** VISUAL LEVEL SENSORS**
1. SIGHT GLASS
The sight glass is an important method for visually determining level. The sight glass is a transparent tube of glass or plastic mounted outside the vessel and connected to the vessel with pipes. The liquid level in the sight glass matches the level of the liquid in the process tank. In process systems that contain a liquid under high pressure a reflex sight glass is used.
Gauge glass or Level gauge is used in high pressurized tank level indication. Two main types of glass level gauges are the REFLEX TYPE and TRANSPARENT TYPE.
Reflex glass level gauges working principle is based on the light refraction and reflection laws. Reflex glass level gauges use glasses having the face fitted towards the chamber shaped to have prismatic grooves with section angle of 90°. When in operation, the chamber is filled with liquid in the lower zone and gases or vapors in the upper zone; the liquid level is distinguished by different brightness of the glass in the liquid and in the gas/vapor zone. The reflex level gauges do not need a specific illumination: the day environmental light is enough. Only during the night an artificial light must be provided.
The liquid is contained between the two sight glasses and under normal conditions the level of the liquid is visible as it is through naked eye. Transparent Level Gauges employ two transparent glasses and a liquid chamber. The liquid level is indicated as a result of difference in the transparent properties of the two media. For water / steam applications, an illuminator is mounted on the rear side of the gauge with its light rays deflected upward into the water column. This enables the observer to see the illuminated surface of the water as the light rays impinge on the surface of separation between water and steam are reflected back to the eye of the observer. For High Temp-application mica shield is used. Special feature is that liquid column in the chamber is illuminated using illuminators.
Dipsticks offer a simple means of measuring level approximately. The ordinary dipstick is the cheapest device available. This consists of a metal bar on which a scale is etched. The bar is fixed at a known position in the liquid-containing vessel. A level measurement is made by removing the instrument from the vessel and reading off how far up the scale the liquid has wet. As a human operator is required to remove and read the dipstick, this method can only be used in relatively small and shallow vessels. Two applications for which dip sticks are commonly used are to measure the oil level in internal combustion engines and to determine fuel quantities in underground storage tanks.
Float devices operate by float movement with a change in level. This movement is then used to convey a level measurement.
FLOAT AND TAPE METHOD
Many methods have been used to give an indication of level from a float piston. The most common of these uses a float and cable arrangement, in which the float is connected to an indicating device with measurement graduations. As the float moves with the level in the tank, a counterweight moves along a graduated scale.