21
Fig 12 Setpoint adjustment from the display mode
To adjust an alarm setpoint select 'SP1' or 'SP2'
and press
P
which will reveal the current setting.
Each digit of the setpoint may be adjusted using
the
Up
and
Down
push-buttons, and the
P
button
to move to the next digit. When the required
setpoint has been entered, pressing
E
will return
the display to the 'SP1' or 'SP2' prompt from which
the other setpoint may be selected, or the indicator
may be returned to the display mode by pressing
E
again.
Direct access to the alarm setpoints is only
available when the
menu is enabled - see section
9.2.11
9.3 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.
9.3.1 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.
