BEKA BA524D, Manual

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8.3 Lineariser
The indicator can be supplied with a sixteen point
lineariser which may be adjusted to compensate for
almost any non linear variable. For example, a
level signal from a horizontal cylindrical tank may
be linearised by the indicator to display the tank
contents in linear volumetric units.
The addition of linearising software does not affect
the intrinsic safety of the indicator.
Fig 13 shows a typical linearising characteristic.
Up to sixteen break-points may be programmed to
occur at any input current between 4 and 20mA.
The slope between adjacent break-points may be
set anywhere between -1250 and +1250 display
counts per milliamp. Greater slopes may be
programmed, but the indicator performance will be
degraded. A linear characteristic can be obtained
by programming just two points, one at 4mA and
the other at 20mA.
Fig 13 Typical indicator characteristic
The lineariser software does not change the main
programme menu, but the 'CAL' and 'SEt' functions
are extended as shown in Fig 14. As with a
linear indicator, calibration may be performed with
an external calibrator using the 'CAL' function, or
from the internal references using the 'Set'
function.
8.3.1 Calibration using an external current
source
This method allows direct calibration with a current
source, and is preferred 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-points required should first be
entered using the 'Add' and 'dEL' functions. In
both these functions the indicator initially displays
the current break-point and the total number of
break-points being used. e.g.
5 13
current total number of
break-point break-points
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 select 'CAL' from the main
menu and press
P to enter the 'Add' function;
press P again to reveal the current and total
number of break-points. Each subsequent
operation of the
P push-button will introduce an
additional break-point. When adding a break-point
to a calibrated indicator, the insertion position can
be selected using the
Up and Down push-buttons.
The delete break-point function 'dEL' operates in a
similar manner to the 'Add' function described
above.
When the required number of break-points has
been entered, return to the sub-menu by pressing
E. The indicator will display 'Add' or 'dEL'
depending upon the last function used. Each
break-point can now be programmed.
Select 'PtS' from the sub-menu and press P which
will select the first break-point '0 n', where n is the
total number of break-points entered.
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