Application Note

2 Fluke Corporation Abridged HART Transmitter Calibration
HART calibration
is required!
A common misconception is
that the accuracy and stability
of HART instruments eliminate
the need for calibration. Another
misconception is that calibra-
tion can be accomplished by re-
ranging field instruments using
only a HART communicator. Still
another misconception is that
the control system can remotely
calibrate smart instruments.
These are not true. All instru-
ments drift. Re-ranging with
just a communicator is not cali-
bration. A precision calibrator
or standard is required. Regular
performance verification with a
calibrator traceable to national
standards is necessary due to:
1. Shifts in performance of
electronic instruments over
time, due to exposure of the
electronics and the primary
sensing element to tempera-
ture, humidity, pollutants,
vibration, and other field
environmental factors.
2. Regulations governing occu-
pational safety, consumer
safety, and environmental
protection.
3. Quality programs such as ISO
9000 standards for all instru-
ments that impact product
quality.
4. Commercial requirements
such as weights, measures,
and custody transfer.
Regular calibration is also
prudent since performance
checks will often uncover
problems not directly caused
by the instrumentation, such as
solidified or congealed pressure
lines, installation of an incor-
rect thermocouple type, or other
errors and faults.
A calibration procedure con-
sists of a verification (As Found)
test, adjustment to within
acceptable tolerance if neces-
sary, and a final verification (As
Left) test if an adjustment has
been made. Data from the cali-
bration are collected and used
to complete a report of calibra-
tion, documenting instrument
performance over time.
All instruments, even HART
instruments, must be cali-
brated on a regular, preventive
maintenance schedule. The
calibration interval should be
set short enough to insure that
an instrument never drifts out
of tolerance, yet long enough
to avoid unnecessary calibra-
tions. Alternatively, the interval
may be determined by critical
process requirements, e.g., cali-
bration before each batch.
How are HART
instruments properly
calibrated?
To calibrate a HART instrument
consistent with its application, it
is very helpful to understand the
functional structure of a typical
HART transmitter.
HART instruments consist
of three distinct sections (see
Figure 1). Proper HART calibra-
tion may involve either or both
sensor trim and output trim.
Adjusting range values (LRV
and URV) without a calibrator
is not calibration. Performing
an output trim while ignoring
the input section is not proper
calibration. Adjusting range
values with a calibrator may be
a practical calibration alterna-
tive for instruments operated in
4-20 mA analog mode, provided
that the PV and PVAO are not
used for process control.
Sensor
Sensor Trim LRV/ URV Adjust Output Trim
PV PVAO 4-20 mA
Input
Section
Conversion
Section
Output
Section
Analog
Input
(digital
input)
(digital
4-20 mA)
Analog
mA Output
Figure 1.