15
7. LINEARISER
A sixteen segment, seventeen breakpoint (0 to 16)
lineariser may be selected in the ‘FunC’ section of
the configuration menu. The starting point and
slope of each straight line segment are fully
adjustable allowing the indicator to display most
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 adjusted 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 uing the 'LtAb' function
described in section 6.11.
7.1 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 to a
calibrated indicator, the insertion position for the
new segment can be selected using the
▲
and
▼
push buttons. Each subsequent operation of the
P
push button will introduce an additional break-
point up to the maximum of 'n:16'.
