How to Interpolate a 4-20 mA Signal to Any Other Scale

In industrial automation and process control, the 4-20 mA current loop is a widely used standard for transmitting sensor data. The 4 mA typically represents the minimum value (e.g., 0% or a lower range value), while 20 mA represents the maximum value (e.g., 100% or an upper range value). However, in many applications, you may need to convert this 4-20 mA signal into a different scale, such as 0-100 or 25-475, for display, control, or further processing.

In this blog post, we’ll walk you through the process of interpolating a 4-20 mA signal to any desired scale using a simple mathematical formula.

Understanding the 4-20 mA Signal

The 4-20 mA signal is linear, meaning it changes proportionally with the measured parameter. For example:

• 4 mA corresponds to the minimum value of the measured parameter.

• 20 mA corresponds to the maximum value of the measured parameter.

To convert this signal into a different scale, we need to perform a linear interpolation.

The Interpolation Formula

The general formula to convert a 4-20 mA signal to any desired scale is:

4-20 mA to Scaled Span Formula

Where:

• XX = The current value in mA (between 4 and 20).

• YY = The corresponding value in the desired scale.

• YminYmin​ = The minimum value of the desired scale.

• YmaxYmax​ = The maximum value of the desired scale.

Example 1: Converting 4-20 mA to 0-100

Let’s say you want to convert a 4-20 mA signal to a 0-100 scale (e.g., for a percentage display).

Using the formula:

0-100 (units) Formula

Simplify:

0-100 (units) Simplified

Example Calculation: If the current X=12 mA X=12mA:

Calculating value at 12mA in the 0-100 (unit) forumla

So, 12 mA corresponds to 50 on the 0-100 scale.

Example 2: Converting 4-20 mA to 25-475

Now, let’s convert a 4-20 mA signal to a 25-475 scale.

Using the formula:

Simplify:

Example Calculation: If the current X=16 mA X=16mA:

So, 16 mA corresponds to 362.5 on the 25-475 scale.

General Steps for Interpolation

1. Identify the Input Range: The input is always 4-20 mA.

2. Define the Desired Output Range: Determine the minimum (YminYmin​) and maximum (YmaxYmax​) values of the new scale.

3. Apply the Formula: Plug the values into the interpolation formula to calculate the corresponding output.

4. Validate the Results: Test the formula with known values (e.g., 4 mA and 20 mA) to ensure accuracy.

Practical Applications

• Process Control: Convert 4-20 mA signals to engineering units (e.g., temperature, pressure, or flow rate).

• Display Scaling: Show sensor data on a display or HMI in a user-friendly format.

• Data Logging: Store scaled values in a database or SCADA system for analysis.

Conclusion

Interpolating a 4-20 mA signal to any desired scale is a straightforward process once you understand the underlying linear relationship. By using the interpolation formula, you can easily convert the signal into meaningful values for your specific application. Whether you’re working with a 0-100 scale, a 25-475 scale, or any other range, this method ensures accurate and reliable results.

If you have any questions or need further assistance, feel free to leave a comment below!