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9.5 Tare function
The tare function is a factory fitted software
accessory, primarily intended for use with weighing
systems.
When the tare software is installed, pushing and
holding the P button for more than approximately
3 seconds sets the indicator display to zero,
irrespective of the input current, and activates the
tare annunciator on the indicator display.
Subsequent operation of the P push-button for less
than 3 seconds will toggle the indicator between
the normal gross display and the net display with
the tare annunciator activated.
The tare function allows an operator to quickly zero
the indicator display at any input current so that
subsequent readings only show the change in
input since the P push-button was operated.
When used with a weighing system, it allows the
weight of a container to be automatically
subtracted from the total gross weight so that only
the net weight of the contents is displayed.
9.6 Display backlights
The BA327C and BA328C can be supplied with
one of two different backlights. The loop powered
backlight produces green background illumination
enabling the display to read at night or in poor
lighting conditions. No additional power supply, IS
interface or field wiring are required, but the
indicator voltage drop in increased. Alternatively,
the separately powered backlight has a bright
orange output which enhances daylight viewing,
but an additional IS interface and field wiring are
required.
9.6.1 Separately powered backlight
The separately powered backlight is segregated
from the measuring circuit and has been certified
as a separate intrinsically safe circuit, but it does
not comply with the requirements for simple
apparatus.
This backlight must be powered from the safe area
via a Zener barrier or a galvanic isolator as shown
in Fig 15. Any certified device may be used,
providing the output parameters do not exceed:
Uo
=
28V dc
Io
=
110mA
Po
=
0.77W
The EC-Type Examination Certificate specifies that
the maximum equivalent capacitance and
inductance between terminals 12 and 13 is:
Ci
=
0.045µF
Li
=
0.02mH
To determine the maximum permitted cable
parameters, these figures should be subtracted
from the maximum permitted cable capacitance
and inductance specified by the certificate for the
Zener barrier or galvanic isolator powering the
backlight.
Fig 15 Separately powered backlight
The display brilliance depends upon the current
flowing through the backlight. This is determined
by the supply voltage and the end-to-end
resistance of the Zener barrier or output resistance
of the galvanic isolator. Brilliance will not be
significantly reduced until the current falls below
20mA.
The BA327C backlight requires a minimum supply
voltage of 14V and the larger BA328C backlight a
minimum supply voltage of 18V. The backlight
current can be calculated from:
For a BA327C
Backlight mA = Vsuppl
̲̲
y –14 _
End-to-end resistance of barrier
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