25-09-2012, 12:29 PM
Level Sensors
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Abstract
A wide variety of level measurement systems are available to address the broad spectrum of applications, accuracy needs, installation requirements, and practices. Measurement technologies are made available in different versions to address a wide range of measurement needs or sometimes to address just one specific application. This subsection will attempt to define some of the general selection considerations of many available technologies, the general forms of these technologies, and some of their general advantages and disadvantages. As always, one must consult the specifications from the various manufacturers for specific products and users’ experiences in different installations to truly determine their applicability to measurement situations.
The family of level measurement systems can be divided into many categories: liquids or solids level measurement, point or continuous level measurement, electromechanical or electrical/electromagnetic level measurement, or contacting or noncontacting /nonintrusive level measurement.
INTRODUCTION
Level measurement is defined as the measurement of the position of an interface between two media. These media are typically gas and liquid, but they also could be two liquids. Level sensors detect the level of substances that flow, including liquids, slurries, granular materials, and powders. All such substances flow to become essentially level in their containers (or other physical boundaries) because of gravity. The substance to be measured can be inside a container or can be in its natural form (e.g. a river or a lake). The level measurement can be either continuous or point values. Continuous level sensors measure level within a specified range and determine the exact amount of substance in a certain place, while point-level sensors only indicate whether the substance is above or below the sensing point. Generally the latter detect levels that are excessively high or low.
There are many physical and application variables that affect the selection of the optimal level monitoring method for industrial and commercial processes. The selection criteria include the physical: phase (liquid, solid or slurry), temperature, pressure or vacuum, chemistry, dielectric constant of medium, density (specific gravity) of medium, agitation, acoustical or electrical noise, vibration, mechanical shock, tank or bin size and shape. Also important are the application constraints: price, accuracy, appearance, response rate, ease of calibration or programming, physical size and mounting of the instrument, monitoring or control of continuous or discrete (point) levels.
Differential Pressure
Differential-pressure level measurement, also known as “hydrostatic,” is based on the height of the liquid head. Level measurement in open tanks is based on the formula that the pressure head is equal to the liquid height above the tap multiplied by the specific gravity of the fluid being measured. In closed tanks, the true level is equal to the pressure measured at the tank bottom minus the static pressure above the liquid surface. To compensate for that static pressure, a leg is connected from the tank top to the low side of the differential pressure transmitter . Two options are available: dry leg and wet leg. In dry leg applications, it is expected that the low side will remain empty (i.e., no condensation). [1]
If condensation takes place, an error will occur because a pressure head will be created on the low side. This error is avoided by intentionally filling the low side with a liquid—hence the term wet leg. Where filled systems (with diaphragm seals) are used between the transmitter and the tank, calibration of the transmitter should allow for the specific gravity of the fill fluid. The user should refer to the vendor’s instructions when setting the zero and span values. [
Displacement
A displacer , which can be either partially or totally immersed, is restricted from moving
freely with the liquid level. It transmits its change in buoyancy (mechanical force) to a
transducer through a torque-tube unit. Sometimes the term float is used instead of
displacer. [1]
Advantages/Disadvantages
Displacers are simple, dependable, and accurate. They may be mounted internally or
externally. These level measurement can only be used for liquids with fixed specific gravity,
where accuracy is not required. A suitable drain is provided at the low point and a vent valve
at the highest point.
Displacers are difficult to calibrate and have many mechanical components. Therefore,
displacer, linkages, or levers should be free to move. Boiling liquid may cause violent
agitation at the liquid surface, so stilling wells
may be required where turbulence exists. the accuracy is also affected by coating, buildup,
or dirt.
Float Level Sensors
The principle behind magnetic, mechanical, cable, and other float level sensors involves the opening or closing of a mechanical switch, either through direct contact with the switch, or magnetic operation of a reed. With magnetically actuated float sensors, switching occurs when a permanent magnet sealed inside a float rises or falls to the actuation level. With a mechanically actuated float, switching occurs as a result of the movement of a float against a miniature (micro) switch. For both magnetic and mechanical float level sensors, chemical compatibility, temperature, specific gravity (density), buoyancy, and viscosity affect the selection of the stem and the float. For example, larger floats may be used with liquids with specific gravities as low as 0.5 while still maintaining buoyancy. The choice of float material is also influenced by temperature-induced changes in specific gravity and viscosity - changes that directly affect buoyancy. [2]
Float-type sensors can be designed so that a shield protects the float itself from turbulence and wave motion. Float sensors operate well in a wide variety of liquids, including corrosives. When used for organic solvents, however, one will need to verify that these liquids are chemically compatible with the materials used to construct the sensor. Float-style sensors should not be used with high viscosity (thick) liquids, sludge or liquids that adhere to the stem or floats, or materials that contain contaminants such as metal chips; other sensing technologies are better suited for these applications.