Configuration
3.7
Primary Variable Current Trim
When the microprocessor generates a 0% signal, the Digital to Analog converter and associated
electronics are supposed to deliver a 4 mA output. If the signal is 100%, the output should be 20 mA.
There might be differences between the Smar current standards and your current plant Standard. In this
case, the Current Trim adjustment shall be done with a precision ammeter as measurement reference.
Two Current Trim types are available:
4 mA TRIM: this is used to adjust the output current value corresponding to 0% of the
measurement;
20 mA TRIM: this is used to adjust the output current value corresponding to 100% of the
measurement;
The Current Trim shall be carried out as per the following procedure:
Connect the transmitter to the precision ammeter;
Select one of the Trim types;
Wait a while for the current to stabilize and inform the transmitter the current readout of the
precision ammeter.
NOTE
The transmitter presents a resolution that makes it possible to control currents as low as
microamperes. Therefore, when informing the current readout to the transmitter, it is recommended
that data input consider values up to tenths of microamperes.
Transmitter Adjustment to the Working Range
This function directly affects the transmitter 4-20 mA output. It is used to define the transmitter working
range; in this document it is referred to as the transmitter calibration. The LD301 transmitter includes
two calibration features:
CALIBRATION WITH REFERENCE: this is used to adjust the transmitter working range, using a
pressure standard as reference;
CALIBRATION WITHOUT REFERENCE: this is used to adjust the transmitter working range,
simply by having user-informed limit values.
Both calibration methods define the Working Range Upper and Lower values, in reference to some
applied pressure or simply informed by entered values. CALIBRATION WITH REFERENCE differs from
the Pressure Trim, since CALIBRATION WITH REFERENCE establishes a relationship between the
applied pressure and the 4 to 20 mA signal, and the Pressure Trim is used to correct the measurement.
In the transmitter mode, the Lower Value always corresponds to 4 mA and the Upper Value to 20 mA. In
the controller mode, the Lower Value corresponds to PV=0% and the Upper Value to PV=100%.
The calibration process calculates the LOWER and the UPPER values in a completely independent way.
The adjustment of values does not affect one another. The following rules shall, however, be observed:
The Lower and Upper values shall be within the range limited by the Minimum and Maximum
Ranges supported by the transmitter. As a tolerance, values exceeding such limits by up to 24% are
accepted, although with some accuracy degradation;
The Working Range Span, determined by the difference between the Upper and Lower Values, shall
be greater than the minimum span, defined by [Transmitter Range / (120) for models: D, M, H, A4,
A5, and Transmitter Range / (2,5), (25), or (50) for A1, A2, and A3, respectively]. Values up to 0.75
of the minimum span are acceptable with slight accuracy degradation.
NOTE
Should the transmitter operate with a very small span, it will be extremely sensitive to pressure
variations. Keep in mind that the gain will be very high and that any pressure change, no matter
how small, will be amplified.
If it is necessary to perform a reverse calibration, that is, to work with an UPPER VALUE smaller than
the LOWER VALUE, proceed as follows:
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