Construction and Working of Level Gauge Technique

A sight glass instrument consists of a graduated tube of toughened glass which is connected to the interior of the tank at the bottom whose water level gauge is to be measured.

The image below shows a simple sight glass for an open tank in which the liquid level gauge in the sight glass matches the level of liquid in the tank. As the level of liquid in the tank rises and falls, the level gauge in the sight glass also rises and falls accordingly. Thus by measuring the level gauge in the sight glass, the level of liquid in the tank is measured. When it is desired to measure a liquid level with the liquid under pressure or vacuum, the sight glass must be connected to the tank at the top as well as at the bottom, otherwise the pressure difference between the tank and the sight glass would cause false reading.

In this case, the glass tube is enclosed in protective housing, and the two valves are provided for isolating the level gauge from the tank in case of breakage of the sight glass. The smaller valve at the bottom is provided for blowing out the level gauge for cleaning purpose.

How Doed The Fuel Tank Gauge Work?

The inconvenience of being stranded on a road/street/highway when the tank runs dry is only surpassed by the embarrassment that comes with it. In any case, there is no excuse for running out of fuel in a small city like Singapore, because there’s always a petrol/diesel station within a 15-minute drive, and it’s open all day and night.

On the face of it, the fuel tank gauge is a simple device. As a rule of thumb, it doesn’t even indicate/calculate the quantity of fuel in litres (or gallons), since all that is needed to alert the driver is the amount of fuel relative to the tank’s capacity.

In this regard, fuel tank gauges have just two main markings – F for Full and E for Empty. Some have a “½” marking, too, to show half-tank. The exact quantum in litres is a piece of information that is not critical to the driving, and the info is probably meaningless to the driver anyway, even someone obsessed with his car’s FC (fuel consumption).

The basic mechanical fuel tank gauge consists of very few parts, and its operation relies on the same simple principle as a voltmeter (voltage-measuring analogue meter).

Essentially, the fuel tank gauge just needs to measure voltage across a variable resistor. But said resistor must somehow vary in direct proportion to the amount of fuel in the tank. To do this, within the fuel tank is a float at one end of a lever arm-and-slider running over a strip of resistor at the other end. Known as the Sending Unit, it is a vital part of every vehicular fuel tank and works flawlessly.

As the float moves vertically with the changing fuel level, the opposite end of the lever slides across the resistor, causing a change in voltage supply at the tank gauge. Finally, in direct response, the indicating needle moves to the appropriate position on the tank gauge.

Because of their simplicity and reliability, the fuel measurement and its display have remained pretty much unchanged over the decades. However, like almost everything in the modern motorcar, some electronic devices have been included, not as a replacement for the proven classical arrangement but to complement it.

Incidentally, the fuel tank gauge does not provide any input to the car’s on-board computer, except in some cases to reset the average fuel consumption after the tank has been refuelled.

Where electronics are used, the Sending Unit feeds into a tiny electronic circuit to trigger a “Fuel Low” or “Please Refuel” warning light at a preset level, usually accompanied by a fuel dispenser symbol. It provides a clear visual notification that the vehicle can travel only 30km to 60km more with the remaining fuel. In fact, most fuel tank gauges are notoriously pessimistic, so as to provide plenty of “allowance” for the optimistic driver running on empty.

In certain cars, digital tank gauges with LCD or LED readouts have replaced the mechanical instrument and its analogue needle. The function of such tank gauges is the same, and the good old float arrangement continues to serve as the measuring device. Only the display method is different.

An electronic processor receives the signal from the Sending Unit, converts it into digital signals and updates an electronic module that controls the display. Some of the systems employ a bar graph, while the more recent ones reproduce a graphic image of the analogue fuel tank gauge.

Note that whatever the display, none indicates numerical values in litres or gallons – just the all-important E and F.

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The Benefits of Tank Gauge System

Utilizing a tank gauge system has perfectly replaced the manual dip-stick measuring as the preferred stock control answer for retail fuel sites with underground fuel storage tanks. A tank gauge system can be linked to probes and electric line leak detectors over various tanks to manage a whole fuel site’s inventory. The tank probes precisely measure the amount and temperature of any fuel or water in a tank and report it back to the tank gauge system.

