BEKA BA374NC, Manual, Page: 5 / 27

1. DESCRIPTION
The BA374NC is an Ex n certified 4/20mA loop
powered transmitter incorporating a large easy to
read display. The instrument may be programmed
on-site to accept most common thermocouples
and resistance thermometers, and will provide a
linear 4/20mA output proportional to temperature,
plus a display in
o C or o F. Voltage inputs may be
scaled allowing the BA374NC to display variables
other than temperature.
Optional alarms provide two galvanically isolated
solid state outputs which may be independently
programmed as high or low trips. For installations
in poorly illuminated areas an optional display
backlight is available.
The instrument has been certified Ex N IIC T5 by
BASEEFA to BS6941:1988. In addition the
instrument complies with the European ATEX
Directive 94/9/EC for Group II Category 3G
equipment. This allows it to be installed in a Zone
2 hazardous area without the need for Zener
barriers, galvanic isolators or additional
mechanical protection.
The BA374NC transmitter is available in a glass
reinforced polyester (GRP), or an epoxy painted
aluminium enclosure, both provide IP66 protection.
Fig 1 Simplified block diagram
2. OPERATION
Fig 1 shows a simplified block diagram of the
transmitter. The input signal, which may be from a
thermocouple, resistance thermometer or a dc
voltage, is digitised and transferred to the
instrument processor via an optical isolator. The
processor scales the signal and for temperature
measurement applies linearisation using look-up
tables stored in permanent memory. The resulting
temperature is shown on the internal display. The
4/20mA output current is proportional to the
displayed temperature.
The transmitter and the optional alarms are
powered from the 4/20mA current loop, but the
optional display backlight requires a separate
supply.
Each time power 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 2.5
seconds.
. . . . For 1 second
0 For less than 0.5
seconds.
Input signal Using calibration
displayed information stored in
instrument memory.
2.1 Controls
All functions of the transmitter can be programmed
on-site via four push-buttons which are located
behind the enclosure cover - see Figs 5A and
5B. In the display mode i.e. when the transmitter
is displaying the input signal, these push-buttons
have the following functions. The 4/20mA output
current is not affected by operating these
push-buttons, and when released, the input signal
will again be displayed.
Button Function
Down While this button is pushed the
transmitter will show the display
corresponding to a 4mA output.
Up While this button is pushed the
transmitter will show the display
corresponding to a 20mA output.
For transmitters fitted with alarms
E + Up While these buttons are pushed the
transmitter will display setpoint 1.
E + Down While these buttons are pushed the
transmitter will display setpoint 2.
It is also possible to adjust the two alarm setpoints
from the display mode - see section 11.4
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