To adjust the 20mA output current reference select
the 'SPAn' prompt by operating the
Up
or
Down
button. Pressing
P
will change the display to
'.020A' and the transmitter output current to
approximately 20mA. The transmitter output current
shown on the external current measuring instrument
should be adjusted to 20.000 ±0.001mA using the
transmitter
Up
and
Down
buttons. When correct
press the
E
push-button twice to return to the 'OP'
prompt. This completes calibration of the output
current reference.
7. TRIM MENU
The Trim Menu is the preferred method of routine
calibration. It enables the BA378C transmitter to be
adjusted using an external voltage, resistance or
temperature calibrator. For maximum accuracy the
complete loop including the thermocouple or
resistance thermometer may be calibrated by
heating or cooling the primary element.
Before using the Trim Menu all the functions in the
'InPut' and 'CAL' sections of the programme menu
must be defined, including the 4/20mA output
range. For voltage inputs, both the millivoltage
input and the corresponding display range must be
entered.
If a voltage calibrator is used to simulate a
thermocouple the transmitter cold junction
compensation must be turned off. Alternatively, the
temperature at the BA378C input terminals may be
measured and the voltage calibrator corrected for
ambient temperature, although this is likely to
produce less accurate results.
7.1 Calibration procedure using Trim Menu
The structure of the Trim Menu is shown in Fig 9.
Access to the menu is obtained by operating the
P
and
Down
push-buttons simultaneously. If the
transmitter is not protected by a security code the
first parameter 'ZEro' will be displayed. If the
transmitter is protected by a security code, 'COdE'
will be displayed first. Pressing
P
again will enable
the security code to be entered digit by digit, using
the
Up
and
Down
buttons to change the flashing
digit, and the
P
push-button to move to the next
digit. If the correct code is entered pressing
E
will
cause the first parameter 'ZEro' to be displayed.
If an incorrect code is entered, or if no button is
pressed for ten seconds, the transmitter will
automatically return to the display mode.
Fig 9 Trim Menu
The 'ZEro' sub-menu allows the transmitter display
and output current to be independently trimmed at
the bottom of the input range. To make the adjust-
ment connect a calibrator to the transmitter input
terminals to simulate minimum input, or place the
primary element in a vessel at the minimum
temperature. With 'ZEro' displayed, pressing the
P
push-button will cause the temperature display to
return. If the display does not correspond with the
input calibrator or the temperature of the primary
element, operate the
Up
or
Down
button until the
transmitter display is correct. Immediately the
Up
or
Down
button is operated the annunciator in the
top left hand corner of the display will be activated
to warn that any changes in the input signal will be
ignored while this display correction is being made.
When the transmitter display is correct, press
P
which will lock the transmitter display and, after
briefly displaying 'Ent', allow the 4/20mA output
current to be trimmed using the
Up
or
Down
push-
button. The output current should be measured
with an accurate DVM or another instrument within
the loop. Operate the
Up
or
Down
buttons until the
required output current is shown on the DVM or
other instrument. When correct, press the
E
button
to store the trim correction in permanent memory
and return the BA378C transmitter to the 'ZEro'
prompt. This completes re-calibration of the
BA378C transmitter at minimum input.
18