Fuel site proprietors utilizing a tank gauge system can precisely assess the amount of fuel at present available for dispensing in each tank. By checking the inventory every day, a proprietor is able to comprehend their site’s business cycles, how much stock they require on hand, and when they have to schedule deliveries. Tight inventory control can help pick-up fluctuations in fuel amounts that may be caused by erroneous deliveries, malfunctioning equipment, or theft. In many states there is an administrative prerequisite that owners report their ten day inventory reconciliation.

By connecting all the tank gauge system on a fuel system to the systems, owners can see the inventory status and differences of every one of their sites at once. The cutting edge reporting features allows further insight into stock cycles and delivery scheduling over the whole system.

Leak Detection

The SABS orders that each fuelling site perform some strategy of leak detection and file a passing outcome once every month. A tank gauge system can perform this test by taking product level and temperature readings almost constantly for a set timeframe to figure out whether a changing product level might be because of a leak. Some tank gauge system perform Continuous Statistical Leak Detection (CSLD) by taking stock readings throughout the month at whatever when point there is no pumping action. The framework statistically examines the information to figure out whether the tank is tight or leaking.

When all the tank gauge system on a fuel system are connected to systems, a proprietor will consequently get an alert if any sites on the network detects a leak. The proprietor will likewise have leak test results readily available.

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Advantages of Servo Gauge

Servo gauge are typically AC synchronous brushless gauge with built-in positional feedback. Servo gauge are used in closed-loop motion control systems where angular position, speed and torque can be very accurately controlled. The rotor of a servo motor uses permanent magnets and can be infinitely positioned between magnetic poles on the stator by varying the voltage and current between windings.

Servo gauge are well suited to precision to applications where accurate control of motor speed, position and/or torque is required.

Servo gauge can be used in many different applications. The advantages listed above means that servo gauge are used more and more in industrial applications to replace conventional AC gauge and stepper gauge as well as hydraulic and pneumatic systems. There are many applications where servo gauge have an advantage.

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Servo Gauge For Use In Safety Applications

Servo Gauge has received SIL-2 certification from TUV Rheinland for use in safety applications, such as tank overfill protection, that help prevent hazardous incidents in bulk-liquid storage facilities.

The approval enables terminals and storage facilities utilizing existing Servo Gauge technology to comply with stringent environment and safety regulations without installing additional or new hardware. There are more than 30,000 Servo Gauges installed that can become SIL-2 compliant for overfill protection with a simple software upload. Additionally, new users can integrate the Servo Gauge SIL option seamlessly into their existing infrastructures by integrating to safety systems.

Servo gauges are widely used in bulk-liquid storage facilities around the world to assess holding tank content and manage tank inventories. TUV Rheinland also determined the Servo Gauge exceeded SIL-2 requirements for quantified safety loops, therefore customers that use redundancy can also utilize the device in SIL-3 applications.

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What is hybrid tank gauge system?

Hybrid tank gauge system is mostly used in petroleum industries, where online measurement of level, density and temperature are required.

Design of hybrid tank gauge system:

To calculate the density of liquid in the tank, pressure sensors are installed in the tank. Therefore, if the tank has atmospheric pressure, the hybrid tank gauge system has one pressure sensor and two pressure sensors if the tank is not ventilated freely.

Pressure measurement, combined with level, provides true average density measurement over the entire product level height

A typical HTMS tank gauge system comprises of either a radar or servo gauge and one hydrostatic head measuring pressure transmitter-the fluid product column pressure in tanks. For those tanks where the gas or vapor pressure the pressure in the empty part of the tank is not ambient, for example, in cases where the tank is not open to the atmosphere, an additional pressure transmitter must be added.

Hydrostatic Tank Gauging:

There is an extra pressure sensor and no level tank gauge system in the Hydrostatic Tank Gauge (HTG) scheme. Only the density between the P1 and P2 sensors calculates the product density in the HTG scheme.

