BEKA BA374C, Manual

Page: 17 / 27

6.8.3 3-wire RTD reference '3rtd'
This adjustment, which is only necessary if the
transmitter is used with a three wire resistance
thermometer, should be performed after the
voltage reference has been calibrated. To adjust
the 3-wire RTD reference the resistance
thermometer should be replaced by a 100 ohm
standard resistor or calibrator, with the
compensation loop remaining connected between
terminals 3 and 4. It is preferable to make this
substitution as close to the RTD as possible, but
transmitters may be calibrated in a workshop.
Select '3rtd' from the 'Cond' sub-menu and press
P. The transmitter will display scrolling decimal
points while the reference is being updated
followed by 'Ent' prior to returning to the '3rtd'
prompt. This completes adjustment of this
reference.
6.8.4 4-wire RTD reference '4rtd'
This adjustment, which is only necessary if the
transmitter is used with a four wire resistance
thermometer, should be performed after the
voltage reference has been calibrated. To adjust
the 4-wire RTD reference the resistance
thermometer should be replaced by a 100 ohm
standard resistor or calibrator, with both
compensation loops from terminals 1 and 4
connected at the standard resistor or calibrator. It
is preferable to make this substitution as close to
the RTD as possible, but if necessary the
transmitter may be calibrated in a workshop.
Select '4rtd' from the 'Cond' sub-menu and press
P. The transmitter will display scrolling decimal
points while the reference is being updated
followed by 'Ent' prior to returning to the '4rtd'
prompt. This completes adjustment of this
reference.
6.8.5 Output current reference 'OP'
Warning: ensure plant safety
before performing this adjustment
This adjustment is only necessary if the transmitter
4/20mA output current is used. A method of
accurately measuring the transmitter output
current, such as DVM, is required to calibrate this
reference.
Select 'OP' from the 'Cond' sub-menu and press P ,
the transmitter will display 'SurE'. If it is safe to
change the transmitter output current between 4
and 20mA continue by pressing
P . The transmitter
will display 'ZEro', pressing P again will change the
display to '.004A' and the transmitter output current
to approximately 4mA. The transmitter output
current shown on the external current measuring
instrument should be adjusted to 4.000 ±0.001mA
using the transmitter Up and Down buttons. When
correct press the E push-button to return to the
'ZEro' prompt.
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
adjustment of this reference.
7. TRIM MENU
The Trim Menu is the preferred method of routine
calibration. It enables the BA374C 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 BA374C 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.
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