We are facing a calibration problem with a Fisher 249 displacer-type leveltrol installed side-mounted in a cage with a DVC controller. The application is crude oil (SG ≈ 0.8), used for both oil level and interface level measurement in a separator. During calibration, we are using water, but the instrument refuses to accept the span and exits calibration mode. What is the correct procedure?
Why Is My Fisher 249 Displacer Leveltrol Not Accepting Span Calibration When Using Water?
We are having trouble calibrating a Fisher 249 displacer-type leveltrol that is placed on the side of a cage with a DVC controller. The application is crude oil (SG ≈ 0.8), which is utilized to measure both the oil level and the interface level in a separator.
You can’t use water to calibrate a displacer made for oil (SG 0.8).
The problem with your calibration is directly linked to a disparity in specific gravity (SG).
The buoyancy force, which depends on the density of the fluid, is used to calibrate a Fisher 249 displacer leveltrol. Because your instrument was made and calibrated for crude oil with an SG of 0.8, using water (SG 1.0) during calibration makes the displacer float more, which makes its travel less effective.
Why Calibration Fails
- Oil is less buoyant than water.
- The displacer doesn’t go down to the right lower-span position.
- The lift force on the torque tube is more than what was planned.
- When you try to change the span, the controller sees a mechanical force that is out of range and rejects the calibration and exits.
Correct Calibration Procedure
- Always use a test fluid that has the same specific gravity as the design fluid (≈0.8).
- If you can’t get actual crude, you can use any clean oil with a specific gravity of about 0.8.
- Use the right fluid to slowly fill the cage and do zero (dry), 4 mA (LRV), and 20 mA (URV) checks.
- Check that the displacer’s size, weight, and torque tube rating are the same as what is on the original datasheet.
- Only after making sure the mechanical span is accurate should you recalibrate the DVC/positioner.
A displacer leveltrol cannot be calibrated using a fluid with a higher density than its design fluid. Use oil (SG 0.8) instead of water, and the unit will accept the span.