31
The maximum equivalent capacitance and
inductance of the optional 4/20mA output is:
Ci
= 2nF
Li
= 8µH
To determine the maximum permissible cable
parameters, these figures must be subtracted
from the maximum cable capacitance and
inductance specified by the system certificate
of the circuit connected to the BA334D 4/20mA
output – usually a Zener barrier or galvanic
isolator.
9.5.2 Programming and adjustment
The main programme menu is extended as
shown in Fig 19 to accommodate the 4/20mA
calibration functions.
The ‘4-20 OP’ programme function allows the
4/20mA output current to correspond to any
rate display. e.g. 4mA could represent 10
litres/minute and 20mA 100 litres/minute.
To calibrate the 4/20mA output current, enter
the programme mode by operating the P and E
push-buttons simultaneously. If the instrument
is not protected by a security code the first
parameter ‘UPdAtE’ will be displayed. Enter
the security code if the instrument is protected.
Using the Up or Down push button select the
function ‘4-20 OP’ and press P which will result
in the BA334D displaying ‘4.000’ which is the
prompt for setting the rate display
corresponding to an output current of 4.000mA.
Pressing P will reveal the existing setting with
the most significant digit flashing. The decimal
point will be fixed in the position already
defined for the rate display by the ‘d.P’
function. The value of the flashing digit may
be changed by pressing the Up or Down
buttons. When this digit is set as required,
pressing P will transfer control to the next digit.
When the flow rate corresponding to an output
current of 4mA has been entered, press the E
button to return to the ‘4.000’ prompt.
To enter the rate display corresponding to an
output current of 20.000mA, press the Up or
Down button to select the ’20.000’ prompt
followed by P to reveal the current setting.
This may be changed in the same way as the
4mA setting. When set as required, press E
three times to return to the display mode via
the ‘20.000’ and ‘4-20 OP’ prompts.
The 4/20mA output may be calibrated to
increase or decrease as the rate display
increases, and most instruments will produce a
linear output between 3.5 and 22mA.
Fig 19 4/20mA output programme structure
Note 1:
If the rate display calibration is
changed by adjusting the rate scale factor
SCALE-r, the timebase t-bASE, or the position
of the rate
display decimal point, the 4/20mA
output will automatically be set to give a
constant 3.5mA output irrespective of the rate
display. The 4/20mA output must therefore
always be recalibrated following recalibration of
the rate display.
Note 2:
If the BA334D and the 4/20mA current
sink output are powered from separate
supplies, the 4/20mA output current will
continue to flow when the BA334D supply fails
or is turned off. Powering both from a common
supply as shown in Fig 18 eliminates this
effect.
9.6 External push-buttons
For applications requiring frequent access to
the programme menus the BA334D can be
supplied with an external membrane keypad.
These switches, which maintain the IP66
integrity of the enclosure, allow all functions to
be controlled without removing either of the
enclosure covers.
Summary of Contents for BA334D
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