Servo Tank Gauges Working Principle

Servo level indicator is a Continuous level measurement type The servo powered level gauge uses a  bi-directional motor  attached to a  displacer and cable. A displacer float is attached to the wire which is stored on a measuring drum. The servo motor is controlled by an electronic weighing balance which continuously senses the buoyancy of the partly immersed displacer. In an equilibrium condition, the apparent weight of the displacer balances against the force of the balancing springs when partly immersed in the fluid. A level rise or fall causes a variation in buoyancy. The detector controls an integration circuit in the bi-directional motor which turns the measuring drum, thus raising or lowering the displacer until the balance position is restored.

Servo tank gauges are normally mounted on the top of the tank on a stilling well. To maximize accuracy, the displacer must be mounted in a stilling well to prevent horizontal movement of the displacer. If not contained in a stilling well, it can be subject to mounting errors.

Servo level gauges may also be used for interface measurements.  In this case, the displacer is sized for the heavier material and sinks through the upper layer. Factors affecting the system accuracy are: expansion of cable due to temperature changes, mounting location, tank bulging due to liquid head stress resulting in a reference point movement, changes in density of the product, and cable and drum tolerance.

Servo Tank Gauges Application :-

Servo Tank Gauge (STG) is designed for high accuracy measurement in bulk liquid  storage applications. Typical areas of application include: Oil (fuels), LPG/LNG, Chemicals, Water, and Foods.

Range  :- Up to 200 ft (60 m).

Design Pressure :- Surface-detecting solids gauges are used up to 5 PSIG (0.34 bars), standard wire-guided tape gauges are used at 30 PSIG (2 bars), and special designs can be used upto 300 PSIG (20.6 bars).

Design Temperature: – Solids level sensors can operate from − 4 to 176 ° F ( − 20 to 80 ° C), and tape gauge son liquid tank farms can handle from cryogenic services to up to 300 ° F (150 ° C)

Price :-  $6500 for 40-ft floating roof tank w/temp. measurement

Installation : Simple installation but Avoid an installation too close to an agitator. The cables/ rods may become entangled.

Servo level gauge advantages and disadvantages

Advantage of servo level transmitter  :-

  • Easy to installed
  • The servo powered level gauge provides a direct level measurement with good instrument

     level accuracy (± 0.5 mm).

  • Some servo gauges can be remotely activated to hoist or lower the level sensing displacer for an overall repeatability and performance check or calibration.
  • By lowering the displacer, it is also possible to measure density and/or detect a water interface below the product surface in the bottom of the tank.

Disadvantage / Limitations of servo level transmitter 


  • To maximize accuracy, the displacer must be mounted in a stilling well to prevent horizontal movement of the displacer.
  • The gauge has many moving parts that are susceptible to mechanical wear and sensitive to dirt and product buildup.
  • Density change of the measured product may affect the immersion of the sensing element in the equilibrium condition.

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Servo level transmitter Technical specification :

Sl No.ParameterMinimum Requirements
1Tag No.LT-501
2Tank No.6754/Tank
3ApplicationTank overfill protection
4ServiceESD- tank overfill protection
5TypeServo displacer type
6LocationOn tank roof / tank top
7Area classificationZONE 1, Gr IIA/IIB,T3
8Process connection6” ANSI 150 class RF
9SIL requirementSIL 2 as per IEC 61508
10Built in diagnostic/ self testingRequired
12Sensor materialSS 316/ SS 316 L
13Other wetted partsSS 316/ SS 316 L
14Enclosure housingIP 65 min, Exd
15Power supply24 V DC/ 230 V AC



2 Nos. Relay Contact  or 4-20mA as well communication to tank side indicator


Switch type

2 X SPDT/ 4-20 mA


Switch rating

230 V AC, 5 Amp


Contact open on level

Safe fill height


Local Display/ Transmitter 

Tank side indicator to be provided for proof testing / 4-20 mA HART 2 wire


Lightning protection




2” Flange





CCOE approval

Yes as per project requirement





Model No.


Servo level transmitter Installation procedure :


Mounting without Guide System servo level indicator

In this case, servo level gauge is mounted on a nozzle of the tank roof without any guide system . The mounting preparations require the observance of some recommendations for setting the nozzle and the minimum measuring level.

Servo level indicator Mounting with Stilling Well

Pipe diameter

The pipe diameter that is required to protect the measuring wire without disturbing its
operation depends on the tank height. The pipe could either be constant diameter, or thinner at its upper part and thicker at its lower part. shows two examples of the latter case, namely an asymmetric pipe and a concentric pipe.

Servo level transmitter Manufacturer :-

  • Sbem Pvt.Ltd.,
  • Endress + Hauser Gmbh & Co.
  • Enraf B.V.
  • L & J Technologies
  • Tokyo Keiso Co.Ltd.
  • Varec

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