SEEKING THE NEXT LEVEL

There are a number of ways of making a continuous level measurement.

Everyday examples of liquid level measurement devices are the engine oil and gas tank gauges of a car. Another simple device is the level gauge or level glass on a tank or boiler.

The measurement and control of liquid level is essential in a process plant, where a wide variety of liquids take part in both batch and continuous processes.

The accurate measurement of level is important for environmental protection, for example, tank overflow to drains; plant safety; product quality; and inventory control.

Almost all liquid level devices measure by way of the position or height of the liquid above a zero or lowest point, or the hydrostatic pressure or head.

Direct and indirect look

The level measurement may be expressed either in units of length or volume, or in percentage of total volume. There are two methods to measure a liquid level: the direct method and the indirect or inferential method.

The direct method measures the liquid height above the zero point by any of the following techniques:

* Direct visual observation of the height by means of sight glass, level gauge, or dip stick

* A float, which is mechanically linked or electrically connected to an indicator or alarm device

* An electrical probe in the liquid

* Reflection of sonic waves from the liquid surface or from the bottom

The indirect or inferential method of measurement uses the changing position of the liquid surface to determine level. The techniques of this method include the following:

* The buoyant force on a float or displacer, which is partially or completely immersed in liquid

* Hydrostatic pressure of the liquid

* The amount of radiation passing through the liquid

* Electric systems by which liquid level may be inferred

Visual level eye sense

A sight glass is a device that connects to a tank in such a way that the liquid level in the tank is visible to the naked eye right through the glass. These devices are very common in the process industry. The two types of sight glass gauges are tubular and flat glass. Sight glasses usually install with shutoff valves and a drain valve, mainly for purposes of maintenance, repair, and replacement. Graduations engraved on the glass or housing help one compare the level with a certain value-for example, between 0% and 100%.

For an open vessel, a simple, openend tubular level gauge is used. For pressure and vacuum vessels, the upper end of the tuhe connects to the vessel to maintain an equilibrium.

Flat gauges work in industry for a wide range of pressure and temperature applications. There are two basic designs: reflex and transparent. The reflex design is the choice for nonviscous, colorless liquid. The transparent gauge is for colored, viscous, and corrosive liquids.

Float devices are many, and one that leverages direct level measurement is the float-cable-pulley-weight arrangement. It operates by having a cable attached to a float pass over several pulleys. The movement of the float raises or lowers a counterweight on a scale attached to the side of the tank. This type usually works on storage tanks and for inventory purposes.

Liquid level is also measurable using a float or displacer along with a mercury switch or microswitch. Here you can see a ball float switch that detects an increase (or decrease) in level beyond a set value.

The ball float has an internal mercury switch and a counterweight in the top part of the float. When the float does not rise atop the liquid, the mercury switch is in a tilted position, and the mercury remains in the bottom of the glass tube-both ends of the glass tube have an electrical contact point. When the liquid rises, the float lifts and tilts such that the mercury in the tube spreads and connects with both contact points on the end of the glass tube, thus completing an electrical circuit.

Magnetic devices are more versatile, because they can detect both high and low level conditions if needed. This capability is a result of a permanent magnet attaching to the mercury switch, along with an attracting sleeve attached to the displacers to tilt the mercury switch. If the level falls, the sleeve drops below the magnetic position, the magnet pulls away from the sleeve area, and the mercury switch tilts in the opposite angle, activating the contacts to sound an alarm or start a pump. The displacers may be stainless steel or porcelain, depending on the type of material sensed.

So far, the discussion has dealt only with level switches, which indicate when a maximum or minimum level condition exists. This is point level measurement. It is also important to know values of level between the maximum and minimum values. There are a number of ways of making a continuous level measurement.

Displacement devices

The displacement level transmitter works well for continuous level measurement. It works on the buoyancy principle of Archimedes, which states that a body immersed in a liquid will rise up by a force equal to the weight of the water displaced. The displacer body has a cylindrical shape. As a result, for each equal increment of submersion depth, an equal increment of buoyancy change will result. This gives a linear, proportional relationship, which is desirable.