Fluke HART 744 - Manual (Page 21)

Documenting Process Calibrator HART Mode
Understanding HART Calibration
13
Understanding HART Calibration
An analog transmitter has one stage of electrical
conversion from a measured physical parameter to a
4-20 mA current loop output. A HART transmitter has the
three stages shown in Figure 3.
Depending on how the transmitter is used in your
application, you may need to test and adjust the Input
stage, the Output stage, or both. For example, if your
application requires the Primary Variable (PV) to be
correct when read by a host computer, you must calibrate
the Input stage.
If your application requires that the 4-20 mA current
output value accurately reflect what the Input stage is
measuring, you must calibrate both the Input and Output
stages.
Transmitters in multidrop systems, in which more than
one is wired in parallel, do not use their Output stages at
all. Their analog outputs are all held at an idling level of 4
mA no matter what the Input stage is measuring.
744 HART Mode Menus for Adjustment
In 744 HART mode, adjusting the Input stage is called
Sensor Trim . Adjusting the Output stage is called Output
Trim . Both adjustments are made from the Service menu.
For pressure transmitters, there is another adjustment,
called
Pressure Zero Trim . This adjustment is the same as
trimming the lower sensor point at zero. All three
operations are run from the 744 HART mode
Service
menu.