Application Note

Application Note

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

Troubleshooting and the

need for speed

Instrument and automation

technicians are constantly chal-

lenged to keep instrumentation

loops and I/O working at peak

efficiency while using the least

possible time to do it.

When Fluke first released

the 771 mA Clamp Meter in

Better ways to

troubleshoot automation

and process control loops

2007, technicians found that

measuring loop current without

breaking the circuit saved a

great deal of time. Now, the new

772 and 773 models can save

even more time.

By incorporating the functions

of a loop calibrator, these

more advanced tools allow

technicians to troubleshoot on

the spot.

Tracing control loop

problems

Often the first indication of

a control loop problem comes

from the operator: “I think we

have a bad valve” or “this loop

isn’t responding the way it

used to.” In either case, it’s the

technician’s signal to begin

troubleshooting.

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Better ways to troubleshoot automation and process control loops Application Note Troubleshooting and the need for speed Instrument and automation technicians are constantly challenged to keep instrumentation loops and I/O working at peak efficiency while using the least possible time to do it. When Fluke first released the 771 mA Clamp Meter in 2007, technicians found that measuring loop current without breaking the circuit saved a great deal of time.
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The first step is to measure the 4-20 mA signal, either by breaking the loop connecting in series with a DMM, or by using a mA clamp meter like the Fluke 771 and verifying the loop current value. If the loop current measured is not as expected, there are three likely causes: broken/disconnected/shorted wires, a bad loop power supply, or faulty instrumentation. If no problem is found in the wires, use a DMM (or the 773 clamp meter) to check the loop power supply.
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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 movement. 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 similarly, by setting the meter at 20 mA, ~19.9 mA and ~20.