This webinar will explore: -What a control loop is and how it works. -The most common culprits that can negatively impact the performance of a loop – and how to avoid them. -Different methodologies and tools to calibrate and troubleshoot 4 to 20 mA control loops.

1. Testing and Troubleshooting 4
mA to 20 mA Control Loops
Process Troubleshooting
Webinar Series

2. What will be covered in this session
• What makes a 4 to 20 mA control loop “tick”
• What things can go wrong with a 4 to 20 mA loop?
• How do you troubleshoot? What tools are of most use?
• Measuring 4 to 20 mA signals and interpreting the measurements
– Multiple measurement methods
• Testing PLC, DCS and indicator inputs with 4 to 20 mA sourcing
tools
– Multiple methods and sourcing approaches
– Testing 1V to 5 V and 0 V to 10 V I/O
©2010 Fluke Corporation Troubleshooting 4 mA to 20 mA loops 2

3. What will be covered in this session
• Measuring loop power supplies, troubleshooting suspect power
supplies
– Testing and isolating a transmitter with substitution power supply
• Troubleshooting suspect transmitters
– mA simulate used for substitution testing
• Using a HART smart transmitter as a mA source
– Using the Loop Test HART command to test a loop
• Testing an I/P with 4 to 20 mA sourcing
• Testing a control valve with a 4 to 20 mA source
©2010 Fluke Corporation Troubleshooting 4 mA to 20 mA loops 3

4. 24 V loop
supply
+–
What makes a 4 to 20 mA control loop “tick”
Indicators/Controllers:
Interpret the 4 to 20 mA signal as the temperature
or pressure being measured in the process. Often
issue commands to a final control element such as
a valve to regulate the process temperature or
pressure to within acceptable limits
Inputs to these devices is frequently a
1 V to 5 V input signal rather than 4 to 20 mA
ZERO SPAN
2200 ºC
4 to 20 mA
2 Wire Transmitter
Sensor input
• Temperature
• Pressure
• Flow
• Frequency
• PH
Readout/Controller
DCS/PLC/Recorder
250 ohm input shunt 
4 to 20 mA signal 

24 V loop power supply:
Provides power for the loop. Transmitters
regulate the 4 to 20 mA signal in the
circuit drawing power from this supply.
Transmitters:
Convert the measured temperature or
pressure to a 4 to 20 mA signal.
Typically are passive devices and
draw their power from an external 24 V
loop power supply.

5. 2200 ºC
Example current loop
ZERO SPAN
4 to 20 mA
2 Wire Transmitter
Sensor input
• Temperature
• Pressure
• Flow
• Frequency
• PH
Readout/Controller
DCS/PLC/Recorder
250 ohm input shunt 
4 to 20 mA signal 

24 V loop
supply
+–
• 4 to 20 mA (dc) signal is proportional to sensor input or PV
• Series circuit dictates the current at one location must be
identical to other locations
• Big advantage sending in sending mA signals over long
distances compared to voltage or pressure signals

6. Temperature transmitters convert
measured temperature (PV) 4 to 20 mA signals
Measured PV:
The PV or primary/process
variable in this example is the
temperature of the process fluid
being measured by the
temperature transmitter.
Temperature transmitter:
Typically sense the temperature
of the process via either a
thermocouple or RTD sensor and
convert the measurement to a
4 to 20 mA signal. The
transmitter in this example has
an input span (range) of 0 °C
to 300 °C. The relationship of
the measured temperature and
mA signal is shown is the table.
Temperature
input
Current
output
Percent
of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %
Temperature transmitter

7. Temperature transmitters convert
measured temperature (PV) 4 to 20 mA signals
Measured PV:
The PV or primary/process
variable in this example is the
temperature being measured by
the temperature transmitter.
Temperature transmitter:
Typically sense the temperature
of the process via either a
thermocouple or RTD sensor and
convert the measurement to a
4 to 20 mA signal. The
transmitter in this example has
an input span (range) of 0 °C
to 300 °C. The relationship of
the measured temperature and
mA signal is shown is the table.
Temperature
input
Current
output
Percent
of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %

