Fluke 718, 753, 754, 707, 87V Service Guide

HART® Transmitter

Calibration

Application Note

Introduction

In today’s process plants, most
new field instruments are smart
digital instruments. Smart
implies a microprocessor-based
instrument with extra function­ality and digital compensation,
supporting multiple sensor types
or multiple variables. These
instruments generally offer
better accuracy, long-term sta­bility, and reliability than con­ventional analog instruments.

The most common class of
smart instruments incorpo­rates the HART protocol, with
more than five million HART
instruments in use in 100,000
plants worldwide. HART, an
acronym for Highway Address­able Remote Transducer, is an
industry standard that defines
the communications protocol
between smart field devices and
a control system that employs
traditional 4-20 mA wiring.

Two capabilities are required
to properly service HART instru­ments: precision analog source
and measure capability and
digital communication capabil­ity. Until recently, this required
two separate tools, a calibrator
and a communicator. Today, the
capabilities of those two tools
are available in a single HART
Documenting Process Calibra­tor that can help you quickly
and effectively service HART
instruments.

From the Fluke Digital Library @ www.fluke.com/library

HART calibration

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

is required!

A common misconception is
that the accuracy and stability
of HART instruments eliminate
the need for calibration. Another
misconception is that calibra­tion can be accomplished by re­ranging 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 instru­ments drift. Re-ranging with
just a communicator is not cali­bration. 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 tempera­ture, humidity, pollutants,
vibration, and other field
environmental factors.

2. Regulations governing occu­pational safety, consumer
safety, and environmental
protection.

3. Quality programs such as ISO
9000 standards for all instru­ments 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 incor­rect thermocouple type, or other
errors and faults.

A calibration procedure con­sists of a verification (As Found)
test, adjustment to within
acceptable tolerance if neces­sary, and a final verification (As
Left) test if an adjustment has
been made. Data from the cali­bration are collected and used
to complete a report of calibra­tion, documenting instrument
performance over time.

All instruments, even HART
instruments, must be cali­brated 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 calibra­tions. Alternatively, the interval
may be determined by critical
process requirements, e.g., cali­bration 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 calibra­tion 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 alterna­tive for instruments operated in
4-20 mA analog mode, provided
that the PV and PVAO are not
used for process control.

Figure 1.

2 Fluke Corporation Abridged HART Transmitter Calibration

Model Number

PV

(Primary Variable)

PVAO

(Digital representation

of the Primary Variable

Analog Output)

Analog Measure Value
Analog Source Value

Tag ID

PV LRV
(Primary Variable
Lower Range Value)

PV URV
(Primary Variable
Upper Range Value)

Figure 2.

New tool speeds
calibration

Today, instrument maintenance
is moving out of the shop and
into the field. This reduces
process interruptions and
avoids the time and expense
of returning instruments to the
shop. Portable communicators
and calibrators are often used

3 Fluke Corporation Abridged HART Transmitter Calibration

together to complete field cali­brations. However, the desire to
carry less equipment and to per­form maintenance in the field
has created a need for a new
class of calibration tool.

The new 754 Documenting
Process Calibrator from Fluke
is a powerful yet easy-to-use
tool for field calibration of HART
instrumentation. Pressing a

single key enters the HART
mode and displays the essential
HART information in the Active
Device Screen, shown in Figure

2. Additional HART functional­ity is accessed with only a few
more keystrokes, per the menu
tree in Figure 3.