12
8.3.1 Calibration example with root extractor
The BA304C is required to display rate of flow in
gallons per minute, with a resolution of 0.1 gallons.
The differential flowmeter has an output of 20mA
at a flow rate of 140.0 gallons per minute.
i.e.
A span of 1400 ignoring the decimal point
A decimal point in position 00.0
The following adjustments are required:
Step 1
Put the suppression / elevation links
in the elevation position.
Step 2
Put the zero link in the 0 to 500
position.
Step 3
The required span is 1400, therefore
the span links should be placed in the
1000 to 1500 position.
Step 4
The decimal point is required between
the least two significant digits,
therefore the decimal point link should
be placed in the 00.0 position.
Step 5
With 4.160mA input current adjust the
zero potentiometer until the indicator
displays 14.0 (10% of flow). If there is
insufficient adjustment to achieve this,
put the elevation/suppression links in
the suppression position and continue
with the calibration procedure.
Step 6
With 20.000mA input current adjust
the span potentiometer until the
indicator displays 140.0
Step
7
Repeat steps 5 and 6 until both
calibration points are correct.
8.4 Display backlight
The BA304C may be supplied with two different
backlights. The loop powered backlight produces
green background illumination enabling the display
to be read at night and in poor lighting conditions.
No additional power supply, IS interface or field
wiring are required, but the indicator voltage drop
is 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.
8.4.1 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 8. 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 8 Separately powered backlight