Several forms of level gauges exist in the industry. Depending upon the types of liquids such as water, oil, diesel etc. measured by level gauges, they can be classified into following categories:
- Tank level gauges: These level sensing devices generally determine volume of liquid enclosed in the tank.
- Fuel level gauges: These devices mainly provide fuel level readings in case of automobiles and other equipments consuming fuel.
- Water level gauges: These level gauges are basically applied to determine the rise or fall occurring in water level.
- Mechanical level gauges: This category of level gauges is mainly employed in underground storage tank applications for measuring the level of liquid.
Some other kinds of level gauges are mentioned below:
Magnetic level gauges
“These types of level Gauges are used for highly hazardous/toxic liquids where gauge glasses are susceptible to attack by the process medium.”1
Their maximum working temperature and pressure range can be 4000C and 220 bar respectively. Use of these devices is mainly recommended for combustible, risky and corrosive fluids. Besides, they are capable for inter-phase level detection. Liquids involving extreme temperature and pressure can also measured by these gauges. They can be installed in underground storage tanks also. These types of level sensors work upon two fundamental principles. One is the Archimedes Principle which states that any object submerged in a fluid tends to experience the same buoyant force as the weight of the liquid put out of place. The other principle is the phenomena of attraction and repulsion between opposite poles and similar poles of permanent magnets respectively.
Reflex Level Gauge
These types of gauges are employed for detection of level in a vessel. Their working temperature and pressure range is upto 4000 C and 400 bar respectively. Major materials used for their construction includes carbon steel and stainless steel. Their operating principle depends upon the dissimilarity in the index of refraction between fluids and vapors. “The liquid column is contained within the recess of the liquid chamber behind the sight glass i.e. a transparent tube which is clamped to the gauge body. The sight glass has prismatic right angled grooves on the side facing the liquid and vapor space.”2
Based upon the presence of liquid or vapor area, external light waves incoming towards the reflex level gauge will undergo either absorption or reflection. In case of vapors, the light radiations running into the groove surface are entirely reflected and returned back to the direction of observation. Thereby, the vapor area gives out silvery white appearance. On the other hand, the light radiations which come across the surface of the grooves in the liquid region are fully absorbed owing to which the liquid at the back of the glass emerges black in color.
Transparent Level Gauge
The functional temperature and pressure range of these level gauges is similar to that of reflex level gauges. Also their constructional features and materials used are almost same.
These level control gauges consist of two transparent glasses which are integrated with the liquid cavities on both sides. The level of the liquid via these devices is detected depending upon the dissimilarity in the transparent characteristics of the two glasses. In case of water & steam applications, an illuminator is usually employed at the back end of the gauge to simplify the viewing of liquid level by the observer. The light rays emitted by the illuminator are directed in upward direction towards the water column owing to which the surface of the liquid gets illuminated. These light radiations tend to fall upon the divided surface between water and steam and then finally reflected back to the eye of the viewer.
Bi – Color Level Gauge
These devices are considered appropriate for measurement of level in boilers since the glasses used in their construction are carefully guarded against the wet steam generated in a boiler drum. This steam protection is provided by putting in high quality mica sheets. Detection of level in bi-color level gauges depends upon the distinction in index of refraction between steam and water. They can work with temperatures as high as 120kg/cm2 and well suited for saturated steam applications.
“Bi-colour gauge is in fact a transparent gauge with its liquid chamber in a wedge shape section and fitted on a rear side with an illuminator.”3
Two color filters, one red in color and the other green in color are positioned directly facing the light source in the illuminator, hence the name bi-color gauge. In cases where red color light radiations hit upon the water, they gets averted in one side and then absorbed whereas when they encounter the steam region, they tends to pass through without any constrictions and emerges red in color. On the other hand, reverse action takes place for light radiations passing via green filters.
Tubular Level Gauge
These types of level gauges detect the level of liquid in the process vessel via direct communication means. These devices mainly find their application in low pressure, non-toxic and non-sticky liquid level measurement applications.