Instruction Manual
D102748X012
DLC3010 Digital Level Controller
Configuration
May 2018
69
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 SG
is provided in the Manual Setup > Process Conditions > Fluid menu when you are in Liquid 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 error curves 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 factor 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. At DD Rev 4, the Two-Point, Min-Max and Weight calibrations are able to
back-compute a theoretical Zero Reference Angle, even in Interface mode, from the two data points provided.
Therefore, the overweight displacer issue is only of concern if it becomes necessary to use the Capture Zero
procedure.
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
sensitivity to the knowledge of a fluid density is maximum at the process condition where that fluid covers all of the
displacer, zero at the opposite extreme process condition, and varies linearly between those points.
If the fluid density changes are batch‐related or very gradual, it may be practical to keep track of the SG of the fluid and
periodically reconfigure the transmitter memory to match the actual process condition. Frequent automatic updates
to this variable are not advised, as the NVM location where it is stored has an expected lifetime of about 10,000 write
operations. If changes are only a function of temperature, the characteristic of the fluid can be loaded once in the NVM
table, and an RTD connected to measure the process temperature and drive the correction table. If temperature is not
the driving influence, the best that can be done is to calibrate for the widest potential differential SG. (This will keep