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Tank gauge for LPG LNG instrument

Mechanical tank gauge for LPG LNG for accurate and reliable level measurement.

Tank gauge for LPG LNG has been developed for all areas of industry. Many years of operation in a wide variety of applications have proven its reliability.

Level indication of petroleum products such as crude oil, kerosene, light and heavy oil, vegetable oil, palm oil, seed oil and animal oil, highly viscous liquid such as asphalt, certain chemicals (corrosive/toxic vapor can be sealed using a liquid seal pot).


  • No electrical power is required to perform the measurement
  • Built-in repeatability check
  • Direct reading at the tank side allows you to check the level indication easily
  • Can be installed on a variety of tanks including fixed roof, floating roof, covered floating roof and spherical tanks
  • Can be used in conjunction with analog/digital transmitter to perform tank inventory management
  • LT5 has completely integrated internal mechanisms
  • Standardized guide wire and anchor design allow easy customer installation

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Types of Level Gauge

Magnetic level gauges

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.

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.

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Introdution to Automatic Tank Gauge

A compact, intelligent and reliable automatic tank gauge. As tank gauging has evolved, the automatic tank gauge servo level gauge has become an industrial standard all over the world. Very reliable, versatile and accurate automatic tank gauge with a minimum of moving parts, meeting all international standards, regulations and recommendations.

The multi-functional instrument is modularly constructed. The automatic tank gauge is equipped with a Servo Auto Test feature which further increases the safety integrity of the servo and increases the diagnostic coverage. That means that the Servo can be used in overfi ll protection loops to prevent spillage.

The increased diagnostics in the gauge will detect failures inside the gauge or in the application and report this to higher level systems for further action. With the added diagnostics, the safety prooftest interval can be extended to 5 years. This will signifi cantly bring down the operational cost compared to other solutions that require a proof test to be carried out every year.

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A simple approach to tank terminal automation

We help lower the cost of ownership while improving the efficiency and performance of tank terminal automation operations. Our solution provides automation and management for a variety of bulk liquids, which includes functionality for tank storage monitoring and control, facility access control, truck loading and offloading, order entry, transaction management, and accounting ledgers and inventory reconciliation.

Real-Time Accountability

With the Accounting application, facilities can improve the visibility and accountability of all products stored at the tank terminal automation. Terminal operators are able to reconcile book versus physical inventory for all managed products, including additives, blends and co-mingled products. They can lock down accounting periods, automatically create close-out reports and quickly identify variances. The daily management of product inventory allows for improved billing cycles. All of this data is also available at the enterprise to provide greater insight into an organization’s total stock position.

Efficient Throughput

The Throughput Calculator application helps manage the throughput of products from receipt, to the tanks and issues at the load rack. It provides greater insight into the product received and stored at the tank farm, while also improving the flow of vehicles through the load rack. At the gate, driver ID and credentials are quickly validated to allow entry. Pre-load stations allow drivers to verify load authorization, training qualifications and product availability prior to pulling onto the load rack. The pre-load station eliminates wasted time at the load rack and improves overall wait times in the tank terminal automation. Driver and vehicle credentials are validated at the load rack ensuring only those authorized can load products.. These features simplify the driver’s tasks and expedite the time each driver spends at the tank terminal automation.

Facility Access and Control

The Load Rack Automation application helps operators control facility access, and facilitates efficient, safe, and secure loading operations by interfacing to the 8620 Driver Entry Tank Terminal Automation. It is web-based and there is no client software to install. Interfaces are also available for 3rd-party automation and safety equipment, such as electronic presets, ESD and grounding systems, or load rack control equipment. The application is also compatible with TWIC identity verification standards.

Increased Safety

We have extensive alarm and event recording capabilities for movement tracking, tank gauging and loading operations. It is able to interface with safety devices, such as alarm sensors, overfill prevention, ground systems, vehicle identification, and leak detection systems These interfaces and integration drives automation actions throughout the terminal based on inputs, outputs and status.

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