There are several methods to measure the level.There are Direct and Indirect methods.example for Indirect methods are Ultrasonic Level Switches ,Interface Level Measurement ,Radiation level measurement techniques etc .in this session we are gonna discuss about some of the Direct level measuring techniques
Simple Sight Glasses
A visual indication of the level can be obtained when part of the vessel is constructed from transparent material or the liquid in a vessel is bypassed through a transparent tube. The advantage of using stop valves with the use of a bypass pipe, is the ease in removal for cleaning.
Direct reading is possible
Not suitable for automated control.
Maintenance – requires cleaning
Fragile – easily damaged
These are not highly suited for industrial applications as manual viewing and transmission of information is required by the operator. Applications of such level measuring devices can be seen in tanks for the storage of lubricating oils or water. They provide a very simple means of accessing level information and can simplify the task of physically viewing or dipping a tank. They are, however, generally limited to operator inspection. Sight glasses are also not suitable for dark or dirty liquids. This type should not be used when measuring hazardous liquids as the glass-tube is easily damaged or broken. In installations where the gauge is at a lower temperature than the process condensation can occur outside the gauge, impairing the accuracy of the reading.
Float and Tape Systems
It is a widely used method for the direct measurement of level. One common form of level measuring system uses a tape which is connected to a float. The height can be read as the float moves with liquid level. Other systems use the float method by sensing the position of the float magnetically or electrically. Float systems can also be used when measuring granular solids as well as liquids. It is employed in open vessels only.
Construction and working
The float rests on the surface of liquid, supported by buoyant force. The float is made with a sloping top to avoid contamination. The float is connected to the drum by a thin, light weight flexible tape or cable.
Float and shaft type systems
It is employed either in open vessels or in pressure vessels. It is suitable for a wide range of liquids and semi-liquids. The float rests on the surface of the liquid, and the motion of the float is taken through the stuffing box by the shaft. The counter weight is adjustable, so that the float can be made to ride half-submerged.
fig below shows a ball and shaft type level indicator
Large level measurement
Float and tape systems have a common problem with the tape hanging up. This often occurs if the long guide pipes are not perfectly vertical, where the tape rubs against the inside of the pipes. Another common problem is with corrosion or dirt, where the tape can be held in place while the float is moving. These problems are more likely to result in a lower than actual reading.