8. Temperature Transmitters convert
measured temperature (PV) 4 to 20 mA signals
Indicator/Controller:
In this example, the temperature
indicator/controller is programmed
to interpret a 4 mA signal as 0 °C
and a 20 mA signal as 300 °C.
Input/Output or I/O:
Refers to the input/output of the
control system or controller. In this
example the input to the controller
is the 4 to 20 mA signal. The
output from the controller is the
signal that controls the control
valve.
Flow Control Valve
Final control element. Opens and
closes based on commands from
the controller to increase or
decrease gas supplied to the
burner as required to maintain
temperature at the setpoint value.
Temperature
input
Current
output
Percent
of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %
Temperature transmitter

9. Pressure Transmitters convert measured
pressure (PV) to 4 to 20 mA signals
Measured PV:
The PV or primary/process variable in this
example is the pressure being measured by the
pressure transmitter.
Pressure Transmitter:
Senses the pressure of the process directly and
converts the measurement to a 4 to 20 mA
signal. The transmitter in this example has an
input span (range) of 0 psi to 100 psi. The
relationship of the measured pressure and mA
signal is shown is the table.
Indicators/Controller:
In this example, the pressure indicator/controller
is programmed to interpret a 4 mA signal as 0
psi and a 20 mA signal as 100 psi.
Pressure relief valve:
Final control element. In this example if the
measured pressure is too high, the controller
instructs the valve to open to reduce the
pressure in the vessel.
Pressure
input
Current
output
Percent
of span
0 psi 4 mA 0 %
25 psi 8 mA 25 %
50 psi 12 mA 50 %
75 psi 16 mA 75 %
100 psi 20 mA 100 %
Pressure transmitter

10. What can go wrong with a 4 to 20 mA loop?
Pressure
input
Current
output
Percent
of span
0 psi 4 mA 0 %
25 psi 8 mA 25 %
50 psi 12 mA 50 %
75 psi 16 mA 75 %
100 psi 20 mA 100 %
Pressure transmitter
Wiring problems:
Bad terminations,
compromised insulation,
corrosion and contami-
nation can cause wiring
to negatively impact 4 to
20 mA loop operation.
24V loop
power supplies:
Noisy, defective or
overloaded power
supplies can cause
erratic mA loop
operation or failures.
Bad I/O into
the controller:
If the mA signal is correct
and the controller does
not interpret the mA signal
correctly the control of
the process is lost.
Temperature input Current output Percent of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %
Temperature transmitter

11. What can go wrong with a 4 to 20 mA loop
Pressure
input
Current
output
Percent
of span
0 psi 4 mA 0 %
25 psi 8 mA 25 %
50 psi 12 mA 50 %
75 psi 16 mA 75 %
100 psi 20 mA 100 %
Pressure transmitter
Bad transmitter:
If the transmitter does
not change the mA
signal to correctly to
respond to the
measured PV the
control system will not
correct to adjust the
PV correctly.
Bad sensor or
clogged capillary:
If the temperature
sensor is defective the
transmitter cannot
sense the temperature.
In a pressure
transmitter if the
connection to the
process is clogged the
transmitter cannot
measure the pressure
accurately.
Temperature input Current output Percent of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %
Temperature transmitter

12. How do you troubleshoot?
What tools are of the most use?
What can be
measured or
sourced
Measurement and
sourcing tool
What it tells
the technician
Measure 4 mA to 20 mA
signals
DMM. Loop calibrator, mA clamp,
ProcessMeter
If the measured mA value is the
expected value
Source 4 mA to 20 mA
signals
Loop calibrator, mA clamp,
ProcessMeter
If the I/O or other mA input device is
working correctly
Simulate 4 mA to 20 mA
signals
Loop calibrator, Fluke 772 or
773 mA Clamp, ProcessMeter
If the power supply, wiring and I/O is
working correctly, perform transmitter
substitution test
Measure 24 V loop
voltage
DMM, Loop calibrator,
Fluke 773 mA Clamp, ProcessMeter
If the full 24V supply available, if it is
defective or being loaded down?
Supply 24 V loop
voltage
Loop calibrator, mA clamp,
Fluke 789 ProcessMeter
If a substitution test for installed supply
fixes the problem
Source 0 V to 10 V,
1 V to 5 V
Loop calibrator with voltage source (715)
or specialized mA Clamp (773)
If the I/O or other voltage input device is
working correctly
Continuity
measurements
DMM, ProcessMeter, some multifunction
process calibrators
Find open circuits, bad terminations,
resistive connections and mis-wires

