Tuesday, October 7, 2014

Types of Level Instruments in Process Control

Tuning fork level
Tuning fork level controls
(form of echo level sensing)
In many industrial processes, the measurement of level is critical. Depending on the nature of the material being measured, this can be a simple or complex task. Several different technologies for sensing level are briefly explained here.

Sight Glasses or Sight Gauges
A very simple type of level measurement device for direct visual measurement of level in a tank or vessel. A sight gauge is usually a clear glass or plastic tube connected to the a vessel at the top and bottom of the level range needing to be viewed. As long as interface (different fluids with different densities floating on each other) the fluid level inside the vessel will be at the same hight as the level in the tube.

Floats are another very simple approach to level measurement for fluids or solids. The float rests on top of the process media and is visually, magnetically, or electronically coupled to the level inside the vessel. It is important that the float material be compatible with the process media and that it freely moves on top of the process.

Hydrostatic Pressure
One of the most common ways to measure level because of the relationship between pressure of a fluid column with the changing level inside the vessel. Sensing is done via pressure sensor (gauge or transmitter) which is attached to the bottom of a tank or vessel, and measures the static column of liquid. The pressure signal is then calculated and conveyed to read level.

Bubbler Systems
Basically a variation of the hydrostatic pressure method, a bubbler system measure the pressure required to push bubbles of a purge gas out of the end of a dip tube. Basically, the deeper the tip of the dip tube is, the greater pressure required to produce bubbles. The amount of pressure can be correlated to level. Bubblers are a good alternative when sensing corrosive liquids.

Displacer Level
Displacer Level
Displacer Level Control
Displacement level devices are based upon the laws of buoyancy, and the amount of liquid displaced by an object. A float, which can be located inside its own isolated vessel (or cage), or can be hanging directly in the process, is calibrated the displacement of the liquid being measured. A displacer, which is usually a sealed metal tube, hang's in place in the process media and as more of the displacer’s volume becomes submerged, the buoyant force is increased, making the displacer "lighter" and changing the tension of a rod or linkage.

Echo (Ultrasonic, Radar, Laser)
Level measured by bouncing some wave form (sound, light, magnet) off the surface of liquids and measuring their time of flight.

Capacitance Level
Capacitive level instruments measure the electrical capacitance of a conductive rod inserted vertically into a process vessel. As process level increases, the capacitance between the rod and the vessel walls increases, causing a signal change in the instruments circuitry.

Level is measured by knowing the empty weight of a vessel and the full weigh of a vessel, and calibrating important points between.