Testing 4 20mA

Testing and troubleshooting

4 mA to 20 mA control loops

Process Troubleshooting

Webinar Series

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

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

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.

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

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

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 ValveFinal 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

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

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

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

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

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

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

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

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.

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

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.

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

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 or Fluke 744)

Loop test output trim

Smart transmitter output

• 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

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 %

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 %

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

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4 mA to 20 mA control loops:

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