Owner's manual

A-5
Applications
Figure A-6. Analog Output Function
Block Diagram Example
ARITHMETIC (ARTH)
FUNCTION BLOCK
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 A-6. Relative Temperature
Effects on Level
AI
Function
Block
PID
Function
Block
AO
Function
Block
BKCAL_IN
BKCAL_OUT
OUT
CAS_IN
OUT CAS_IN
Output
OUT
fieldbus-fbus_09a
100%
0%
Level
Percent
Elevated Temperature
Level
Indicated Level
Calibrated at
Ambient
Temperature
xMa4mA 20mA
Milliamp Signal