If the d/p cell is not located at an elevation that corresponds to 0% level in the tank, we must calibrate it to account for the difference in elevation.
This calibration adjustment is “zero elevation” when the cell is located above the lower tap; it is “zero suppression” when the cell is located below the lower tap.
To calculate the amount of elevation or suppression, we must measure the distance between the zero reference level and the high-pressure port of the d/p cell. Taking into account that 27.68″ H2O = one (1) psig, calibrate the transmitter for the distance measured in the direction necessary to result in an accurate output.
When the d/p cell is at the same elevation as the zero level in the tank, calibrate the transmitter for “true zero.”
The correct answer is D, calibrated for suppressed zero.
Reference: Mike Cable, Calibration: A Technician’s Guide, ISA Press.