Rosemount OCX 8800, Instruction Manual

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Instruction Manual
IM-106-880, Rev 2.1
April 2017
OCX 8800
7-56
This allows a ratio to be set up that increases the level indication at block
output for an increase in the tank temperature relative to ambient
temperature.
Figure 7-25. Arithmetric
Function Block Diagram
Example
ARITH_TYPE = FLOW
COMP-LINEAR
This application can be applied to very large storage tanks whose contents
are subject to thermal expansion and contraction during seasonal changes in
temperature.
Advanced Topics
Arithmetric Types
The parameter ARITH_TYPE determines how PV and the compensation
terms (t) are combined. User may select from nine (9) commonly used math
functions, depicted below. COMP_HI and COMP_LO are compensation
limits.
Flow Compensation Linear Flow Compensation Square Root
func = PV ·
f
func = PV ·
f
COMP_HI
COMP_HI
f =
t(1)
t(2)
f =
t(1) t(3)
√
t(2)
COMP_LO
COMP_LO
If there is a divide by zero and the numerator is positive, f is set to COMP_HI;
if the numerator is negative, then f is set to COMP_LO.
The square root of a negative value will equal the negative of the square root
of the absolute value. Imaginary roots are not supported.
Flow Compensation Traditional Multiply
Approximate BTU Flow and Divide
func = PV
·
f
func = PV
·
f
func = PV
·
f
COMP_HI COMP_HI
COMP_HI
f = t(1) t(2) t(3) 2 f = t(1) - t(2)
f =
t(1)
+ t(3)
t(2)
COMP_LO COMP_LO
COMP_LO
If there is a divide by zero and numerator is positive, f will be limited to
COMP_HI; if the numerator is negative, f will be limited to COMP_LO.
Compensation inputs which are not usable are not included in the calculation.
PV is always included.
TANK_LEVEL
LIQUID_TEMP
AIR_TEMP
IN
ARTHM
Block
IN_1
OUT
IN_2
3
9
9
3
0
0
2
1
3
9
9
3
0
0
2
9
3
9
9
3
0
0
2
2