13. Measure the 4 mA to 20 mA signal
In series,
“Break the loop”
Temperature input Current output Percent of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %
Temperature transmitter
Pressure
input
Current
output
Percent
of span
0 psi 4 mA 0 %
25 psi 8 mA 25 %
50 psi 12 mA 50 %
75 psi 16 mA 75 %
100 psi 20 mA 100 %
Pressure transmitter
Compare
measured mA
signal to the
expected
value on the
display

14. Measure the 4 mA to 20 mA signal;
Don’t break the loop
Don’t “Break
the loop”
Temperature input Current output Percent of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %
Temperature transmitter
Pressure
input
Current
output
Percent
of span
0 psi 4 mA 0 %
25 psi 8 mA 25 %
50 psi 12 mA 50 %
75 psi 16 mA 75 %
100 psi 20 mA 100 %
Pressure transmitter
Compare
measured mA
signal to the
expected
value on the
display

15. Source a 4 mA to 20 mA signal
Temperature input Current output Percent of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %
Temperature transmitter
Pressure
input
Current
output
Percent
of span
0 psi 4 mA 0 %
25 psi 8 mA 25 %
50 psi 12 mA 50 %
75 psi 16 mA 75 %
100 psi 20 mA 100 %
Pressure transmitter
Tests the
indicator,
controller or
PLC/Control
system I/O
directly. Verify
the value on
the display

16. Simulate a 4 mA to 20 mA signal
Simulate a
transmitter in a
loop, regulate
current
Temperature input Current output Percent of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %
Temperature transmitter
Pressure
input
Current
output
Percent
of span
0 psi 4 mA 0 %
25 psi 8 mA 25 %
50 psi 12 mA 50 %
75 psi 16 mA 75 %
100 psi 20 mA 100 %
Pressure transmitter
Tests the wiring,
power supply,
indicator,
controller or
PLC/Control
system I/O.
Verify the value
on the display.
Perform a
transmitter
substitution test.

17. Measure 24V loop power
Temperature input Current output Percent of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %
Temperature transmitter
Pressure
input
Current
output
Percent
of span
0 psi 4 mA 0 %
25 psi 8 mA 25 %
50 psi 12 mA 50 %
75 psi 16 mA 75 %
100 psi 20 mA 100 %
Pressure transmitter
Is the full 24 V
available or
is the power
supply being
loaded down

18. Test with external 24 V loop power supply
Temperature input Current output Percent of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %
Temperature transmitter
Pressure
input
Current
output
Percent
of span
0 psi 4 mA 0 %
25 psi 8 mA 25 %
50 psi 12 mA 50 %
75 psi 16 mA 75 %
100 psi 20 mA 100 %
Pressure transmitter
Supply 24 V
loop power,
simultaneously
measure
mA signal.
Substitution
test for the
power supply.

19. Using a HART smart transmitter as a mA source
• Using loop test, the
Fluke 744 issues the mA
output command to the
transmitter over the
HART cable
• Transmitter sources a
mA signal into the I/O
• Verify correct indication
on the display
• Compare commanded
output value to mA
signal read by Fluke 754
• Tests transmitter
output, power supply,
wiring and I/O
HART
cable
Temperature input Current output Percent of span
0 °C 4 mA 0 %
75 °C 8 mA 25 %
150 °C 12 mA 50 %
225 °C 16 mA 75 %
300 °C 20 mA 100 %
Temperature transmitter
Instrument
Output
4 to 20 mA
output to
PLC or DCS
PVAO command
(from communicator, Fluke 744 or 754)
Loop test output trim
Smart transmitter output

