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5.8 Calibrating the internal references

WARNING!

Changing the internal references will
alter the instrument performance. Do
not not attempt to recalibrate these
references unless you have adequate
test equipment and have read the
following instructions. For routine
calibration of the the instrument use the
Trim Menu described in section 6.

If the internal references are used for calibrating
the BA574C transmitter they may require periodic
checking against external standards. Annual checks
will produce acceptable performance, although less
frequent checks are usually adequate.

If the BA574C transmitter is calibrated against an
external voltage, resistance or temperature
calibrator using the Trim Menu, the internal
references should never need re-calibrating.
Details of the Trim Menu are contained in
section 6.

There are four internal references:

Voltage input
3-wire RTD
4-wire RTD
Current output

The input voltage reference must be adjusted first
irrespective of the transmitter application, but it is
only necessary to adjust the other three references
if the function to which they relate is being used.
For example, a BA574C with a thermocouple input
from which the 4/20mA output current is being used
would require the voltage input and the current
output reference adjusting.

5.8.1 Conditioning internal references: 'Cond'

WARNING!

These functions are contained in a protected
sub-menu to minimise the possibility of accidental
adjustment. Select 'Cond' from the menu and press

. The transmitter will display '0000'.

Using the

and

Down

buttons and the

button to

move to the next digit enter the password 'SurE.
(Note that 'S' is entered as '5') There is a ten
second time-out on password entry. Pressing

will

then give access to the sub-menu containing the
four refences.

5.8.2 Voltage input reference 'U.IP'

With the BA574C input terminals connected to an
accurate voltage calibrator, select 'U.IP' from the
'Cond' sub-menu and press

The transmitter will

respond by displaying 'Lo' which is a request for an
accurate -75.000mV input. When the input is
correct press

to store this figure in permanent

memory. The transmitter will display scrolling
decimal points while the reference is being updated
and will then return to the 'Lo' prompt.

Select 'Hi' from the sub-menu which is a request for
an ac75.000mV input. When the input is
correct press

to store this figure in permanent

memory. The transmitter will display scrolling
decimal points while the reference is being updated
and will then return to the 'Hi' prompt. This
completes adjustment of the voltage input
reference.

5.8.3 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. Although it is preferable to
make this substitution as close to the RTD as
possible, transmitters may be calibrated in a
workshop.

Select '3rtd' from the 'Cond' sub-menu and press

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.

5.8.4 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.