In this section, examples of typical calculations for determining the liquid Density measurement Calculations with Constant Tank Level and single Bubble Tube 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
H1 = Height of the bubble tube immersed inside the process liquid
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 = Constant level of the tank (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 Constant Tank Level and single Bubble Tube
The DP transmitter is mounted in an open tank with single bubble tube and also has the following specifications to determine density of the process liquid.
Process liquid Specific gravity varies between 1.0 and 1.8
Constant level of the tank is
H= 60 inches (Head between LRV and URV)
Find the calibration range of the transmitter for measuring the density?
So, SG1 = 1.0
Span value calculation
Span = (H)*(SG2 – SG1)
Span = (60)*(1.8 – 1.0)
Span = (60)*(0.8)
Span value of the transmitter = 48 inchH2O
LRV calculation (4mA)
HW at LRV = (H*SG1)
HW at LRV = (60 *1.0)
HW at LRV= 60 inchH2O = Suppression
URV calculation (20mA)
HW at URV = (H*SG2)
Hw at URV = 60 * (1.8)
HW at URV = 108 inchH2O
Calibration range of the transmitter = 60 to 108 inch of H2O