6. LINEARISER
A sixteen segment, seventeen break-point (0 to 16)
lineariser may be selected in the ‘FunC’ section of the
configuration menu. The position of each break-point
is fully adjustable so that the slope of the straight line
between break-points can be set to compensate for
input non-linearity, thus allowing the indicator to
display a non-linear process variables in linear
engineering units. Each break-point must occur at a
current greater than the preceding break-point and
less than the following break-point, in the range 3.8 to
21.0mA. If this requirement is not observed when
configuring the lineariser the indicator will display
'FaiL' and the configuration adjustment which
produced the error message will be ignored. Fig 9
shows a typical linearised indicator characteristic.
Fig 9 shows a typical linearising characteristic
Selecting ‘Lin’ in the ‘FunC’ section of the
configuration menu activates the lineariser, this does
not change the configuration menu shown in Fig 8, but
the 'CAL' and 'SEt' functions are extended as shown
in Fig 10. As with a linear indicator, calibration of the
lieariser may be performed with an external current
source using the 'CAL' function, or with the internal
reference using the 'SEt' function.
The lineariser calibration is retained irrespective of
how the indicator function ‘FunC’ is subsequently
changed. It is therefore possible to select and
deselect the lineariser without having to reconfigure it
each time.
The lineariser calibration may be reset to the factory
default settings without changing the indicator
configure using the 'LtAb' function described in section
5.11.
6.1 Lineariser calibration using an external
current source.
This method allows direct calibration of the
lineariser with an external current source and is the
preferred method when traceability is required. If
the exact system non-linearity is unknown, this
method also allows direct calibration from the
variable to be displayed. e.g. the output from a
level sensor in an irregular tank may be displayed
in linear volumetric units by filling the tank with
known incremental volumes and calibrating the
indicator to display the sum of the increments at
each break-point.
The number of break-point required should first be
entered using the 'Add' and 'dEL' functions. In
both these sub-functions the indicator initially
displays the current break-point and the total
number of break-points being used as shown
below.
Display
Description of function
'Add'
Add a break-point
Adds a new break-point before the
displayed break-point. The calibration
of existing break-points is not changed,
but the identification number of all
subsequent break-points is increased
by one.
'dEL'
Remove a break-point
Removes the displayed break-point and
joins the preceding break-point to the
following break-point with a straight
line. The identification number of all
subsequent break-points is decreased
by one.
To add a break-point use the
▲
or
▼
button to
select 'CAL' from the configuration menu and press
P
which will result in the
'Add' sub-function prompt
being displayed. To enter the sub-function press
P
which will reveal the current break-point and the
total number of break-points which have already
been entered. When adding a break-point the
insertion position can be selected using the
▲
and
▼
push buttons followed by
P
push button to
insert the additional break-point. In previously
uncalibrated linearisers each new break-point
should be added in front of the highest existing
break-point, if this sequence is not followed a 'FaiL'
message will occur in the 'Pt5' function. See
example in section 6.1.1.
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