Replacing Mercury Manometers With LevelBAR Indicators


Pressure measurement is important in many fluid mechanics related applications. From appropriate pressure measurements velocity, aerodynamic forces and moments can be determined. Pressure is measured by the force acting on unit area.

Comparing Manometers and LevelBAR Indicators



The device used to know about the pressure difference in the pipeline, it is simple in construction, the basic law of physics is applied for calculation of the pressure drop. It is a glass or metal tube with a ‘U’ bend providing with two legs. Manometeric fluids as mercury or carbon tetrachloride etc., where the density should be higher than the fluid which flows through the pipe, manometeric fluid will be filled in the tube for the value, the two legs are connected to the points on which we are interested to calculate the differential pressure, when this done the fluid which flows in the pipe or tube will enter into both the legs, the pressure on the leg will differ showing the deflection of height in the manomertic fluid.


Principle: all manometers work on the effect of the hydrostatic pressure exerted by a liquid column. In manometer unknown pressure is determined by balancing it against some known pressure or vacuum.

Construction and working:

The U-tube manometer consists of glass U-tube partially filled with a suitable liquid like water, mercury etc. one of the arms or legs of the manometer, is connected to unknown pressure tap to be measured while other is connected to other pressure tap or it is left open to atmosphere.

When there is a difference of pressure between two arms of the manometer, liquid levels in the two arms of the manometer, liquid levels in the two arms do not match. This level difference in the two arms of the manometer represents differential pressure (P1-P2). The static balance equation is:

P2-P1=h ρ g

h=height difference
ρ=mass density of manometer liquid
If the fluid over manometer liquid has appreciable density, then static balance equation can be written as:
P2-P1= h (ρm – ρl) g

h= height difference
ρm = mass density of manometric liquid
ρl = mass density of fluid over manometric liquid

Manometers – Advantages and Limitations

The manometer in its various forms is an extremely useful type of pressure measuring instrument, but suffers from a number of limitations.

  • While it can be adapted to measure very small pressure differences, it cannot be used conveniently for large pressure differences – although it is possible to connect a number of manometers in series and to use mercury as the manometric fluid to improve the range. (limitation)
  • A manometer does not have to be calibrated against any standard; the pressure difference can be calculated from first principles. ( Advantage)
  • Some liquids are unsuitable for use because they do not form well-defined menisci. Surface tension can also cause errors due to capillary rise; this can be avoided if the diameters of the tubes are sufficiently large – preferably not less than 15 mm diameter. (limitation)
  • A major disadvantage of the manometer is its slow response, which makes it unsuitable for measuring fluctuating pressures.(limitation)
  • It is essential that the pipes connecting the manometer to the pipe or vessel containing the liquid under pressure should be filled with this liquid and there should be no air bubbles in the liquid.(important point to be kept in mind)

LevelBAR Indicator

The KING-GAGE SYSTEMS Used By CDC Innovations in Installations


Tank liquid level and inventory monitoring using KING-GAGE® systems provide you with reliability, consistency and accountability. Here you will find cost-effective solutions for beverage, food, dairy and chemical plants in liquid level measurement or inventory tank gauging encompassing new installations, upgrades and retrofit applications.

The LevelBAR multi-segment LED column display includes an individualized scale calibrated to the actual tank capacity. Marked in any specified unit(s) of measurement, it permits direct reading of total liquid weight or volume. This unique analog column indicator works as a local or auxiliary remote display in new or retrofit applications.

Pneumatic input models include an integrated differential pressure (D/P) element. This allows LevelBAR to be used with bubblers or other air operated systems as a direct replacement for mercury tank gauges or other fluid-filled manometers. It even provides 4-20 mA output compatible with other receivers or to use for process control functions.

The KING-GAGE LevelBAR (v2) used by CDC Innovations in its installations delivers a bright multi-segment LED column to graphically indicate tank level. The system monitors tank level by reading the proportional 4-20 mA signal output from compatible two wire level transmitters. The available D/P option alternatively accommodates direct pneumatic pressure measurement.


LevelBAR indicators can be used singly or in multiples to provide additional display locations simply by extending the signal loop circuit. This tank level display takes the guesswork out of installation and troubleshooting with practical elements such as loop circuit status (on/off/open) and fault (short) indicators. With the available differential pressure (D/P) option, the LevelBAR indicator can be used as a direct replacement for mercury tank gauges or other fluid-filled manometers.


Tank Level Indication KING-GAGE LevelBAR system is designed for liquid level tank gauging. This unique analog column indicator works as a stand-alone system or supplemental remote display in new or retrofit applications. The system will accept standard 4-20 mA transmitter output or can accommodate direct pneumatic pressure measurement. With its available differential pressure (D/P) transducer, it can be used as a direct replacement for mercury tank gauges or other fluid-filled manometers. The multi-segment LED column provides graphic indication of tank levels for operations personnel. A simple adjustment allows full scale ranging of the display when maximum input is less than 20 mA, such as with fixed-range transmitters.

LevelBAR includes a custom scale marked in any specified measurement unit (volume, weight, depth, % full). Pneumatic Input (D/P Option) A high precision piezoresistive pressure transducer provides a proportional 4-20 mAdc output suitable for indication or control functions. With this D/P option, pneumatic-tocurrent (P/I) conversion permits bubblers and other air-driven systems to be retrofitted with an electronic LevelBAR indicator, eliminating older manometer type gages. Electronic Input The

LevelBAR System is designed to work as a tank level monitor. The system accepts a 4-20 mA proportional signal from typical pressure based level transmitters/sensors. It will provide full scale display even if the transmitter does not provide full scale output (such as with fixed range pressure sensors).

Pneumatic System

When used with either an air-driven diaphragm sensor or bubbler, the pressure signal is directed into the LevelBAR (with D/P option). The internal transducer generates a milliamp output in response to the applied pressure. Individual LED segments on the scale illuminate as the signal (e.g., tank depth) increases. The scale itself is custom marked for the actual tank dimensions and pressure range of the gauging application.


Bubbler (Downpipe) System – uses compressed air to purge an open ended pipe extending down into the tank. A regulator or purge control supplies the air which fills the pipe. The resulting downstream pressure (within the pipe) is directly proportional to liquid depth. As depth increases, so does pressure. Conversely, as depth decreases, downstream pressure is equally reduced as excess air flows out the immersed end of the pipe.

Diaphragm Sensor – uses compressed air directed to one side of a sensitive diaphragm to balance it against the force of liquid in the tank. An air flow and backpressure control regulates the supply and exhaust lines feeding the diaphragm sensor. The sensor acts as a force balance creating a pneumatic pressure equal to the force of liquid which is proportional to its depth. As the depth in the tank changes, the pneumatic pressure also changes accordingly.

Installation of the LevelBAR systems is quick and easy. There are no extensive adjustments or calculations to perform. Simply connect power and signal input to make the system operational. It’s rugged stainless enclosure suits any environment. For manometer replacements and levelBAR installations, contact CDC Innovations.

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