Magnetic Flowmeters Flow Reference Section（电磁流量计流参考部分）.pdf

MAGNETIC FLOWMETERS Flow Reference Section INTRODUCTION Magnetic flowmeters are low pressure drop, volumetric, liquid flow measuring devices. The low maintenance design–with no moving parts, high accuracy, linear analog outputs, insensitivity to specific gravity, viscosity, pressure and temperature, and the ability to measure a wide range of difficult-to- meter fluids (such as corrosives, slurries and sludges)–differentiates this type of metering system from other flowmeters. Two basic styles of magnetic flowmeter are currently available from OMEGA Engineering: 1) Wafer-style, where highest accuracy (up to +0.5% of reading) measurements are required; and 2) Insertion-style, for greater economy and particularly for larger pipe sizes. All OMEGA® magnetic flowmeters employ the state-of-the-art dc pulsed magnetic field system. The Figure 1: In-line magnetic flowmeter operating principle following discussion details the principle of operation, as well as the advantages, of dc pulsed type magnetic flowmeters. PRINCIPLE OF OPERATION Faraday’s Law The operation of a magnetic flowmeter is based upon Faraday’s Law, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor. Faraday’s Formula: E is proportional to V x B x D where: E = The voltage generated in a conductor V = The velocity of the conductor B = The magnetic field strength D = The length of the conductor Figure 2: Insertion-type flowmeter operating principle To apply this principle to flow measurement with a magnetic the magnetic field strength (B) and considered as the measuring flowmeter, it is necessary first to the length of the conductor (D) element of the magnetic flowmeter, state that the fluid being measured (which in this instance is the it can be seen that the measuring mus