3
1. DESCRIPTION
The BA324D is a 4½ digit intrinsically safe loop
powered digital indicator which displays the current
flowing in a 4/20mA loop in engineering units. The
indicator only introduces a 1.1V drop which allows
it to be installed into almost any 4/20mA current
loop. No additional power supply or battery are
required.
The BA324D is a third generation instrument
incorporating a separate terminal enclosure.
Complying with the European Explosive
Atmospheres Directive ATEX, the instrument has
been certified intrinsically safe for use in explosive
gas and combustible dust atmospheres. ATEX
dust certification is an option - See Appendix 2.
For installation in the USA and Canada, FM and
cFM intrinsic safety and nonincendive certification
are also available. See Appendix 1.
All versions have IECEx certification.
The main application of the BA324D is to display a
measured variable or control signal in a hazardous
process area. The zero and span of the display
are independently adjustable so that the indicator
can be calibrated to display any variable
represented by the 4/20mA current, e.g.
temperature, flow, pressure or level.
The BA324D has been certified intrinsically safe by
ITS to the European ATEX Directive 94/9/EC. The
EC-Type Examination Certificate specifies that
under fault conditions the voltage, current and
power at input terminals 1 and 3 will not exceed
those specified for
simple apparatus
in Clause 5.4
of EN50020:1994.
The indicator is housed in a robust IP66 glass
reinforced polyester (GRP) enclosure with an
armoured glass window.
2. OPERATION
Fig 1 shows a simplified block diagram of a
BA324D. The 4/20mA input current flows through
resistor R1 and forward biased diode D1. The
voltage developed across D1, which is relatively
constant, is multiplied by a switch mode power
supply and used to power the instrument. The
voltage developed across R1, which is proportional
to the 4/20mA input current, provides the input
signal for the analogue to digital converter.
Each time a 4/20mA current is applied to the
instrument, initialisation is performed. After a short
delay the following display sequence occurs:
-1.8.8.8.8
Display test in which all
segments of the display
are activated for 0.5
seconds.
Blank display
For 0.5 seconds.
Decimal points For 3 seconds.
cycled
Input current
Using calibration
display in
information stored in
engineering instrument
memory.
units.
Fig 1 Simplified block diagram of BA324D
2.1 Controls
The BA324D is controlled and calibrated via four
push-button switches which are located behind the
instrument control cover, or as an option on the
instrument cover. In the display mode i.e. when
displaying a variable, these switches have the
following functions:
P
While this button is pushed the
indicator will display the input current in
mA, or as a percentage of the
instrument span depending upon how
the indicator has been programmed.
When the button is released the normal
display in engineering units will return.
The function of this push-button is
modified when alarms or a tare function
are fitted to the indicator.
