Application Note

2 Fluke Corporation Troubleshooting 4 mA to 20 mA controls, ethanol style
Going with the flow
“Ethanol is like other chemi-
cal production, says Yenzer.
“Using corn fermentation and
extracting the alcohol involves,
quite literally, thousands of
transmitters—constantly taking
measurements and reporting
back to the DCS.” The major-
ity of SELC’s operations are
controlled from computers that
are part of a DCS. The system,
he says, can often run auto-
mated, with occasional manual
intervention from facility opera-
tors.
“We know what kind of flow
we should be getting through
a pipe. If an operator notices
that the flow is wrong, but the
DCS says the valve is adjusted
correctly to 50 % open, I can
place the 771 on the signal
lines coming in, verify that they
are correct, and use that data to
verify that the valve is open to
the proper position. Typically,
the signal is not getting to the
valve. We are in a nasty envi-
ronment for electronics—high
moisture and high heat—and
that can cause malfunctions in
the wiring, in the computers,
the terminals, or the connection
to the device.”
Or, he says, the valve itself
might require an adjustment—
essentially an impromptu cali-
bration. The 4 mA to 20 mA
current-to-pressure transducer
at the device would output a
3 psi to 15 psi pressure signal
that actually controls the valve.
“I could use this signal to get a
pneumatic response to control
the valve,” says Yenzer. “Less
frequently, I could have a fuse
blown, could have a bad output
on my computer card, or could
have a bad rack in the card
cage.”
In the loop
The 4 mA to 20 mA current
loop is a common method of
transmitting sensor information
in many industrial process-
monitoring applications—
typically in systems monitoring
pressure, temperature, pH, flow,
or other physical factors. These
systems employ the famil-
iar two-wire, 4 mA to 20 mA
current loop, in which a single
twisted-pair cable supplies
power to a module and also
carries the output signal.
The loop’s operation is
straightforward: a sensor’s
output voltage is first converted
to a proportional current, with
4 mA normally representing
the sensor’s zero-level output
and 20 mA representing the
sensor’s full-scale output. A
reading of 20 mA means that a
direct-acting valve, for example,
is 100 % open, and a read-
ing of 4 mA means that it is
closed (and the opposite for a
reverse-acting valve). Read-
ings between the maximum
and minimum values indicate
that the circuit is controlling the
valve.
Transmitting sensor infor-
mation via a current loop is
particularly useful when the
signal has to be sent over long
distances—1,000 feet typically,
or more. The use of basic two-
wire technology makes the
installation both inexpensive
and simple to wire, maintain
and troubleshoot.
Key among the advantages
of 4 mA to 20 mA current loop
technology is that the accuracy
of the signal is not affected by
voltage drop in the intercon-
necting wiring, and that the
loop can supply operating
power to the device. Even if
there is significant electri-
cal resistance in the line, the
current loop transmitter will
maintain the proper current, up
to its maximum voltage capabil-
ity.
Back at the plant
“Our team considers this clamp
meter indispensable,” says
Yenzer. “Once I expose a posi-
tive or negative lead, all I have
to do is clamp on the circuit
and take a measurement.” The
771 display, he says, provides
an Open or Close indication—
meaning that, even if the
mechanic is not able to diag-
nose the reading, the tool will
tell him, for example, that 8 mA
= 25 % open. If the reading
doesn’t agree with the DCS, it
means the mechanic needs to
make an adjustment.
“After I received my Fluke
771, I ordered another one.
We’ve cut troubleshooting and
device calibration or alignment
by 70 % to 80 %. For us, the
biggest advantage of a clamp
meter is that it allows analysis
without interrupting the circuit.
And honestly, I have people
with little or no electrical back-
ground using the Fluke 771. It
couldn’t be easier.”
Fluke Corporation
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
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Web access: http://www.fluke.com
©2008 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 7/2008 3358397 A-EN-N Rev A
Modification of this document is not permitted
without written permission from Fluke Corporation.
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Checking a Fieldvue control on a Fischer valve for a condensate
collection system.