- 33 -
Use of the Green Keys - TRIM - 4 to 20 mA Trimming
The scope of this operation is to adjust the digital to
analog converter to cover exactly the range 4 to 20 mA.
This operation requires that a high accuracy digital
meter is connected to the current loop output : it is
possible to use either a precision milliammeter with a
resolution of 1
µ
A or a voltmeter with a 1/20000
resolution. In the case that the voltage reading is selected
particular attention should be paid to the accuracy and
thermal stability of the resistor connected in series with
the current loop. Please check that the transmitter is
adequately powered before doing this operation. This
can be done by performing the Loop Test operation to
exercise the output to a full 21 mA, thereby ensuring that
the power supply is capable of providing the necessary
current.
If the transmitter output is out by more than
±
5 % then
a multiple trimming operation should be done ( see
section: Multiple Trim Generation).
A third option is foreseen, Factory Trim, that allows the
user to restore the calibration done during the
manufacturing process: the parameters of this calibration
cannot be changed by the user.
This option is available for 600T, 600T EN and 652/653
S Transmitters.
Starting from the top display press the F2 key ( 4-20 TRIM ) : the display
at the side appears; the screen then displays two methods for specifying
the output current trimming value:"OTHER SCALE and 4 to 20 mA".Press
F2 for current reading or F1 (OTHR SCALE ) for voltage reading: If "Other
Scale" will be selected the following warning message is then displayed.
"WARNING-Control loop should be in manual - PROCEED". Press
the PROCEED key to acknowledge the message and continue and the
following sequences of display ask you to define the appropriate scale.
A first display indicates the actual trimming scale, by default -0.0 to 0.0.
Pressing F3 for Changing, a new display sets the initial point of the scale
corresponding to an output of 4 mA. A blinking cursor appears on the first
character of the figure within square brackets: using the cursor and the
numerical keypad enter the value ( for instance 1.000) and then press the
F4 key for ENTER.
The display now shows the final point of the scale corresponding to an
output of 20 mA. Proceed as above to set this value (for instance 5.000).
This ends the procedure of setting any other scale than 4 to 20mA.
The message in the display below asks you to connect the reference
meter, i.e. the precision milliammeter or voltmeter. When you have done
this press F4 to proceed. The Communicator is now ready to set the
output to 4 mA and the following message is displayed:
"About to set Xmtr output to 4 mA - PROCEED".
Pressing the F4 key to proceed the display at the side appears. In the top
right corner a T blinks for some seconds to indicate that communication
with the transmitter is in course to read the actual value of the output: this
value is then displayed within square brackets as in the example. As usual
a blinking cursor appears in the first digit. Using the cursor and the
numerical keypad enter the value read on the reference meter ( for
instance 4.025) and then press the F4 key for ENTER.
Note: the maximum allowed change is
±
5% of the output range,
corresponding to
±
800
µ
A: if this value ( i.e. the difference between
the displayed value and the meter reading ) is exceeded an appropriate
message is displayed, the operation is aborted and an operation of
multiple trimming should be performed (see next page).
The value is transmitted to the connected instrument and a message is
displayed for some seconds:"Making changes permanent - PLEASE
WAIT". The Communicator reads and displays the new value and then
queries if the reading is sufficiently close to the reference meter reading.
If the reading (within
±
3
µ
A) is not sufficiently close, press the NO key to
repeat the above procedure, otherwise press the YES key and the
procedure will be repeated for the 20 mA setting.
4-20 m A T R I M - CHOOSE
INTERNAL SCALE
OTHR 4 TO FACT
SCALE 20mA TRIM
S e t s c a l e - 4 mA p o i n t
4 mA : [ - 0.000 ]
< - - - - > C L R ENTR
S e t s c a l e - 20 mA p o i n t
20 mA : [ 0.000 ]
< - - - - > C L R ENTR
C o n n e c t r e f e r e n c e
meter
PRO
CEED
Input present meter T
V a l u e : [ 4.000 ] mA
< - - - - > C L R ENTR
S c a l e d o p u t : 4.000
e q u a l t o r e f. m e t e r
NO YES
T R I M M I NG A N D S E T U P
SNSR 4 - 20 LOOP CALI
TR I M TR I M TEST BRA
T r i m w i l l b e s c a l e d :
- 0.000 to 0.000
CHNG ENTR
