M.S. Beck

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.

Process tomography : a European innovation and its applications

Recent developments in the design and application of tomographic sensors for measurements in industrial processes are reviewed. The principal sensing methods and their relative performance are summarized. Industrial scale applications, future prospects and limitations of the technology are discussed.

Development of capacitance tomographic imaging systems for oil pipeline measurements

UMIST is one of the establishments where process tomography techniques were first developed. This paper introduces the development of capacitance tomographic imaging systems at UMIST for oil pipeline measurements. The principles of electrical capacitance tomography (ECT) are briefly presented and two ECT systems based on charge/discharge capacitance measuring circuits are described. These systems, a PC‐based 8‐electrode system and a Transputer‐based 12‐electrode system, have been used to visualise oil pipelines. Future developments of ECT technology are discussed, including a low‐cost PC‐based ECT system, a twin‐plane ECT system which can measure the velocity profile of a pipeline flow, a multi‐frequency ECT system using ac capacitance measuring circuits and an ECT system with a rotating parallel excitation field.

Design of capacitance electrodes for concentration measurement of two-phase flow

Capacitance electrode systems for the measurement of the volumetric concentration of two-phase flows have inherently non-uniform sensitivity distributions over the pipe cross section and therefore have different responses to different flow regimes (e.g. core, annular and stratified flows). In processes with varying flow regimes, this can lead to significant measurement errors. An electrode system which has a uniform sensitivity distribution and consequently a response independent of flow regime is therefore desirable. The system performance parameters of surface electrode systems used with stray-immune measurement transducers have been characterised quantitatively using a two-dimensional finite-element model. The electrode system design parameters have also been identified and their effects on system performance parameters have been investigated. As a result, optimum electrode designs are proposed, with responses to different flow regimes having a minimum deviation from linearity.

Correlation in instruments: cross correlation flowmeters

The basic principles are briefly explained, showing how cross correlation can be used to identify dynamic characteristics of a wide range of telecommunication, structural and process systems. The main emphasis of the paper is on cross correlation flowmeters, which are developing to a stage where they can successfully solve industrial and environmental measurement problems, ranging from the flow of highly polluted liquids in pipes to the flow of gas from volcanic jets. Cross correlation flowmeters are based on measuring the transit time of a tagging signal (turbulence, clumps of particles, etc.) in the flow between two axially separated sensors. The transit time is measured by a cross correlator. The design of the various subsystems is discussed in some detail and the various sensing techniques required for specific applications are described. Cross correlation flowmeters have become a realistic proposition because of the reducing cost of large scale integrated circuits and microprocessors from which the correlator may be realised, a number of different designs of cross correlator are described and their relative merits discussed.

Real time capacitance imaging of bubble formation at the distributor of a fluidized bed

Abstract This paper describes the use of a transputer-based 8-electrode capacitance tomography system for imaging gas bubbles in a fluidized bed in the vicinity of an air distributor plate. The quantitative results show how the solid concentration distribution varies as a function of time for three different flow regimes: bubbling, slugging and the transition to turbulent. Bubble shape, length and coalescence can be observed.

Electrical tomography techniques for process engineering applications

Abstract This paper summarizes the characteristics of electrical tomography techniques, highlights their current applications and gives an indication of their future applications in the chemical process engineering environment.

Determination of composition and motion of multicomponent mixtures in process vessels using electrical impedance tomography-I. Principles and process engineering applications

Abstract A method of mapping composition profiles and phase boundaries within process reactors and pipelines using robust, low-cost and non-intrusive electrical sensors is described. The method involves utilising differences in electrical resistivity between phases in a disperse system to chart their momentary distribution in a cross-section of the vessel. Sensing electrodes are placed at known locations around the periphery of the vessel and the sensor signals are fed into a suitable algorithm to solve the boundary value problem. The algorithm solves Laplaces equation (∇·ϱ−1∇V=0) inversely: i.e. a computed image of the resistivity profile of the two-dimensional cross-section is formed using electrical measurements obtained from the sensing electrodes. The technique is analogous to that used in medical tomography, but the sensing system employed in this method is solid-state, of much lower cost and can be used in a process plant environment. Composition maps can be obtained in real time. The principles, scope and limitations of electrical impedance tomography and some practical details of its implementation on process-scale vessels are discussed and illustrated by two case studies.

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.

A multi-interface level measurement system using a segmented capacitance sensor for oil separators

A multi-interface level measurement system using a 64-segment capacitance sensor has been developed for use in oil separators. It can not only locate the multi-interfaces between different phases, such as gas/oil, oil/water and water/sludge, but also detect the foam layer between gas and oil. Experimental results show that the interfaces of gas/oil and oil/water can be easily identified by using a simple normalized difference algorithm and the foam signal has a unique spectrum that can be acquired by fast Fourier transform analysis.