermocouples to measure temperature at different locations in the cell two capacitive hygrometers on the evaporator a humidity controller designed to deliver a maximum vapor flow rate of lOkg/h have been installed. A liquid flowmeter to measure the volumetric flow rate of the aqueous desiccant solution at the membrane contactor outlet was used. All instruments are connected to a data logger. 

All reductions were carried out on a glass vacuum system under static H2 (Linde 99.999%) which was dried by passing through Drierite and molecular sieves prior to exposure to the sample. H2 uptakes were monitored using a capacitance manometer (MKS Instruments Inc.) N2 isotherms at 77 K were performed on the same vacuum system using pre-purified grade N2 (Linde) which was dried prior to use. Oxidations were performed under flowing 02 (Linde 99.999%) at 773 K and under static O2. The samples were evacuated at 623 K to a residual pressure of less than 5 X 10 5 torr prior to reduction or N2 isotherm measurements. 

The chapter begins with the fundamental measurements of resistance, capacitance, charge, and particle force. We proceed with flow measurements with various probes followed by a listing of some commercial electrostatic instruments. Nonelectrosatic measurements in multiphase flow such as the laser-Doppler anemometer, radioactive tracers, and stroboscopic techniques (Polaskowski, et. al, 1995 Soo, 1982) have not been discussed unless in relation to an electrostatic effect. 

To illustrate density measurement by capacitive methods, the performance of the ANL capacitive mass flowmeter is described. Instrument evaluation tests were conducted at the ANL Solid/Liquid Test Facility (SLTF), shown schematically in Fig. 6.19. The SLTF was designed as a specialized instrument-testing and calibration-loop facility for various liquid and liquid/solid media. The facility can provide volumetric flow rates that range from 0 to 10 L/s and flow speeds up to 6 m/s in 2-in. Schedule 40 pipe. It is equipped with an acoustic cross-correlation flowmeter, a PNA system to... 

In conclusion, the capacitive mass flowmeter can be a reliable instrument for measuring the flow of dilute suspended solids. For the flow of high concentrations of suspended solids, the velocity measurement becomes inaccurate. The inaccuracy may be caused by the velocity profile effect, but further study is required to confirm the observation. 

Gamio, J.C., Castro, J., Rivera, L., Alamilla, J., Garcia-Nocetti, R, and Aguilar, L. (2005), Visualization of gas-oil two-phase flows in pressurized pipes using electrical capacitance tomography, Flow Measurement and Instrumentation, 16(2-3) 129-134. 

The induction of experimental dermatitis by means of model irritants represents a method for reproducing ICD in a standardized way and can be employed both for evaluating skin reactivity in high-risk subjects and for monitoring the response and adaptation to the occupational milieu. Skin reactivity to exogenous substances may vary with respect to both the intensity and course of barrier damage and the inflammatory response. Transepidermal water loss (TEWL) and capacitance measurements and instrumental evaluation of skin blood flow, erythema and edema represent the methods for the quantification of different aspects of experimentally induced irritation. 

Here -AT/R is the principal heat flow rate (which is roughly equivalent to that of the TAI2920 model, which uses AT/R, where R is the thermal resistance of the constantan heat leak disk), [C, - Cs dTJdt is the thermal capacitance imbalance, and finally, -Cr dAT/dt) is the heating rate imbalance (TA Instruments 1993). 

The development in recent years of noninvasive instrumental techniques has considerably increased the level of discrimination between products or surfactants. Significant differences in terms of interaction of products with skin surface are now detected much earlier than clinical signs of irritation. Different test protocols have been described in the literature [86] and have been developed in order to induce no or minimal clinical irritation and compare the effect of siufactants by means of instrumental measurements. Transepidermal water loss (evaporimetry), skin capacitance/conductance (skin surface electrical measurements), and vascular status (laser Doppler flowmetry) measurements seem among the most sensitive bioengineering methods for such a purpose. They assess the effect of the surfactants on alteration of the skin barrier function, skin surface hydration, and microvessels blood flow, respectively. 

As a result of cavitation problems encountered in flow and pressure control valves used with liquid oxygen, it became necessary to develop some type of instrumentation which would accurately indicate the presence or absence of cavitation in the liquid oxygen flow through these valves. Various resistive, capacitive, and inductive principles were investigated in the development of this electronic cavitation detecting instrumentation, In addition to the normally low response of such instrumentation to the detection of density gradients in a liquid flow, the solution of this problem was further complicated by the difficulties encountered at the extreme low temperatures. 

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