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 BA338C 4/20mA output – usually
a Zener barrier or galvanic isolator.
9.6.2
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 BA338C 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 will be 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 will be 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 BA338C and the 4/20mA current
sink output are powered from separate supplies,
the 4/20mA output current will continue to flow
when the BA338C supply fails or is turned off.
Powering both from a common supply as shown in
Fig 18 eliminates this effect.
