Application Note

2 Fluke Corporation Troubleshooting process loops in potentially explosive atmospheres
With the calibrator in control of
the current, you can accurately
set the current between 4 and
20 mA, controlling the loop and
the device connected to it. This
makes it possible to test valve
positioners (see below), mechani-
cal position indicators, flow
indicators and mA signal condi-
tioners.
For testing devices requir-
ing a voltage input, like a signal
conditioner or PLC, you can use
the source mode of a calibrator
in conjunction with a precision
resistor to generate accurate volt-
ages.
Simply taking a precision
resistor and placing it across the
output leads of a loop calibra-
tor creates a voltage across the
resistor that is directly controlled
by the source current from the
calibrator. For example, placing a
250 Ω resistor across the source
output jacks and driving it with a
4 to 20 mA current will produce
1 to 5 V across the resistor. Place
this voltage on the input of a sig-
nal conditioner and we have cre-
ated a test system to set linearity
as well as the zero and span
points of the conditioner. With a
DMM or ProcessMeter™ instru-
ment like the Fluke 789,
you can
measure the output of the signal
conditioner and ensure the proper
levels are coming out of the con-
ditioner with a corresponding
input voltage.
Testing valve positioners
Electronic valve positioners
should receive periodic in-field
calibrations as part of preventa-
tive maintenance programs. Fluke
loop calibrators are the ideal pro-
cess tool for these checks. Valve
positioners vary in design and
valve type and should be cali-
brated using specific instructions
from the individual manufacturer.
Quick operational checks
can be performed using a field
calibrator as a signal source
while observing the valve stem
position, mechanical position
indicators, or flow indicators as
input changes are made. Fluke
loop calibrators provide a conve-
nient source for simulating
the controller output to a valve
positioner.
The following example (using
a Fluke 707Ex calibrator) shows
a general method for an in-field
operational check of a valve fit-
ted with an electronic valve
positioner. These methods may
be adapted to various types of
valves, however manufacturer’s
specific instructions should
always be consulted for proper
and appropriate techniques. In
the following example, valve
operation and movement is
checked either by feel or by
observing valve stem movement.
Step 1: Basic set-up. Setting
the Fluke 707Ex Loop
Calibrator current output.
Place the calibrator in the 4-20
mA output current mode. Connect
the 707Ex to the input terminals
of the valve positioner.
Step 2: Zero adjustment
Set the 707Ex to an output of
4 mA and allow some time for
valve stem movement to stabilize.
Quickly decrease the current from
4 mA to 3.9 mA by depres-sing
and turning the vernier knob
in a counter-clockwise direc-
tion. You can operate the 707Ex
with one hand 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 set-
tings by using the Zero adjust-
ment on the positioner.
Increase and decrease current
from 4 mA to 4.1 mA using the
vernier knob in the depressed
position. Insure that the valve
stem just begins movement
above the 4.1 mA setting and
fully closed at 4 mA.
Step 3: Span (full open) posi-
tion check
Using the 25 % button, step the
valve input to 20 mA and allow
the valve to stabilize. Step the
input to 24 mA while watching
or feeling for movement of the
valve stem. Minimize this move-
ment using the Span adjustment
on the valve positioner.
Using the vernier knob in the
depressed position, adjust current
up and down between 20.10 mA
and 19.9 mA. There should be
no movement of the valve stem
above 20 mA and a slight move-
ment below 20 mA.
Step 4: Check zero and span
again
Many positioners have interactive
Zero and Span controls. This step
will help ensure proper valve
position adjustment.
Repeat Step 2 and Step 3.
Step 5: Linearity check
For valves with linear action, lin-
earity can be checked by setting
the 707Ex to 4 mA and stepping
current to 12 mA (50 %) while
observing valve travel. If your
valve is of a nonlinear type, refer
to the valve manual for proper
operational checks.
Step 6: Stroking the valve
Checking for smooth valve opera-
tion is easy to accomplish using
the Slow Ramp function of the
707Ex.
•
Set the calibrator to mA source
mode and select the Slow
Ramp function ( ) by depress-
ing the 25 % and 0-100 %
buttons simultaneously.
•
Allow the calibrator to ramp
through several cycles while
watching or feeling for any
abnormal operation of the
valve such as sticking in one
position momentarily or erratic
movement.
Summary
Calibrating process loops is an
essential part of regular plant
maintenance made all the more
difficult in potentially explosive
environments. Using intrinsically
safe tools and the proper proce-
dures to safeguard the working
area minimizes the risks involved.
Fluke Corporation
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©2003-2010 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 7/2010 2098403B A-EN-N
Modification of this document is not permitted
without written permission from Fluke Corporation.
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