Excessive Mechanical Gain; Determining The Sg Of An Unknown Fluid; Accuracy Considerations; Effect Of Proportional Band - Fisher FIELDVUE DLC3010 Instruction Manual

Instruction Manual
D102748X012
These steps will provide an approximate PV calibration to get a system operational. Further refinements can then be
made when it is possible to manipulate and observe the level and instrument output.

Excessive Mechanical Gain

If the displacer/torque tube sizing provides more than 4.4 degrees of torque tube rotation for a full span change in
process input, It may be difficult to obtain a valid calibration with the normal coupling procedure. In such a case, you
can utilize the full mechanical span of the DLC3010 by coupling the instrument to the torque tube at the 50% travel
condition, instead of at the lowest process condition. When coupled at the 50% travel condition, the travel limits of the
249 hardware will be the constraining factor. If the 249 travel limit is reached before full process input travel is
achieved, the hardware is either improperly sized for the application, improperly assembled, or damaged.

Determining the SG of an Unknown Fluid

If the instrument has been calibrated with weights or by using a test fluid with a well­known SG, it is possible to use the
instrument to measure the SG of an unknown fluid, or the differential SG between two fluids. A procedure called
'Measure Density' is provided in the Manual Setup ­ Process Fluid menu when you are in Level measurement mode. The
procedure presents the measured value and allows you either to automatically move it into the instrument
configuration, or to manually record it for later use.

Accuracy Considerations

Effect of Proportional Band

If you are operating at low Proportional Band [PB = 100% times (full span torque tube rotation) / (4.4 degrees)], you
can expect a degradation factor of about (100%)/(PB%) on the Transmitter accuracy specifications.
Note
This formula is most correct for linearity errors that are relatively steep‐sided. If the linearity error curve shape is simple with
relatively gradual slope, the net effect of reducing span may be less. Instruments such as the DLC3010, that use a compensation
technique to reduce the residual mechanical or electrical non‐linearity, will generally have a complex shape for the net‐error curve.
If this is too much degradation, an improvement of 2.0 can be obtained by using a thin‐wall torque tube. Additional
gain can be achieved by increasing the displacer diameter. Available clearance inside the cage, and the need to keep
the net displacer weight at the highest and lowest process conditions within the usable range of the torque tube /
driver rod combination, place practical limits on how much the sizing can be adjusted.
With an overweight displacer, the calibration process becomes more difficult, (because the zero buoyancy condition
will occur with the linkage driven hard into a travel stop). In interface measurement mode it becomes impossible to
Capture Zero. One simple and effective solution is to use Level measurement mode. Capture Zero at the lowest
process condition instead of zero buoyancy, and enter the differential SG = (SGlowerfluid - SGupperfluid). The
algorithm then computes level correctly.

Density Variations in Interface Applications

A high sensitivity to errors in the knowledge of fluid density can develop in some interface applications. For example:
Suppose the whole input span is represented by an effective change in SG of 0.18. Then a change in the actual SG of
the upper fluid from 0.8 to 0.81 could cause a measurement error of 5.6% of span at the lowest interface level. The
Configuration
October 2014
65