Emerson Rosemount OCX 8800, Instruction Manual

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Section 7: Foundation
Fieldbus
April 2017
Instruction Manual
IM-106-880C, OI
15
4
Compensation Input
Calculations
7.13.5 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.
7.13.6 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.