12
When calibrating a BA304D fitted with a root
extractor the indicator zero potentiometer should
be adjusted to give the required display at 10% of
flow (4.16mA). The indicator zero potentiometer
should not be used to set the display to zero with a
4mA input. Zero suppression or elevation may not
be used, i.e. 4mA must correspond to zero flow.
For reference, the following table shows the output
current from a non-linearised differential flowmeter.
% of full flow Current output mA
2.5
4.01
10.0
4.16
25.0
5.00
50.0
8.00
75.0
13.00
100.0
20.00
A root extractor can not be fitted to an instrument
with an internal calibrator.
8.3.1 Calibration example with root extractor
The BA304D 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 two least 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 repeat
step 5.
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 BA304D 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
=
159mA
Po
=
0.8W
The EC-Type Examination Certificate specifies that
the maximum equivalent capacitance and
inductance between terminals 12 and 13:
Ci
=
40nF
Li
=
20µH
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.
For guidance, System Certificates ITS No
Ex99E2006 and 2007 have been issued which list
some of the devices that may be used.
