Addendum For Pv-Scaling Operation - ABB 268D/V Operating Instruction

ADDENDUM FOR PV-SCALING OPERATION

PV-scaling operation can be used to align the "zero" of the
process with the "zero" reading of the transmitter. A configuration
tool must be use to perform this operation through digital
communication.
1) PV scaling for analog + HART Safety version
There are two different ways to perform a PV-scaling.
Method 1: apply to the transmitter a pressure that corresponds
to the scaling value (offset) you have to apply to the
reading and perform the operation using the
configuration tools. The operation is called SET PV
ZERO (see example 1).
Method 2: calculate the scaling value (offset) and apply it to the
transmitter following the operation available on the
configuration tool. With this method it is possible to
perform a scaling operation even for a value different
then zero. The operation is called SET PV VALUE
(see example 2).
Effect of the PV-scaling operation:
An example can better explain the effect of the scaling action.
°
Example n 1
the transmitter is calibrated at:
LRV = 0 mbar
URV = 200 mbar
the transmitter model has the following limits of operation:
LRL = -400 mbar
URL = +400 mbar
For the effect of a transmitter's capillary, connected to a tank,
there is a pressure of 80 mbar when the tank is empty, i.e. the
transmitter's reading is 80 mbar.
In order to eliminate the pressure caused by the fluid inside the
capillary, you can perform a PV scaling for compensating/
scaling the reading for these 80 mbar. The result of this
operation is:
the transmitter's reading is now 0 mbar.
offset is -80 mbar and must be considered that while the
limits of the transmitter remains:
LRL = -400 mbar
URL = +400 mbar
and the calibration does not change
LRV = 0 mbar
URV = 200 mbar
The configuration tools allows you to evaluate the new
operative limits:
operative LRL = -480 mbar
operative URL = +320 mbar
°
Example n 2
the transmitter is calibrated at:
LRV = 0 mbar
URV = 200 mbar
the transmitter model has the following limits of operation:
LRL = -400 mbar
URL = +400 mbar
the transmitter is reading:
PV = 100 mbar
and you know the process value is 50 mbar.
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You can apply this 50 mbar for your PV scaling operation, with
similar effect as per the previous example:
PV reading = 50 mbar
offset = 50 mbar so that while the limits of the
transmitter remains:
LRL = -400 mbar
URL = +400 mbar
with no change for the calibration, the configuration
tools allows you to display the new operative limits:
operative LRL = -450 mbar
operative URL = +350 mbar
When requested it is possible to reset the value actually
applied as offset.
When an offset is defined, the trimming operations are
disabled and can be rehabilitated only by eliminating the
scaling, i.e. setting the offset to 0.
WARNING. In order to ensure the correct operation
of the transmitter, after the calibration procedure the
device must be put in operating condition as described in
Section Commissioning and Configuration Issues.