LD301 - Operation and Maintenance, Instruction Manual
3.6
Primary Variable Trim - Pressure
Pressure, defined as a Primary Variable, is determined from the sensor readout by means of a
conversion method. Such a method uses parameters obtained during the fabrication process. They
depend on the electric and mechanical characteristics of the sensor, and on the temperature change to
which the sensor is submitted. These parameters are recorded in the sensor's EEPROM memory. When
the sensor is connected to the transmitter, such information is made available to the transmitter
microprocessor, which sets a relationship between the sensor signal and the measured pressure.
Sometimes, the pressure shown on the transmitter display is different from the applied pressure. This
may be due to several reasons, among which the following:
The transmitter mounting position;
The user pressure standard differs from the factory standard;
Sensor original characteristics shifted by overpressure, over temperature or by long-term drift.
NOTE
Some users prefer to use this feature for zero elevation or suppression when the measurement refers
to a certain point of the tank or tap (wet leg). Such practice, however, is not recommended when
frequent laboratory calibrations are required, because the equipment adjustment refers to a relative
measurement, and not to an absolute one, as per a specific pressure standard.
The Pressure Trim, as described on this document, is the method used in order to adjust the
measurement both in relation to the applied pressure and the user's pressure standard. The most
common discrepancy found in transmitters is usually due to Zero displacement. This may be corrected
by means of the zero trim or the lower trim.
There are four types of pressure trim available:
LOWER TRIM: Is used to trim the reading at the lower range. The user informs the transmitter the
correct reading for the applied pressure via HART
configuration tool.
NOTE
Check on section 1, the note on the influence of the mounting position on the indicator.
For better accuracy, the trim adjustment should be made in the lower and upper values of the
operation range values.
UPPER TRIM: Is used to trim the reading at the upper range. The user informs the transmitter the
correct reading for the applied pressure via HART
configuration tool.
WARNING
The upper pressure trim shall always be applied after the zero trim.
ZERO TRIM: is similar to the LOWER TRIM, but is assumed that the applied pressure is zero. The
reading equal to zero must be active when the pressures of differential transmitter cameras are
equalized or when a gage transmitter opens to atmosphere or when the absolute transmitter is
applied to the vacuum. Therefore, the user does not need to enter with any value.
NOTE
The pressure taps on the transmitter must be equalized when zero trim is applied.
CHARACTERIZATION: this is used to correct any possible intrinsic non-linearity to the conversion
process. Characterization is done by means of a linearization table, with up to five points. The user
shall apply pressure and use the HART
configuration tools to inform the pressure value applied to
each point of the table. In most cases, characterization is not required, due to the efficiency of the
production process. The transmitter will display "CHAR", thus indicating that the characterization
process has been activated. The LD301 is fitted with an internal feature to enable or disable the use
of the Characterization Table.
WARNING
The characterization trim changes the transmitter characteristics. Read the instructions carefully
and make sure that you are working with a pressure standard with 0.03% accuracy or better,
otherwise the transmitter accuracy will be seriously affected.
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