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.
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
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