Displacement Level Transmitter Calibration
Buoyant force acts on a displacer that is submerged in a liquid. The displacer is reduced in weight by the weight of the amount of fluid it displaces. This movement of the displacer is typically translated and converted to an instrument signal.
Input and Output Measurement Standards
One method is to use actual liquid level as the input for calibrating a displacement level transmitter. The most appropriate liquid for replicating process conditions is a safe liquid with the same specific gravity as the process fluid.
Connect a milliammeter as the output standard and a 24 V DC power supply in a series circuit with the transmitter.
Determine the setting for the calibration dial by multiplying the specific gravity of the liquid by the correction factor. Then, set the pointer to the compensated value.
Displacement level transmitters are classified as direct or reverse acting. With direct action, an increase in level, increases the output signal, and a decrease in level decreases the output signal. With reverse action, an increase in level, decreases the output signal, and a decrease in level increase the output signal.
When the chamber is empty, the corresponding output should be 4 mA. If the milliammeter displays a value that is greater than or less than 4 mA, adjust the zero.
To correct span, fill the chamber to the upper range value, and turn the span adjustment until 20 mA is produced.
Linearity is not always adjustable on this type of transmitter, check to manufacturers specifications.
Adjust both zero and span until transmitter performs within specifications.
This same sensing principle used to measure liquid vapor interface, can be used to locate the interface between two liquids.
The heavier of the two liquids exerts more buoyant force, so as the lower phase rises or falls, the displacer travels with it.
To create calibration conditions that replace the process, use liquids that have the same specific gravity as the process fluids. Fill the chamber with the lighter phase to check and adjust the zero. Check zero by filling the chamber to 100% with the lighter fluid.
Check span by filling the chamber 100% full of the heavier phase. Adjust span until 20 mA output is produced.
Check mid-range output and recheck zero and span.