Process Troubleshooting Webinar Series
Testing and troubleshooting 4 mA to 20 mA control loops
What will be covered in this session •
What What make makes s a 4 to to 20 20 mA mA cont contro roll loo loop p “ti “tick ck””
•
What things can go wrong with a 4 to 20 mA loop?
•
How do you troubleshoot? What tools are of most use?
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Measuring 4 to 20 mA signals and interpreting the measurements – Multiple measurement methods
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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
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
What makes a 4 to 20 mA control loop “tick” Transmitters:
Indicators/Controllers:
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.
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
Sensor input •
Temperature
•
Pressure
•
Flow
•
Frequency
•
PH
4 to 20 mA signal
22 00 ºC
250 ohm input shunt
–
+
4 to 20 mA
ZERO SPAN 24 V loop supply
2 Wire Transmitter
Readout/Controller DCS/PLC/Recorder
24 V loop power supply: Provides power for the loop. Transmitters regulate the 4 to 20 mA signal in the circuit drawing power fro m this supply.
Example current loop •
•
•
4 to 20 mA (dc) signal is proportional to sensor input or
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
Sensor input •
Temperature
•
Pressure
•
Flow
•
Frequency
•
PH
PV
4 to 20 mA signal
22 00 ºC
250 ohm input shunt
–
+
4 to 20 mA
ZERO SPAN 24 V loop supply
2 Wire Transmitter
Readout/Controller DCS/PLC/Recorder
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 transmitter 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 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 %
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 transmitter 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 %
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 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 %
What can go wrong with a 4 to 20 mA loop? Temperature transmitter
Wiring problems: Bad terminations, compromised insulation, corrosion and contamination 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.
Pressure 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 %
Te mpe ra tur e i nput
C ur re nt out put
P er ce nt of s pa n
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 %
What can go wrong with a 4 to 20 mA loop Temperature 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.
Pressure 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 %
Te mpe ra tur e i nput
C ur re nt out put
P er ce nt of s pa n
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 %
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
Measure the 4 mA to 20 mA signal Temperature transmitter
In series, “Break the loop”
Compare measured mA signal to the expected value on the display
Pressure 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 %
Te mpe ra tur e i nput
C ur re nt out put
P er ce nt of s pa n
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 %
Measure the 4 mA to 20 mA signal; Don’t break the loop Temperature transmitter
Don’t “Break the loop”
Compare measured mA signal to the expected value on the display
Pressure 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 %
Te mpe ra tur e i nput
C ur re nt out put
P er ce nt of s pa n
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 %
Source a 4 mA to 20 mA signal Temperature transmitter
Tests the indicator, controller or PLC/Control system I/O directly. Verify the value on the display
Pressure 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 %
Te mpe ra tur e i nput
C ur re nt out put
P er ce nt of s pa n
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 %
Simulate a 4 mA to 20 mA signal Temperature transmitter
Simulate a transmitter in a loop, regulate current
Tests the wiring, power supply, indicator, controller or PLC/Control system I/O. Verify the value on the display. Perform a transmitter substitution test.
Pressure 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 %
Te mpe ra tur e i nput
C ur re nt out put
P er ce nt of s pa n
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 %
Measure 24V loop power Temperature transmitter
Is the full 24 V available or is the power supply being loaded down
Pressure 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 %
Te mpe ra tur e i nput
C ur re nt out put
P er ce nt of s pa n
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 %
Test with external 24 V loop power supply Temperature transmitter
Supply 24 V loop power, simultaneously measure mA signal. Substitution test for the power supply.
Pressure 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 %
Te mpe ra tur e i nput
C ur re nt out put
P er ce nt of s pa n
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 %
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 744 • Tests transmitter output, power supply, wiring and I/O
Smart transmitter output Loop test output trim Instrument Output
4 to 20 mA output to PLC or DCS
PVAO command (from communicator or Fluke 744)
HART cable
Temperature transmitter Te mpe ra tur e i nput
C ur re nt out put
P er ce nt of s pa n
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 %
Troubleshooting, sourcing 1 V to 5 V and 0 V to 10 V •
•
•
•
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
Ohms law
0.004 x 250 = 1.0 V 0.020 x 250 = 5.0 V
22 00 ºC
Many older chart recorders and HVAC systems have 1 V to 5 V and 0 V to 10 V I/O Readout / Controller DCS / PLC / Recorder
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 Pressure output
4-20 mA current input
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 %
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
Pressure input
Current output
Percent of travel
Pressure input
Current output
Percent of travel
3 psi
4 mA
0%
3 psi
4 mA
0%
6 psi
8 mA
25 %
6 psi
8 mA
25 %
9 psi
12 mA
50 %
9 psi
12 mA
50 %
12 psi
16 mA
75 %
12 psi
16 mA
75 %
15 psi
20 mA
100 %
15 psi
20 mA
100 %
Closed
Closed
Open
Open Open
Closed
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
Next steps • Visit us to get more information: – I want a demonstration – Notify me about upcoming Process Troubleshooting Presentations • Download these application notes on 4 mA to 20 mA control loops: • Troubleshooting process loops » • In field valve positioner checks with the Fluke-789 ProcessMeter » • Better ways to troubleshoot automation and process control loops » – See mA process videos