20. • The input to many
control systems and
PLCs is a voltage signal
• 1 V to 5 V is most
common as 4 to 20 mA
through 250 ohms is
1 V to 5 V
• Some low power
transmitters have
1 V to 5 V outputs
• Many older chart
recorders and HVAC
systems have 1 V to 5 V
and 0 V to 10 V I/O
2200 ºC
Readout / Controller
DCS / PLC / Recorder
Troubleshooting, sourcing
1 V to 5 V and 0 V to 10 V
0.004 x 250 = 1.0 V
0.020 x 250 = 5.0 V
Ohms law

21. Testing I to P devices
I to P converts a 4 mA to 20 mA signal to a 3 psi to 15 psi pneumatic signal
•Often used with control valves
•Used as a bridge between 4 mA to 20 mA loop and 3 psi to 15 psi pneumatic technology
•Typically operate from a 20 psi or greater pressure supply
•Source 4 mA to 20 mA, verify/measure correct pressure output
4-20 mA
current
input
Pressure output
Supply
pressure
~20 PSI
Current input Pressure output Percent of span
4 mA 3 psi 0 %
8 mA 6 psi 25 %
12 mA 9 psi 50 %
16 mA 12 psi 75 %
20 mA 15 psi 100 %

22. Testing control valves
• Flow control, final control element
• 4 mA to 20 mA or 3 psi to 15 psi input
• Normally closed or open
–Normally closed fails closed with loss of power
–Normally open fails open with loss of power
• Apply a 4 mA to 20 mA signal and check for operation
per the applicable table below
Normally Closed Normally Open
Closed
ClosedOpen
Open
Closed Open
Pressure
input
Current
output
Percent
of travel
3 psi 4 mA 0 %
6 psi 8 mA 25 %
9 psi 12 mA 50 %
12 psi 16 mA 75 %
15 psi 20 mA 100 %
Pressure
input
Current
output
Percent
of travel
3 psi 4 mA 0 %
6 psi 8 mA 25 %
9 psi 12 mA 50 %
12 psi 16 mA 75 %
15 psi 20 mA 100 %

23. Testing HART control valves
• Testing HART valves require simultaneous HART
feedback, while energizing the valve and monitoring
travel.
• Underperforming valves can disrupt a process
unexpectedly, leading to expensive unplanned downtime.
• Even valves with no visible issues, may be contributing to
lowered quality and higher cost.
• Simple tools exist for quick verification of valve health to
avoid some common issues.
• Often control valves are critical components in safety &
shutdown systems
What can go wrong with a
Control Valve?
Mechanical Issues: Electrical Issues: Equipment Issues:
Stiction
Hysteresis
Dead Band
Stick-Slip
Over/Under Travel
PV Translation
Control Power
Loss
Out of Tolerance
Positioner
Communication
Transmitter
Malfunction
Under/Over-Sized
Valves
Nonlinearity
Applications

24. Review: Current loop devices and test methods
• Transmitters
–Apply input stimulus (temperature, pressure, etc)
–Measure for correct 4 to 20 mA output
–Use mA simulate for substitution testing
• 24 V loop power supplies
–Measure for correct voltage, substitution test to verify
• I to P, 4 mA to 20 mA input, 3 psi to 15 psi output
–Source 4 to 20 mA, verify/measure correct pressure output
• Control valves
–Source 4 mA to 20 mA, verify position indication
–Use mA signal ramping to test for smooth operation
• PLC, DCS, indicators, controllers, flow computers, and
chart recorders analog inputs
–Source 4 mA to 20 mA into the input and verify correct indication

25. Special Offers
• For a limited time only, Save $500 on Fluke 710 mA Loop Calibrator

26. Questions or Comments?
Email Nicole VanWert-Quinzi nvanwert@Transcat.com
Transcat: 800-828-1470
www.Transcat.com
For related product information, go to:
www.transcat.com/brand/fluke-store