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A broad range of instruments are used for monitoring and maintaining levels within industrial fluid systems. Through the principles of visual, ultrasonic, microwave, or electromagnetic detection, liquid level gages can track and measure variations in liquid levels, enabling operators to set controls and keep systems within preferred performance parameters. While glass liquid level gages feature relatively straightforward design and operational characteristics involving direct line-of-sight monitoring and measurement, they can be less effective in applications that include hazardous or toxic fluids due to the potential risk of fracture or leakage in the glass components. Glass designs can also make it difficult to take readings at longer distances. In these cases, magnetic liquid level gages are a useful alternative because they are effective in handling toxic substances as well as providing long-distance indications.
Magnetic level gages do not depend on direct viewing of levels and function without the use of transparent glass components. This allows the measuring chamber to be constructed of opaque, welded metal parts, greatly expanding the operating temperature range and improved ruggedness and durability compared to glass chambers. A magnetic gage’s measuring chamber usually has the same coefficient of thermal expansion as the vessel being measured, allowing readings across a wider range of temperatures, which would be impractical if glass materials were incorporated into the system and allowed to interface with the metal chamber. These magnetic liquid level gages are an effective option for a number of fluid monitoring applications.
For magnetic liquid level gages to perform successfully, the metal used to construct the measuring chamber needs to be nonmagnetic material, such as an austenitic stainless steel. A magnetic gage usually relies on a float within the measuring chamber to help provide level indications. This float is typically a permanent magnet and magnetic field detection methods are used to determine the float’s location, which in turn indicates the fluid level in the vessel, so the chamber itself cannot be magnetized for risk of interfering with the magnetic detection process. A magnetic gage float typically needs to be designed with a thick wall in order to function at higher pressures. The common methods for determining the location of the float include magnetostrictive transducers, magnet-operated flags, and magnetic followers.
A magnetic follower is a tracking device that is usually mounted to the side of the gage’s measuring chamber. The permanent magnet inside the float lines up with the follower as the float moves up or down with changes in fluid level. The follower’s position and movements are measured against a scale to produce level readings. The strength of the magnetic attraction between the follower and the float sometimes causes a significant degree of friction against the measuring chamber wall. This may limit the level of resolution that is expressed as a discontinuous motion of the follower responding to level variations.
Like magnetic followers, magnet-operated flags physically track the movement of the float as it travels up and down inside the measuring chamber. The magnet within each flag causes it to flip in one direction when the float passes downward and flip in another direction when the float passes upward. Easily differentiated colors, such as red, orange, and yellow, are often used to mark the flags and to provide a color line for important sections of the level scale. Fluorescent colors may also be used to make the readings easier to see with a flashlight. These flags can, however, flip incorrectly due to bobbing of the float or rapid changes in fluid levels. These errors can often be corrected by passing a magnet externally along the flag, while float position can be determined with a compass if necessary.
A magnetostrictive transducer can be used with a level detection system to provide accurate readings without the limitations presented by glass-based designs, or it can be installed into an existing magnetic liquid level gage to augment the functions of flags or followers. The transducer is a linear device that tracks the position of a magnetic field parallel to the transducer’s own sensing probe. In a magnetic liquid level gage, the float serves as a position magnet to produce the magnetic field. Magnetostrictive transducers are most effective when used in conjunction with a standard nonmagnetic metal tube enclosing a float and magnet system.
The waveguide is the core component in a magnetostrictive transducer. As a current pulse reaches the waveguide circuit, torsional force becomes induced at the point of the position magnet and a timer is activated. This torsional force produces a strain wave that travels through the waveguide until reaching the pickup where it is detected and the timer is stopped. The elapsed time recorded on the timer indicates the position of the magnet. There is usually little or no float friction because the waveguide’s diameter is relatively small, creating no magnetic attraction along the chamber wall. The lack of friction and the transducer’s ability to detect minute positional differences results in highly accurate readings. The measurements can be transmitted locally, remotely, on a standalone indicator, or as computerized input.
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