Users Guide Instruction Manual

IB-23C650
RATIO BIAS CONTROL
correct ratio value. When the streams do not have the same flow range, the
variables must be expressed in engineering units to obtain the correct flow
and signal ratio values.
Most applications require direct ratio; the dependent variable increases as the
independent variable increases. If the signal representing the variable is not
linear, it can be linearized using the square root extraction function.
Linearization is often required in flow ratio applications where the process flow
signal from a differential pressure transmitter is proportional to flow squared.
Some applications require inverse ratio; the dependent variable decreases as
the independent variable increases. Inverse ratio is easily accomplished
because the analog signal output can be configured to be reverse acting.
Bias Function
Some ratio control applications require that the ranges of the independent and
dependent variables have different starting points. This means that the ratio
lines will not radiate from the 0-0 origin. This is usually true when temperature
is used to adjust flow, pressure, or another temperature. A bias adjustment
can be configured to shift the ratio line. The bias is added or subtracted from
the set-point value to shift the operating range of the controller.
Figure 21. Effect of Ratio Adjustment in a Direct Ratio Control System
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