Level Measurement

Bubbler method level calculation – Liquid level calculation (Transmitter Installed with single Bubble tube)

Examples of standard calculations for measuring liquid levels in an open tank with a single Bubble tube Transmitter Installation are provided in this section.

It is important to keep in mind that the upper-range values (URV) and lower-range values (LRV) relate to the highest and minimum measurements that are intended, respectively. The Calibrated Span is equal to the URV minus the LRV. The output at LRV greater than 0% represents the value of the suppression.


The following acronyms are utilized in this directive.

LRV = Lower-range value (a measurement that’s output is 4 mA).

URV = Upper-range value (a measurement with a 20 mA output).

SG = A liquid’s specific gravity (relative density). Water has a specific gravity of 1.00 in both traditional and SI systems.

HW= Equivalent Headlevel pressure of Water

Formulas for Converting Specific Gravity for All Liquids

Specific gravity = Density [lb/ft3 ] / 62.4

Specific gravity = Density [kg/m3] / 1.00

Liquid Level Calculations (Transmitter installed with single Bubble Tube)

    Liquid Level Calculations (Transmitter installed with single Bubble Tube)

    SGL = Specific gravity of Tank Liquid

    H = Height of the bubble tube immersed inside the process liquid

    H1 = Tank level need to be measured.(head between LRV and URV)

    H2 = Head level below minimum level (suppression)

    Calibrated Range of the transmitter = LRV to URV 

    where: SGL = Specific gravity of tank process liquid 

    HW= Equivalent Headlevel pressure of Water 

    Span value of the transmitter  = H1*SGL 

    Equivalent Head level pressure of Water(HW) at Lower range value

    HW at LRV = H1*SGL = Suppression

    Equivalent Head level pressure of Water(HW) at Higher range value

    Hw at URV = (H1 +H2)*SGL

    Example calculation- Transmitter installed with single Bubble Tube for level measurement

    The transmitter installed in open tank with following parameters

    Height level measured between LRV and URV is 

    H1= 2000mm,

    Head level distance below minimum level(LRV) (suppression)

    H2 = 200mm, 

    The process liquid’s specific gravity is 

    SGL = 0.8 

    Find the calibration range of the transmitter?

    Span value calculation

    Span value = H1*SGL 

    Span = (2000)*(0.8)

    Span value of the transmitter = 1600 mmH2O 

    LRV calculation (4mA)

    HW at LRV = H1*SGL 

    Hw at LRV = (200)*(0.8) 

    Hw at LRV= 160 mmH2O = Suppression 

    URV calculation (20mA)

    Hw at URV = (H1 +H2)*SGL

    Hw at URV = (2000 + 200) (0.8) 

    Hw at URV = 1760 mmH2O 

    Calibration Range of the transmitter = 160 to 1760 mmH2O

    Sundareswaran Iyalunaidu

    With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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