There are two methods for calibrating a displacer level transmitter.
1. Wet calibration (by using water)
2. Dry calibration (by using calculated weights)
1. WET CALIBRATION : Wet calibration of level troll means calibration using water. Before we calibrate the displacer level transmitter, we must aware that the transmitter has been set at factory as per process fluid specific gravity (SG) if we will use water as the calibration fluid. When the displacer is operated by actual process fluid, it will show 4mA/3psi at low level (the displacer is not immersed) and 20mA / 15psi at high level (the displacer fully immersed). Thus before we calibrate the transmitter, we need to calculate the equivalence of this buoyancy effect if we use water as the test fluid instead of using actual process fluid.First calculate the equivalence water level. Zero level= Displacer not immersed. Calculate the high level using the equation,
High level = (Spec. Gravity of process fluid/Spec. Gravity of water) * Input Span of Transmitter.
PROCEDURE OF WET CALIBRATION :
1. First close both the primary isolation valves and drain the liquid inside the chamber.
2. Not fill the level transmitter chamber with water and adjust the zero to get 0% output (4mA / 3psi).
3. Connect a transparent PVC tube to the drain point as shown in hook-up range value.
4. Mark the calculated high level and fill the water up to that level.
5. Adjust the span to 20mA / 15psi6. Now fill it up to 50% check linearity
PROCEDURE OF DRY CALIBRATION :
1. Remove the displacer from the torque arm.
2. Apply equivalent weight on the torque arm that is equal to the weight of the displacer 3. Adjust the zero (4mA / 3psi), For span First find out the volume of the displacer using the formula,
V = Volume, π = 3.14,
r = Radius of the displacer,
h = height of the displacer
Calculate the loss of weight of the displacer when it is fully immersed in the liquid.
Weight to be suspended = Wt. of displacer – Wt. of displacer immersed in liquid.
= Wt. of displacer – (Volume х Specific Gravity of process fluid)
4. Apply equivalent weight on the torque arm.
5. Adjust the span to get 100% (20mA) output.
6. To check linearity, apply average of the two weights.