Application Note

3 Fluke Corporation Better ways to troubleshoot automation and process control loops
Fluke Corporation
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
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Web access: http://www.fluke.com
©2009 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 6/2009 3497464A A-EN-N
Modification of this document is not permitted
without written permission from Fluke Corporation.
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increments between 4.0 mA
and ~3.9 mA, while feeling the
valve stem with your free hand
to check for any sign of move-
ment. Adjust for zero movement
between these two current
settings by using the zero
adjustment on the positioner.
Next increase and decrease
current from 4 mA to ~4.1 mA.
Insure that the valve stem just
begins movement above the
~4.1 mA setting and fully closed
at 4 mA.
Span can be checked simi-
larly, by setting the meter at
20 mA, ~19.9 mA and ~20.1 mA,
and linearity can be checked
by using the meter’s 25 % step
function.
Checking loop isolators
To check a loop isolator, apply
a mA input signal to the device
and measure its 4-20 mA output
using the clamp-on current
measuring function. This two
channel simultaneous source/
measure function in the 773
can also be used for valves
that report their position using
4-20 mA.
Checking VFDs
Variable frequency drives (VFDs)
are used to power motors,
blowers and fans in process
applications as well as conveyor
systems and machine tools.
Control inputs are generally volt-
age (1 V to 5 V or 0 V to 10 V) or
current (4 mA to 20 mA). A mA
process clamp meter can feed
in a signal to simulate a normal
input while the technician
observes the result.
Quick calibration
While not classified as loop
calibrators, today’s mA process
clamp meters boast accura-
cies of 0.2 %, and can be used
for quick calibration checks,
while cutting down on the
number of instruments needed.
For example, checking a pro-
cess transmitter on the bench
normally requires (aside from
a pump and separate pressure
standard) a loop power supply
and an instrument for read-
ing the transmitter’s 4-20 mA
output. But with today’s mA pro-
cess clamp meters it’s possible to
both power the transmitter and
read the output. “This tiny little
thing,” says Paul Jusak, Main-
tenance Engineer, Puget Sound
Energy, “allows you to do that
function without having to drag
out a separate power supply.”
Summary
Today’s mA process clamp
meters can save instrumenta-
tion and automation technicians
a great deal of time in trouble
shooting, because they can
replace a number of separate
instruments. The technician no
longer has to spend 15 min-
utes going back to the shop to
get an instrument, because the
one instrument he takes with
him will do all the necessary
functions. And, adds Jusak,
“instead of having two tools in
your pouch you now have one
tool in your pouch for doing all
4-20 mA loop calibrations and
troubleshooting. That to me is
pretty doggone convenient.”