Application Note

An ethanol production plant is
a storehouse of process control
technologies and a great venue
for illustrating the enduring
benefits of 4 mA to 20 mA
technology. Ask Bruce Yenzer,
an instrumentation technician
at the Siouxland Energy & Live-
stock Cooperative (Sioux Center,
IA). He “keeps the gears oiled”
in an ethanol production facil-
ity that manufactures 60 million
gallons of the alternative fuel
annually. Control devices at
the plant measure tempera-
ture, pressure, flow, pH, “and a
whole myriad of parameters,”
says Yenzer. And what all the
control devices have in common
is that they output a 4 mA to
20 mA signal—more or less the
industry standard of control
instrumentation. “I also have
a vast array of proportional
devices—valves—that are fed a
4 mA to 20 mA signal. 4 mA to
20 mA is our lifeblood.”
However, the simplicity of
the 4 mA to 20 mA current
loop masks the complexity of
the challenge to keep a plant
running optimally and reveals a
basic limitation of conventional
electronic measurement tools.
Let’s look deeper.
Measuring current with a
multimeter requires making a
connection within the circuit
under test, which often requires
powering down the circuit. Of
course, that could mean shut-
ting down power to a complex,
dynamic system in which
Field
Applications
Case
Study
Profile: Bruce Yenzer, instrumentation
technician, Siouxland Energy and
Livestock Cooperative
Tool: Fluke 771 Miliamp Process
Clamp Meter
Measurements: Troubleshooting
live 4-20 mA controls for temperature,
pressure, flow, and pH devices and
valves
performance is measured in
thousands of dollars of revenue
per hour. “In a plant like this,
where production is para-
mount—we’re online 24 hours
a day—closing a valve or shut-
ting down a device can have
far-reaching effects throughout
the process and the plant,” says
Yenzer. “If I were using a multi-
meter to troubleshoot a circuit, I
would have to break the circuit
and stop the process, which
would cause the valves to slam
shut and ‘deadhead’ the pump.
That could cause myriad prob-
lems. Taking my measurements
while things are running is the
best of all possible worlds.”
The other limitation of a
multimeter or ammeter in
Yenzer’s application? Insert-
ing the meter within the circuit
changes the resistance of the
circuit—even if slightly—and can
skew a current measurement.
Enter the clamp meter (a name
derived from “clamp-on amme-
ter”). Because a clamp meter
“clamps” around an individual
wire, no disruption to the circuit
is required.
“I use the Fluke 771 Milliamp
Clamp Meter almost daily. Used
to be, I had to break the circuit
and insert my meter, which
meant I had to shut down the
circuit. The 771 allows me to
take my measurement and
to perform calibration while
online, with no interruption of
the circuit whatsoever.”
Troubleshooting 4 mA
to 20 mA controls,
ethanol style
In this operation, downtime is
not an alternative
Application Note
From the Fluke Digital Library @ www.fluke.com/library