Use Of The Green Keys - Trim - Sensor Trimming Option; Zero Trimming; Full Trimming - ABB 691HT Operating Instructions Manual

Use of the Green Keys - TRIM - Sensor Trimming option ( for 600T and 600T EN pressure transmitter only )
The scope of the Sensor Trimming operations is to
correct the characterisation parameters stored in the
transmitter's EEPROM during the last characterisation
process, either the first, factory done, or the subsequent
trimming operation. To understand completely the
reason for this operation it is necessary to know that
during manufacturing the sensor output characteristics
are compared to different pressure input in order to
obtain parameters that are stored in the EEPROM and
represents a sort of "fingerprint" of a specific sensor.
These parameters, are applied to a suitable algorithm
that provides a precise primary output linearisation.
During the use of the transmitter the sensor is submitted
to various influences (ageing, one side overpressure,
shocks, mounting position, etc) that can affect the
original characteristics. The effect of these changes is
that, although through the reranging operation the analog
output matches with the input pressure, the interpreta-
tion of this pressure by the transmitter is incorrect and
the Process Value indication, in engineering units,
doesn't match with the real input pressure.
The full trimming operation forces two points, usually
the zero and full scale values, to match exactly the
primary output: these values are stored in the transmit-
ter in a permanent way.
Using the full trimming operation it is possible, providing
that suitable high accuracy equipment is available, to
restore the original performance or in case that the
transmitter is calibrated to a new range to match exactly

ZERO TRIMMING

LRL
Linearisation curve before
zero trimming
100%

FULL TRIMMING

In both examples the trimming has been applied to a span of
about 75% of the full range.
600T Ver. 5.2 and 600T EN
New
LRV
0%
LTP
- 36 -
Linearisation curve after zero
trimming 600T Release 5.1 and
Deltapi K
New
URV
100%
HTP
the digital reading with the corresponding analog out-
put value. The best accuracy is accomplished trimming
at values corresponding to the LRV and URV or slightly
more. Nevertheless trim with span smaller than sensor
minimum span are not allowed.
The zero trimming operation involves the zero and the
span points of the scale and can be usually done
without the need of external equipment. It can be used
to compensate small errors ≤0.1% (e.g. mounting
position effects). The figures below show the linearity
errors of the transmitter primary output at the maximum
span (URL less LRL) and the different effects of the full
and zero trimming. The full trimming has been applied
to a span of about 75% of the sensor limits. With a 600T
EN Pressure transmitter full trimming and Zero
trimming operations are not accepted if a PV BIAS
VALUE has been introduced. When trying to do a
trimming with the PV bias value not equal to zero the
following message appears: "WARNING: TRIM is not
available when PV is scaled, reset the PV BIAS
before trim".
See next (PV scaling option).
Note for 600T Transmitters Revision 5.2 and 600T EN
The procedure for the sensor trimming of 600T
transmitters slightly differs from the described one: a
fourth 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.
Linearisation curve after zero
trimming 600T Release 5.2 and
600T EN
0%
600T Ver. 5.1 and Deltapi K
New
+0,1%
LRV
0%
LTP
-0,1%
original linearity
after low trim
after low and high trim
+0,1%
URL
-0,1%
100%
+0,1%
New
URV
-0,1%
100%
HTP