Bubbler method liquid density measurement calculations with varying tank level and two bubble tubes

In this section, examples of typical calculations for determining the liquid Density measurement Calculations with varying tank Level and two Bubble tubes  Installation are given.

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.

Abbreviations

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

H = Distance between the end of the two bubble tubes (Head between LRV and URV)

SGL = Specific gravity of the liquid – need to be evaluated

SG1 = Minimum Specific gravity of the liquid

SG2 = Maximum Specific gravity of the liquid

H = Head between LRV and URV

Calibrated Range of the transmitter = LRV to URV 

Where:

HW = Equivalent Head level pressure of Water 

Span value of the transmitter = (H)*(SG2 – SG1)

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

HW at LRV =(H*SG1) = Suppression

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

HW at URV = (H*SG2)

Example calculation – Density Calculations with varying Tank Level and Two Bubble Tubes

The DP transmitter is mounted in an open tank with two bubble tube and also has the following specifications to determine density of the liquid.

Process liquid Specific gravity varies between 1.1 and 2.0

Distance between the end of the two bubble tubes is 

H= 60 inches (Head between LRV and URV) 

Find the calibration range of the transmitter for measuring the density?

So, SG1 = 1.1

SG2 = 2.0

Span value calculation

Span = (H)*(SG2 – SG1)

Span = (60)*(2.0 – 1.1) 

Span = (60)*(0.9) 

Span value of the transmitter = 54 inchH2O 

LRV calculation (4mA)

HW at LRV = (H*SG1) 

HW at LRV = (60 *1.1) 

HW at LRV= 66 inchH2O = Suppression 

URV calculation (20mA)

HW at URV = (H*SG2)

Hw at URV = 60 * (2.0) 

HW at URV = 120 inchH2O 

Calibration Range of the transmitter = 66 to120 inch of H2O