Continuous level monitoring measures the level of the liquid on an uninterrupted basis. In this case the level of the material will be constantly monitored and hence, the volume can be calculated if the cross-sectional area of the container is known.
Single Point Sensing :
In the case of single point sensing the actual level of the material is detected when it reaches a predetermined level, so that the appropriate action can be taken to prevent
Level Gauge are used for the Direct Sensing devices
Types of Level gauges
1) Tubular type LG
2) Reflex Type LG
3) Magnetic Float Type LG
Tubular Type LG These are the simplest and most commonly used indicators of level. These are also called as Glass tube type level Gauges. Their lower cost and easy maintenance are the reason for their popularity.
Designed for liquid level indication in vessel under high pressure and temperature conditions.
Tempered borosilicate glass, resistant to chemicals, thermal & mechanical shocks.
High Pressure & temperature application i.e. Working pressure Appx.65 Kg/cm2(max) and 450°C
Light striking the glass covered by liquid is refracted (absorbed) into the liquid making this portion appear black, whereas light covering the vapor space is reflected back towards the viewers, making it appear silvery-white. Features:
These models consists of Red or black/white magnetic rollers equipped with small rod magnets which are rotated one after another at an angle of 180° by the magnetic field of the permanent magnet put inside the float. The rollers change from white to red on rising level and red to white on falling level Used for high temp and pr applications or if glass parts cannot be used for safety reasons Magnetic Level Indicator
Float and Displacer type of Level Measurement are commonly done for single point sensing where magnetic, mechanical, cable, and other float level sensors involves the opening or closing of a mechanical switch through direct contact with the switch .
These Devices are also used in continuous level measurement .Float types of level sensor are based on the fact that the float rises as the liquid level rises and vice-versa.This displacement is converted into electrical 4-20 mA signal through different methods.
Displacer type are based on archimedes principle which states that a body(displacer) wholly or partially immersed in fluid is buoyed upo by a force equal to weight of fluid displaced.By detecttion of the apparent weight of immersed displacer,level is sensed.
Ultrasonic level sensors are used for non-contact level sensing of highly viscous liquids, as well as bulk solids.
The sensors emit high frequency (20 kHz to 200 kHz) acoustic waves that are reflected back to and detected by the emitting transducer.
Ultrasonic level sensors are also affected by the changing speed of sound due to moisture, temperature, and pressures. So, Correction factors is applied to the level measurement to improve the accuracy of measurement.
Turbulence, foam, steam, chemical mists (vapors), and changes in the concentration of the process material also affect the ultrasonic sensor’s response. Turbulence and foam prevent the sound wave from being properly reflected to the sensor; steam and chemical mists and vapors distort or absorb the sound wave; and variations in concentration cause changes in the amount of energy in the sound wave that is reflected back to the sensor.
Proper mounting of the transducer is required to ensure best response to reflected sound.