SIGHT GLASS The sight glass is normally a graduated glass tube mounted on the side of the tank as shown in Fig This method is very simple and gives a direct reading of level at sight tube.

Fig. 7.1 Sight glass level gauge

BOUYANT FLOAT Many kinds of float-operated devices are available for continuous level measurement. The primary element is the float which, because of its buoyancy, will follow the changing liquid level. The movement of the float is then relayed to a pointer or recorder by using some form of transducer or Converting device. A float level gauge using a counterweight is shown in Fig In this system the float is coupled directly to the indicating element.

Fig Float level gauge with counterweight

Fig Float level gauge with electrical output

Fig. 7.3 Shows an example of a float operated method equipped for electrical transducers. In this system the movement of the float produces an angular rotation of the take-up drum, which is connected via suitable gearing to a rotary potentiometer type displacement transducer. The output voltage from the potentiometer is proportional to the angular movement of the drum and hence the linear float movement.

ULTRASONIC SYSTEM Ultrasonic system uses an ultrasonic signal source (transmitter) and a matched receiver. As shown in Fig. 7.4, the ultrasonic transmitter and receiver are placed above the full level of the tank. In this case two echoes are received. One from the surface of the liquid and one from the bottom of the tank. The time separation between receiving the two echoes is a measure of the liquid level in the tank and the echoes may be displayed on a suitable analogue device such as a cathode ray oscilloscope (CRO)

Fig Ultrasonic level measurement

This method of level measurement is very expensive but can be used for difficult liquids, i. e. corrosive or radioactive, as none of the equipment is in contact with the liquid to be measured.

ELECTRICAL SYSTEM the variable capacitance transducer is the most widely used electrical method for liquid level measurement. A simple capacitor consists of two electrode plates separated by a material called the dielectric. The capacitance of a parallel plate capacitor can be expressed in the following form. Eq. 7.1 Where C is the capacitance, k is a constant; A is the overlapping area of the plates, ζ is the dielectric constant and d is the distance between the plates. A capacitor transducer can be used to measure the level of a liquid in a tank, Fig. 7.5.

Fig Principle of capacitance level gauge

A metal electrode is placed inside the tank and insulated from it. The tank itself is earthed and forms one of the plates. The transducer therefore consists of two concentric metal cylinders and a change in liquid level will alter the dielectric constant and hence the capacitance. The capacitance transducer is connected to one arm of a Wheatstone bridge circuit and changes in capacitance will alter the output voltage from the bridge. The bridge output voltage can therefore be calibrated directly in terms of liquid level. Capacitance transducers can be used to measure levels from a few millimeters to hundreds of meters. The method may be used for corrosive liquids provided a suitable metal electrode is used, e. g. stainless steel.