How to Calibrate 4–20 mA in Electro-Pneumatic Positioners?

In industrial processes, it is important not only for control valves to open and close, but also to be precisely positioned at a specific opening percentage. Electro-pneumatic positioners play a key role in this process by converting the electrical signal received from the control system into pneumatic movement, ensuring that the valve reaches the desired position.

The 4–20 mA signal is widely used in industrial automation systems to control valve position. However, for the positioner to operate correctly, the 4 mA and 20 mA values must be properly matched to the valve’s starting and ending positions.

Incorrect calibration can cause a discrepancy between the signal sent by the PLC or DCS system and the actual valve position. This can reduce the accuracy of process control and, in some applications, lead to problems such as hunting, excessive opening and closing, or unstable operation. UNOX control valves and electro-pneumatic positioner solutions are also used in such precision valve control applications.

What Is 4–20 mA?

4–20 mA is one of the standards used for analog signal transmission in industrial automation systems.

In general:

• 4 mA → 0% valve position
• 12 mA → 50% valve position
• 20 mA → 100% valve position

This scaling is typically applied as follows.

For example, if the control system is required to position the valve at 50% open, a control signal of approximately 12 mA can be sent to the positioner.

The key point here is that 4 mA is considered the zero value. This is known as a “live zero.” This allows certain fault conditions, such as a broken signal line, to be detected by distinguishing between 0 mA and 4 mA.

What Is an Electro-Pneumatic Positioner?

An electro-pneumatic positioner is a device that interprets the electrical signal received from the control system, supplies the necessary air pressure to the pneumatic actuator, and controls the valve position.

A simple system operates as follows:

PLC/DCS → 4–20 mA Signal → Positioner → Pneumatic Actuator → Control Valve

The positioner compares the commanded position with the valve’s actual position and makes the necessary corrections to ensure that the valve reaches the target position.

UNOX’s Rotary Electro-Pneumatic Positioner is used for position control of rotary valves, while the Linear Electro-Pneumatic Positioner is designed to control pneumatic actuators with linear motion.

Why Is 4–20 mA Calibration Important?

Improper calibration of the positioner can cause a discrepancy between the signal sent by the control system and the valve’s actual position.

For example, in the system:

4 mA = %0
20 mA = %100

Even if the valve is configured to fully close at 4 mA and fully open at 20 mA, if it does not fully close at 4 mA or fully open at 20 mA, the calibration should be checked.

Proper calibration;

• More precise control of the valve position,
• Improved synchronization between the PLC/DCS and the valve,
• Stable process control,
• Reduction of unnecessary valve movement,
• Improved control accuracy.

This helps ensure.

What Should Be Checked Before 4–20 mA Calibration?

Before starting the calibration process, it is not sufficient to focus solely on the electrical signal. All mechanical, pneumatic, and electrical connections should be checked.

Positioner and Actuator Compatibility

The positioner must be compatible with the pneumatic actuator and valve being used.

The actuator’s operating direction, stroke or rotation angle, and air connections should be checked.

Instrument Air

The compressed air supply to the positioner must be sufficient and stable.

Fluctuations in air pressure can make it difficult for the valve to remain at the desired position during calibration.

UNOX’s pneumatic actuator selection guide also states that operating pressure is one of the key parameters in actuator selection.

Mechanical Connections

The mechanical connections of the positioner to the valve or actuator should be checked.

In particular:

• Positioner mounting,
• Feedback mechanism,
• Actuator shaft,
• Valve stem,
• Linkage arms,
• Mounting screws.

should be checked.

If there is any play or misalignment in the mechanical connection, the electronic calibration may not produce accurate results.

Electrical Connections

The 4–20 mA signal cables should be checked to ensure they are connected to the correct terminals.

Additionally, it should be ensured that the signal source is correctly generated by the PLC, DCS, or calibrator.

How to Perform 4–20 mA Calibration?

The menu structure and calibration method of positioners may vary depending on the model. Therefore, the following procedure provides a general calibration workflow. The connection and calibration procedure specified in the manufacturer’s instructions for the positioner being used should be followed.

Step 1: Put the Valve in a Safe Condition

Process conditions should be assessed before starting the calibration procedure.

Especially in lines containing pressurized, hot, or hazardous fluids, all necessary safety procedures should be followed before performing the calibration.

It should be kept in mind that the valve will move during calibration.

Step 2: Check the Pneumatic Air Supply

The positioner must be supplied with the required instrument air.

In the air line:

• Adequate pressure,
• Clean air,
• Proper filtration,
• Stable pressure.

Ensure that these conditions are met.

Step 3: Apply the 4 mA Signal

A 4 mA signal is sent from the signal source to the positioner.

