J. Hemp

The weight vector theory of Coriolis mass flowmeters

Abstract In this paper a fuller and clear derivation of the weight vector theory of Coriolis mass flowmeters is given indicating its potential for the prediction of velocity profile effects in both the traditional kind of meter (vibrating tube) and the compact kind (vibrating element inside a duct).

Calculation of the sensitivity of a straight tube Coriolis mass flowmeter with free ends

Abstract The weight vector theory for Coriolis mass flowmeters is applied to a simple theoretical meter configuration consisting of a single unsupported straight tube unattached to adjacent piping. The tube has free ends and vibrates in the fundamental mode. It is shown how the sensitivity of this meter depends in part on the interaction of the flow velocity profile with fluid vibrations occurring near the tube ends. This end effect is negative i.e. the meter reads lower than would be expected if end effects were ignored. On account of the end effect there is a predicted variation in sensitivity with Reynolds number (of the order 1% in a tube 25 diameters long) and this can be minimised by a certain choice of the sensor positions.

A technique for low cost calibration of large electromagnetic flowmeters

Abstract The need for calibration of electromagnetic flowmeters adds considerably to the cost of these instruments especially in the larger sizes. This paper discusses a proposed technique for calibration of large electromagnetic flowmeters which avoids the need for a calibration flow rig. The technique consists essentially of filling the flowmeter tube with water, operating the electromagnet as it would be operated in normal flowmeter operation and measuring the axial component of the eddy current electric field in the water over each electrode. This allows the calibration factor for a flat velocity profile to be determined.

Problems in the theory and design of electromagnetic flowmeters for dielectric liquids: Part 1: experimental assessment of static charge noise levels and signal-to-noise ratios

This paper reports on experimental measurements of electrode signal noise spectra in an electromagnetic flowmeter designed for flow measurement of the dielectric liquid BP180. The design of the flowmeter tube and the detection electronics is described. Dependence of the noise spectra on flow rate, electrode dimensions and particle content of the liquid is reported. Implications for flowmeter tube design and for the choice of frequency of operation to achieve reasonable signal-to-noise ratios are explained.

Problems in the theory and design of electromagnetic flowmeters for dielectric liquids. Part 2b: theory of noise generation by charged particles

Abstract This paper describes an analytical model of the generation of noise by charged particles in electromagnetic flowmeters working with dielectric fluids. Using estimated charge values for the particles, the model predicts that particle noise dominates in the case of dirty liquids, with very small particle concentrations producing high levels of noise. Validation of the theory is done by comparison with available experimental data of noise in BP 180 oil for different flow rates and electrode lengths. The model can be used to study the noise spectrum of a particular meter design when the liquid and particle physical properties are known.

The weight vector theory of Coriolis mass flowmeters — Part 2. Boundary source of secondary vibration

Abstract The purpose of this short paper is to draw attention to a boundary source of secondary vibration in Coriolis mass flowmeters. This is important in the calculation of meter sensitivity using the weight vector theory if the effect of fluid viscosity is to be taken into account in the vibrational flow.

Measurement of pipe flow by an electromagnetic probe

Abstract Electromagnetic (EM) flow velocity probes have some important advantages over normal full scale flow meters when being used in pipe flow measurement. To predict the sensitivity of an electromagnetic probe, with the effect of electrode and magnetic coil arrangement, properties of pipe wall and probe wall, work has been done recently on a mathematical model of an electromagnetic flow velocity probe [1]. This paper gives a brief description of the model. Then the model is used to calculate the pipe wall effect where the pipe wall of different properties is near the sensing head of the probe. The result of the calculation gives a suggested minimum distance between the pipe wall and the electrodes of the probe to keep the error of sensitivity within an accepted degree.

Problems in the theory and design of electromagnetic flowmeters for dielectric liquids. Part 2a: Theory of noise generation by turbulence modulation of the diffuse ionic charge layer near the pipe wall

Abstract This paper describes an analytical model of the generation of noise by turbulence modulation of the background charge distribution in electromagnetic flowmeters working with dielectric liquids. The model is tested for the highly insulating oil BP180 in cases with different flow speeds and electrode lengths, and the results show good agreement with the available experimental data. Comparison of experimental results of filtered and unfiltered oil shows that turbulence modulation of the background charge is responsible for the generation of noise in the filtered case, and that other source of noise is present in the unfiltered case. This model is therefore valid for clean fluids, and can be used to study the characteristics of the noise spectrum of a particular meter design under this condition.

Calculation of the virtual current around an electromagnetic velocity probe using the alternating method of Schwarz

Abstract A semi-analytical method is introduced to solve Laplaces equation in a complex geometry under certain boundary conditions and hence to calculate the virtual current around an electromagnetic flow velocity probe situated near a wall. Contours are given of virtual current potential with insulating and conducting walls with the probe at various distances from the wall.

Problems in the theory and design of electromagnetic flowmeters for dielectric liquids. Part 3a. Modelling of zero drift due to flux linkage between coil and electrode cables

Abstract This paper investigates theoretically the magnitudes of zero offsets and zero drifts originating from magnetic flux linkage between the coils of the electromagnet and the loop formed by the electrode cables in an electromagnetic flowmeter for dielectric liquids. The dependence of such zero offsets on liquid properties, frequency of operation, etc. is explained. The paper gives the results of measurements of the effective electrical conductivity and permittivity of dielectric liquids BP180 and Castrol OX-9 (and mixtures of these) in the frequency range 1 Hz–1 kHz. These are used to predict the zero offsets expected in metering these liquids using phase sensitive detection with the flowmeter tube described in Part 1 of the paper. Precautions needed in flowmeter design to minimise instrument zero offsets and zero drifts are explained. The electromagnetic flowmeter theory was developed at Cranfield and the determination of the electrical properties of the dielectric liquids was carried out at QinetiQ.