Density Applications - With Standard Displacer And Torque Tube; Calibration At Process Conditions (Hot Cut-Over) When Input Cannot Be Varied - Fisher FIELDVUE DLC3010 Instruction Manual

Instruction Manual
D102748X012

Density Applications - with Standard Displacer and Torque Tube

Note
When you change 'PV is' from level or interface to density, the range values will be initialized to 0.1 and 1.0 SGU. You may edit the
range values according to the specify gravity unit.
It is necessary to back out of Manual Setup and re­enter the Manual Setup menu to see the changes being refreshed.
If the displacer is overweight, there is no way to get the output numerically correct in density mode, because the Level
Offset is not available. Therefore, density calibration normally has to begin with the assumption that the displacer is
free moving at zero buoyancy (dry) conditions. Capture Zero accurately at dry displacer conditions, and any of the full
sensor calibration methods (Weight, Min/Max, and Two Point) can be used in density mode. The terminology can be
confusing, because it usually refers to a "level" as the process condition to set up. When using one of these method,
remember that you are in the density mode and enter observed PV in current units of SGU, g/L, lb/in
Weight Based: The Weight Calibration asks you for the lowest and highest density you want to use for the calibration
points, and computes weight values for you. If you can't come up with the exact values asked for, you are allowed to
edit the values to tell it what weights you actually used.
Min/Max: The Min/Max Calibration essentially reverts to level mode during the calibration process. It asks for the SG of
your test fluid first. Then, it has you set up first a dry and then a completely submerged displacer condition.
Two Point: The Two Point Calibration requires you to set up two different process conditions with as much difference
as possible. You could use two standard fluids with well‐known density and alternately submerge the displacer in one
or the other. If you are going to try to simulate a fluid by using a certain amount of water, remember that the amount
of displacer covered by the water is what counts, not the amount in the cage. The amount in the cage will always need
to be slightly more because of the displacer motion. Because of this inconvenience, and the extra work of draining and
flooding with two fluids, the two‐point calibration method is probably the least attractive in density mode.
Note
These calibration methods advise you to trim zero for better accuracy. That command is not available in density mode.
Sensor Calibration at Process Conditions (Hot Cut‐Over) when input cannot be varied
If the input to the sensor cannot be varied for calibration, you can configure the instrument gain using theoretical
information and use Trim Zero to trim the output to the current process condition. This allows you to make the
controller operational and to control a level around a setpoint. You can then use comparisons of input changes to
output changes over time to refine the gain estimate. A new trim zero will be required after each gain adjustment. This
approach is not recommended for a safety‐related application, where exact knowledge of the level is important to
prevent an overflow or dry sump condition. However, it should be more than adequate for the average level‐control
application that can tolerate large excursions from a midspan set point.
There are a number of calibration methods available in the DLC3010 Device Description. Two Point Calibration allows
you to calibrate the torque tube using two input conditions that put the measured interface anywhere on the
displacer. The accuracy of the method increases as the two points are moved farther apart, but if the level can be
adjusted up or down a minimum 5% span, it is enough to make a calculation. Most level processes can accept a small,
manual adjustment of this nature. If your process cannot, then the theoretical approach is the only method available.
Configuration
October 2014
3 3
, kg/m , etc.
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