This value is generally considered the 0% command in the system, corresponding to the valve’s starting position.

he 4 mA input value is matched to the minimum position using the corresponding setting on the positioner.

For example:

4 mA → 0% valve opening

can be adjusted accordingly

Step 4: Check the Valve’s Zero Position

When a 4 mA signal is applied, the valve should be fully closed or at the defined minimum position, depending on the process design.

Here, attention should be paid to the valve type and control characteristic.

The assumption that “4 mA = valve closed” should not be made in every application. In some systems, reverse-acting operation may be preferred.

Step 5: Apply the 20 mA Signal

Then, a 20 mA signal is applied from the signal source.

The positioner’s maximum input value is matched to this signal.

In general practice:

20 mA → 100% valve opening

is set accordingly.

It is checked that the valve reaches its full stroke or full rotational movement.

Step 6: Test the Intermediate Values

Calibration at only the 4 mA and 20 mA points may not be sufficient.

It is also useful to check the intermediate values.

For example:

The valve position is observed at these values.

If the positioner and valve are mechanically suitable, the expected proportional relationship between the input signal and the valve position should be achieved.

Common Mistakes in 4–20 mA Calibration

Certain mistakes made during calibration can cause the positioner to operate incorrectly.

Incorrect Zero Adjustment

If the valve does not reach its actual starting position at 4 mA, the zero adjustment may not have been performed correctly.

Incorrect Definition of Maximum Stroke

If the valve does not reach the fully open position at 20 mA, the maximum stroke setting should be checked.

Incorrect Actuator Direction

If the positioner’s opening and closing direction does not match the actuator’s actual direction of movement, the response to the 4–20 mA signal may be reversed.

In this case, the direction or operating mode settings on the positioner should be checked.

Insufficient Air Pressure

If the air pressure is insufficient, the actuator may not be able to move the valve to the target position.

In this case, instead of repeating the electronic calibration, the pneumatic system should be checked first.

Mechanical Play and Friction

Even if the positioner receives the correct signal, friction and mechanical play in the valve stem, packing, actuator, or linkage mechanism can cause positioning errors.

In control valves, a sticking-friction problem known as stiction can also affect the stability of position control.

How Is Valve Position Calculated from a 4–20 mA Signal?

In a standard 4–20 mA system, the following formula can be used to approximately calculate the valve position:

Valve Position (%) = (Input Current − 4) / 16 × 100

For example, if the input signal is 12 mA:

(12 – 4) / 16 × 100 = %50

As a result, a 12 mA signal corresponds to approximately 50% valve position.

Similarly:

• 4 mA → %0
• 6 mA → %12,5
• 8 mA → %25
• 10 mA → %37,5
• 12 mA → %50
• 14 mA → %62,5
• 16 mA → %75
• 18 mA → %87,5
• 20 mA → %100

This calculation is valid for a linear signal-to-position relationship. If the valve characteristic is linear, equal-percentage, or another characteristic, the actual change in flow may not occur at the same rate.

What Should Be Checked After Calibration?

After calibration is completed, it is recommended to test the system not only at the two end points but also at various intermediate positions.

For example, 25%, 50%, and 75% command values can be applied to observe the valve movement.

At the same time, it should be checked whether the command value displayed on the PLC or DCS screen is consistent with the actual valve position in the field.

If significant deviations in valve position are observed despite the signal being correct in the control system, the issue may not be caused by calibration alone. The positioner settings, actuator, valve mechanism, air supply, and feedback mechanism should be evaluated together.

UNOX Electropneumatic Positioners

The UNOX product range includes Rotary Electropneumatic Positioners and Linear Electropneumatic Positioners.

Rotary positioners are used in rotary valve and actuator applications, while linear positioners are used in applications requiring linear movement. The product pages specify the positioners’ applications in industrial automation and precision valve positioning.

UNOX also offers pneumatic on-off and pneumatic proportional control valves in its product range. Therefore, evaluating the positioner, actuator, and control valve together is important for developing an automation solution suitable for the application.

Conclusion

In electropneumatic positioners, 4–20 mA calibration is an important adjustment that ensures the electrical signal from the control system is correctly matched to the valve position.

In general:

4 mA → Minimum position
12 mA → Approximately 50% position
20 mA → Maximum position

can be established in this way.

However, for proper calibration, adjusting only the 4 mA and 20 mA values is not sufficient. The pneumatic air pressure, actuator direction, valve mechanism, feedback linkage, and positioner settings should also be checked.

Especially in applications involving precise process control, selecting the correct positioner and performing proper calibration are critical to ensuring stable system operation.

For more information about UNOX electropneumatic positioners and actuated valve solutions, it is important to review the product technical specifications and determine the model best suited to the application.