S M Huang

Electronic transducers for industrial measurement of low value capacitances

Reviews existing instrumental techniques suitable for industrial measurement of capacitance in the range 0.1-10 pF, including resonance, oscillation, charge/discharge and AC bridge methods. It shows that in order to achieve high measurement sensitivity and good signal-to-noise ratio, the effects of stray capacitances on the measurement systems must be minimised. Several intrinsically stray-immune measuring circuits are described, including an audio-frequency transformer-ratio-arm bridge and a charge/discharge measuring circuit capable of operating at frequencies up to several MHz. The active guard method and suitable guard driving techniques for reducing the effects of strays on the measurement of grounded capacitances sensors are described. Techniques for reducing the baseline drift of the capacitance measuring circuits, such as auto-balancing and auto-calibration, etc., are also discussed.

Conductivity effects on capacitance measurements of two-component fluids using the charge transfer method

Describes an investigation into the effects of component conductivity on capacitance measurements of two-component fluids using the charge transfer method. Various capacitance sensor systems are represented using three typical equivalent circuit models according to their electrode configurations and the type of mixture between the electrodes. The effects of conductive elements on the measurement of these models are analysed theoretically and tested experimentally using a capacitance transducer based on the charge transfer method. It is shown that the conductance effects can be reduced by using semiconductor switches with low ON resistance and fast commutation time to control the charge and discharge of the sensor. Measurements of glass bead concentration in water-based conductive slurries from an industrial wet peening process and water contentin oil/water mixtures are described as practical applications.

Capacitance-based instrumentation for multi-interface level measurement

An electrical capacitance method is described for the measurement of multi-interface levels of gas/liquid/solid, including the foam layer. It uses a novel multi-electrode capacitance sensing element. A capacitance transducer based on the charge/discharge principle is used, which is stray immune and capable of operating at high frequencies to reduce the effects of the liquid conductivity on the measurement. The presence of foam is detected by processing the fluctuating component of the capacitance signal, which reflects the inherently unstable feature of the foam, whereas the multi-interface levels are reconstructed by processing the steady-state component of the measured capacitances.

Mass-flow measurement of solids using electrodynamic and capacitance transducers

Mass-flow measurement of gas/solids flow is a common requirement in many industrial processes. The authors present a case study dealing with the measurement of returned fines in the recycle line of a fluidised-bed gasifier pilot plant. The problem is solved by measuring two parameters, namely the solids velocity and the volumetric concentration. The former is measured by cross correlating the electrostatic charge fluctuations generated by the conveyed solids, and the latter by using an inherently stray-immune capacitance transducer. The two measurements are then combined to give the solids mass-flow rate. The effects of solids flow characteristics (Reynolds number, flow regime) on the measurement errors of solids velocity and concentration are experimentally investigated. Possible measures to reduce these errors are discussed.

A new capacitance transducer for industrial applications

An absolute capacitance measuring device based on the fundamental charge, capacitance and voltage relationship is described. It consists of a pair of CMOS analogue switches and a current integration amplifier, resulting in a simple capacitance transducer capable of providing a programmable measurement frequency ranging from 100 kHz to 5 MHz. This makes the transducer suitable for component concentration measurement of multicomponent flow processes involving conductive fluids. The initial circuit design is followed by a mathematical model of the transducer. Parametric analysis of this model is used to optimise both the sensitivity and stability of the transducer. A discrimination of 0.01 pF in 10 pF full scale is achieved. In measurements of capacitance with one plate grounded, the problem of stray capacitance arises. A novel switched guard method has been devised which reduces the effect of strays by a factor of 0.0025 at an operating frequency of 1 MHz.

A tomographic flow imaging system based on capacitance measuring techniques

A computerized tomographic method is described for imaging the component distribution in two-component flow processes. It uses eight capacitance electrodes arranged around the flow pipeline to scan the flow, and a linear backprojection algorithm is developed to reconstruct the cross-sectional image of the flow from 28 measured capacitance values. This method has the advantage of being nonintrusive to the flow, inexpensive, and simple to implement, and hence is promising for industrial applications. A prototype system based on this method has been developed and images of static physical models have been obtained.<<ETX>>

A capacitance-based solids concentration transducer with high immunity to interference from the electrostatic charge generated in solids/air two-component flows

Electrostatic charge generated in solids/air two-component flows can cause significant errors in solids-concentration measurement systems based on capacitance techniques, and, in the worst case, can overload or even damage the measuring circuits. A transformer ratio-arm bridge capacitance transducer, incorporating synchronous demodulation and a noise attenuating input amplifier, has been developed for this type of application. The transducer has been tested on several solids-flow plants. It shows a very high immunity to charge interference and it is capable of withstanding large magnitudes of electrostatic interference without being overloaded.