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Saturday, December 15, 2012

Typical Calibration Procedure of Displacer (Buoyancy) Level Transmitter

Operating Principle
The displacer level transmitter (or any other buoyancy level transmitter) uses the effect of buoyancy. When the displacer is immersed in the process fluid, this buoyancy effect will make the displacer lighter than the non immersed displacer. This change of displacer weight is then detected by the sensor (whether use torque tube or LVDT) and then translated as a fluid level.
Displacer Level Transmitter Operating Principle
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), while we will use water as the calibration fluid. When the displacer is operated by using actual process fluid, it will show 4 mA at low level (the displacer is not immersed) and 20 mA at high level (the displacer is 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 (which is in practice are very difficult to get).
Non-Interface Displacer Level Transmitter Calibration Procedure
1. Calculate the equivalence water level
Zero level = displacer not immersed (no need certain adjustment)
Calculate the high level using this equation:
(Process Fluid Operating SG / Water SG)*Transmitter Level Range
2. Set up the Displacer Level Transmitter to column or temporary support as shown in the calibration setup file below.
3. Fill the level transmitter chamber with water up to the centre of the lower part flange of the LIT cage as a zero level.
4. By using handheld HART communicator set this level as zero level (see also in the LCD display of the transmitter, it should show zero level).
5. Read the mA output of the transmitter by using a multimeter. Adjust (if any) through the HART communicator so that the output of the transmitter (on multimeter) is 4 mA.
6. Fill the level transmitter chamber with water up to the calculated equivalence high level above.
7. By using handheld HART communicator set this level as high level (see also in the LCD display of the transmitter, it should show high level).
8. Read the mA output of the transmitter by using a multimeter. Adjust (if any) through the HART communicator so that the output of the transmitter (on multimeter) is 20 mA.
Interface Displacer Level Transmitter Calibration Procedure
1. Calculate the equivalence water level
Calculate zero level using this equation:
(Lighter Process Fluid Operating SG / Water SG)*Transmitter Level Range
Calculate the high level using this equation:
(Heavier Process Fluid Operating SG / Water SG)*Transmitter Level Range
2. Set up the Displacer Level Transmitter to column or temporary support as shown in the calibration setup file below.
3. Fill the level transmitter chamber with water up to the calculated equivalence zero level above.
4. By using handheld HART communicator set this level as zero level (see also in the LCD display of the transmitter, it should show zero level).
5. Read the mA output of the transmitter by using a multimeter. Adjust (if any) through the HART communicator so that the output of the transmitter (on multimeter) is 4 mA.
6. Fill the level transmitter chamber with water up to the calculated equivalence high level above.
7. By using handheld HART communicator set this level as high level (see also in the LCD display of the transmitter, it should show high level).
8. Read the mA output of the transmitter by using a multimeter. Adjust (if any) through the HART communicator so that the output of the transmitter (on multimeter) is 20 mA.
Example of equivalence water level calculation:
Level Transmitter Level Range = 38 inch
Water SG = 1
Process Fluid SG = 0.84
Equivalence high level using water = (0.84/1)*38 = 31.92 inch
It means the displacer should show 20 mA while we fill the chamber up to 31.92 inch with water fluid as test fluid.
Interface Application
Level Transmitter Level Range = 38 inch
Water SG = 1
Lighter Process Fluid SG = 0.695
Heavier Process Fluid SG = 0.994
Equivalence zero level using water = (0.695/1)*38 = 26.41 inch
It means the displacer should show 4 mA while we fill the chamber up to 26.41 inch with water fluid as test fluid.
Equivalence high level using water = (0.994/1)*38 = 37.772 inch
It means the displacer should show 20 mA while we fill the chamber up to 37.772 inch with water fluid as test fluid.
Typical tools required:
  1. 24 VDC power supply
  2. Multimeter digital
  3. Water Supply Connection
  4. HART communicator
  5. Screwdriver set
  6. Wrench set
Note: This typical maintenance procedure is just an illustration of how to regularly service a displacer level transmitter for academic purpose only. This typical procedure shall not be used as day to day operation guidance. The vendor specific maintenance manual shall be used in detail.
Typical Displacer Level Transmitter Calibration Setup

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