Mechanical or Direct Method:
Direct level measurement is simple, almost straightforward and economical. It uses a direct measurement of the distance (usually height) from the datum line, and used promarily for local indication. It is not easily adopted to signal transmission techniques for remote indication or control.
Dip Sticks and Lead Lines:
Flexible lines fitted with end weights called chains or lead lines have been used for centuries by seafarinng men to gauge the depth of water under their ships. Steel tape having plump bob- like weights and stored conveniently in a reel are still used extensively for measuring level in fuel oil bunkers and petroleum storage tanks.
Though crude as this methods seems, it is accurate to about 0.1% with ranges up to about 20 feet.
Although the dipstick and lead line method of level measurement are unrivalled in accuracy, reliability, and dependability, there are drawbacks to this technique. First, it requires an action to be performed, thus causing the operator to interrupt his duty to carry out this measurment. There cannot be a continuous representation of the process measurement. Aother limitation to this measuring principle is the inability to successfully and conveniently measures level values in pressurised vessels. These disadvantages limit the effectiveness of these means of visual level measurement.
Another simple method is called sight glass (or level glass). It is quite straightforward in use, the level in the glass seeks the same position as the level in the tanks. It provides a continuous visual indication of liquid level in a process vessel or a small tank and are more convenient than dip stick, dip rod and manual gauging tapes.
Sight glass A is more suitable for gauging an open tank. A metal ball normal used in the tube to prevent the fluid from flowing out of the gauge. Tubular glass of this sort is available in lenghts up to 70 inches and for pressure up to 600 psi. It is now seldom used.
The closed tank sight glass B, sometimes called a 'relfex glass', is used in many pressurized and atmospheric processes. The greatest use is in pressurised vessel such as boiler drums, evaporators, condensers, stills, tanks, distillation columns, and other such applications.The length of reflex glass gauges ranges from a few inches or eight feet, but like the tube type gauges, they can be gauge together to provide nearly any length of level measurement.
The simplicity and reliability of gauge type level measurement results in he use of such devices for local indication. When level transmitters fail or must be out of service for maintenance, or during times of power failure, this method allow the process be measured and controlled by manual means.
However, glass elements can get dirty and are susceptible to breakage thus presenting a safety hazard especially when hot, corrosive or flammable liquids are being handled.
Chain or Float Gauge:
The visual means of level measurement previously discussed are rivaled in simplicity and dependability by float type measurement devices. Many forms of float type instruments are available, but each uses the principle of a buoyant element that floats on the surface of the liquid and changes position as the liquid level varies. Many methods have been used to give an indication of level from a float position with the most common being a float and cable arrangement. The operational concept of a float and cable is shown in the following diagram.
The float is connected to a pulley by a chain or a flexible cable and the rotating member of the pulley is in turn connected to an indicating devices with measurement graduation. As can be seen, as the float moves upward, the counterweight keeps the cable tight and the indicator moves along the circular scale.
As in the figure A, as the float moves, the weight also moves by means of a pulley arrangement as in the diagram above. The weight which moves along a board with calibrated graduations, will be at the extreme bottom position when the tank is full and at the top when the tank is empty. This type is more commonly used for closed tanks at atmospheric pressure.
When it is desired to add a bit of sophistication to the measured systems for ease of reading the level at one location near the bottom of the tank and to gain a bit more accuracy, the system in the Figure B can be used.
In the system shown in Figure B, a perforated stainless steel tape connects the float to a spring or weight loaded drum that rotates as floats moves up or down. The position of the tape on the drum can be read through a window. This type of level float can also be adapted to remote reading capabilities by installing a transmitter assembly.
Article Source: Level Measurement by N. Asyiddin (wwwpiyushpanchal2007.mynetworksolutions.com)