Coriolis flow meter with adjustable excitation phase

Details Coriolis flow meter with adjustable excitation phase Coriolis flow meter with adjustable excitation phase

1998-12-18

2001-12-25

Patel, Harshad (Department: 2855)

Measuring and testing

Volume or rate of flow

Mass flow by imparting angular or transverse momentum to the...

C073S861357

Reexamination Certificate

active

06332366

ABSTRACT:

Coriolis flow meters have been used for many years to obtain accurate mass flow measurements. The principle behind such flow meters is that a transducer is employed to apply vibration to a conduit containing the fluid whose mass flow rate is to be measured and the vibration in the conduit is measured by two spaced apart sensors, typically either side of the source of vibration. Throughout this specification, the term fluid is intended to encompass both homogeneous fluids such as liquids or gasses and non-homogeneous fluids such as slurries, suspensions or particulate media. In the absence of a fluid flow, the phase of vibration at each sensor location will be approximately the same. However, when a fluid flows through the conduit, there will tend to be a lag in the phase of the upstream sensor and a lead in the phase of the downstream sensor. From the phase difference, a measure of the mass flow rate can be obtained.
Examples of flow meters operating of this principle can be found in U.S. Pat. No. 4,422,338, U.S. Pat. No. 5,423,221, U.S. Pat. No. 4,856,346, U.S. Pat. No. 5394758, U.S. Pat. No. 4,192,184 and U.S. re-issue Pat. No. 31,450, the disclosures of each of which are herein incorporated by reference.
The inventor has appreciated that a problem with conventional mass-flow meters is the need to measure the small phase differences accurately; typically the phase differences induced are only of the order of a few degrees.
The inventor has proposed that the excitation applied to a flow conduit be adjusted to modify the measured phase difference, for example to achieve or maintain a desired phase difference between sensors (preferably a null phase difference) and the mass flow rate derived from the adjustment applied. In this way, it may be easier to obtain accurate measurement as detection of a particular phase difference at a single point may be more accurately achieved than accurate measurement over a range of possible phase differences. The technique can be employed to extend the range or increase the accuracy of measurement, by effectively reducing the range over which a phase difference must be measured.
Accordingly, in one aspect, the invention provides apparatus for obtaining a measure of mass flow rate in a fluid conduit comprising: excitation means for applying vibration to a portion of the conduit; sensor means for detecting a measure of phase difference in the vibration at spaced apart points in the flow conduit; wherein the excitation applied by said excitation means is adjustable to compensate for variation in said phase difference caused by flow of fluid through the conduit;.
The apparatus preferably includes control means for adjusting the excitation provided by the excitation means and preferably further includes means for determining a measure of the mass flow rate through the conduit based on a measure of the adjustment provided by said control means.
Preferably the excitation means comprises two (or more) spaced apart transducers, preferably electromagnetic transducers, and the adjusting means comprises means for adjusting the relative phase and/or amplitude, preferably at least the relative phase, of excitation signals supplied to the transducers.
Preferably the apparatus is arranged so that the control means maintains a substantially constant phase difference, preferably a null phase difference at the sensor locations; this enables an accurate measure of mass flow to be derived directly from the adjustment applied to the adjustment means.
Alternatively, the phase difference may be measured, and the mass flow derived from both a measure of the adjustment applied to the excitation means and the measured phase difference. For example, the excitation may be adjusted in discrete steps and a correction factor may be determined from the measured phase difference.
Another benefit of the invention is that the calibration of the meter can be checked by comparing the measured phase difference with a stored or predicted phase difference for a given adjustment applied to said excitation. For example, a measure of mass flow rate may be determined based on the measured phase difference with no compensating adjustment, and compared to a measure of mass flow rate obtained by adjusting the excitation to produce a substantially null phase difference; if the measures do not agree, this suggests a fault or calibration shift in the meter. Thus, the apparatus may include means for verifying accuracy of the meter based on said measure of adjustment and a measure of the phase difference, preferably based on a stored or calculated relationship between the two.
The inventor has appreciated that the measured phase shift may be a non-linear function of the applied phase shift, depending on further physical properties of the fluid, particularly viscosity. Thus a further benefit of the invention is that it may be possible to obtain a measure of a further property of the fluid, for example viscosity, based on the variation of measured phase shift with applied phase shift. The method preferably includes obtaining a measure of phase shift for a plurality of values of applied excitation and deriving a property of the fluid from the measured variation of phase shift with applied excitation.


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