Fluke 707 Application Note

Loop Calibration and
Introduction
Process instrumentation
requires periodic calibration
and maintenance to ensure that
it is operating correctly. This
application note contains
information to help guide tech-
nicians through some of the
more common loop calibration
tasks using Fluke tools.
Fluke loop calibrators are
ideal for a wide variety of cali-
bration applications. They
include:
•
Fluke 707 Loop Calibrator
•
Fluke 705 Loop Calibrator
•
Fluke 715 Volt/mA Calibrator
•
Fluke 787 ProcessMeter
A summary of the features of
Fluke's loop calibrators and a
glossary of terms are at the end
of this application note.
Field testing loop
powered isolators
The loop-powered isolator and
the two-wire isolating trans-
mitter are two of the more
prevalent devices in use in
4-20 mA control loops today
(see Figure 1). The testing and
troubleshooting procedures for
each are different and need to
be understood by the techni-
cians performing operational
checks on these units in the
field.
Maintenance
Figure 1
Loop-powered isolators
The main purpose of a loop
isolator is to eliminate ground
loops in control systems while
sending the control signal
current to another part of the
system. Loop-powered
isolators, unlike two-wire
transmitters, draw their operat-
ing power from the "input" side
of the isolator (see Figure 1),
which requires a pickup voltage
from 5.5 V to 13.5 V depending
on the manufacturer.
The output of the loop isola-
tor is an electrically isolated
mirror image of the input side
current. The compliance voltage
associated with the output is
greatly reduced from that of the
input side and ranges around
7.5 V. This produces a total loop
loading capability of 350 Ω.
This limited loop drive capabil-
ity is the primary limitation of
the loop isolator.
Application Note
Two-wire transmitters
Isolating two-wire transmitters
provide the same isolating
functionality as loop isolators
with the added advantage that
many provide signal condition-
ing for a variety of inputs such
as thermocouple, frequency, dc
current, RTD, strain gauge, and
other process inputs. The
power supplied to a two-wire
transmitter is connected to the
output side of the transmitter.
The two-wire transmitter mod-
ulates the current from the
power supply between 4-20 mA,
proportional to the input.
Typical power supplies for two-
wire transmitters range from
24 V to 48 V. Power supplies of
this size allow a significant loop
load capability on the output.