Instruction Manual

Instruction Manual
D103434X012
Blocks
November 2014
142
Block Execution
The Arithmetic function block provides range extension and compensation through nine (9) arithmetic types.
There are two inputs (IN and IN_LO) used in calculating PV. PV is then combined with up to three inputs (IN_1, IN_2,
and IN_3) through the user selected compensation function (ARITH_TYPE) to calculate the value of func. A gain is
applied to func and then a bias is added to get the value PRE_OUT. In AUTO, PRE_OUT is used for OUT.
Range Extension and Calculation of PV
When both IN and IN_LO are usable, the following formula is applied to calculate range extension for PV:
PV = G S IN + (1 – G) S IN_LO
G has a range from 0 to 1, for IN from RANGE_LO to RANGE_HI.
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:
D When IN_(k) is usable:
t(k) = GAIN_IN(k) S ( BIAS_IN(k) + IN_(k) )
D 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 Bitstring Parameters topic.
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 can be 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.