| 摘要 |
<p>A magnetic flowmeter utilizes a field coil having a portion extending generally along the longitudinal axis of a flow conduit to generate an integral magnetic field therein, concentrically located about the field coil and perpendicular to the flow. An inner electrode surrounds the longitudinal portion of the coil, and the conduit itself is used as the other electrode across which flow induced voltages are sensed, thereby providing a large surface-area electrode. The magnetic field excitation is accomplished by high current (over 20 amperes), very short duration (less than 10 millisecond) pulses. The inductance of the field coil windings is of low enough value to establish very rapid (less than 2 millisecond) rise and decay time constants so that electrode output voltages may be read within a correspondingly short period of time after the field current pulse is turned on. These electrode voltages are comprised of two components, i.e., a short-pulse, flow induced signal proportional to flow rate superimposed on a d-c voltage that is approximately equal to the galvanic potential at the electrodes. In a preferred embodiment, a pulse transformer, connected in the signal path between the sensing electrodes and the readout circuitry, passes the flow induced component, while at the same time isolating the d-c component, thereby producing at its output a signal representing the actual flow signal. In an alternate embodiment, two voltage readings are sampled, one when the magnetic field is at steady-state on value, the other after the field current is turned off, and then subtracted to produce the actual flow signal. Since these two readings are made within a few milliseconds of one another, no additional circuitry is required to compensate for any drift in the baseline galvanic d-c voltages.</p> |
