Rosemount OCX 8800, Instruction Manual

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Instruction Manual
IM-106-880, Rev 2.1
April 2017
OCX 8800
7-55
Compensation Input Calculations
For each of the inputs IN_1, IN_3, IN_4 there is a gain and bias. The
compensation terms (t) are calculated as follows:
• When IN_(k) is usable: t(k) = GAIN_IN(k) * (BIAS_IN(k) + IN_(k))
• When IN_(k) is not usable, then t(k) gets the value of the last t(k)
computed with a usable input.
Status Handling
IN_x Use Bad
IN_x Use Uncertain
IN_LO Use Uncertain
IN Use Uncertain
For complete descriptions of supported input options, refer to the Option
Bitistring Parameter.
Application Information
The Arithmetic function block can be used to calculate tank level changes
based on greatly changing temperature conditions in devices that depend on
the physical properties of the fluid.
For example, a differential pressure cell’s analog input can be scaled initially
to provide a 4-20 mA signal for 0-100% of level indication. As the temperature
of the system rises, the density of the fluid decreases. For a system that
requires accurate level indication at widely ranging temperature, changing
density proves inconvenient.
The Arithmetic function block allows for the automatic compensation of this
change by incorporating gain and bias adjustments to the temperature signal.
It then applies both the compensated temperature signal and the level signal
to a characteristic system equation. The result is a level that is a true
indication of fluid in the vessel.
Different fluids over the same temperature range have different effects on
level due to their thermal expansion coefficients. Vessel geometry also plays
a major role. As the height of the vessel increases, the effect of thermal
expansion becomes more apparent. The following figure shows the relative
temperature effects on a level signal.
Figure 7-24. Relative
Temperature Effects on Level
100%
Level
Percent
0%
Elevated Temperature Level
Indicated Level
Calibrated at Ambient
Temperature
4mA xMa 20mA
Milliamp Signal
The calculation is done by applying the level signal to the IN connector, the
liquid temperature to the IN_1 connector, and the ambient air temperature to
the IN_2 connector. Select the Arithmetic type (ARITH_TYPE) of Flow
Compensation - Linear.
3
9
9
3
0
0
2
0