4–20 MA Current Loop Communications

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Imagine this scenario: There is a flow meter with 4–20 mA output setup. The switch is flipped, thereby turning it on. Then it is noted that the readings are off, and the required output is not shown. The meter is fiddled with but still the readings are not shown. What is to be done now? This meter should be repaired so that the application is up and running.

The questions that need to be answered are as follows: What is to be done? Why is this even happening?

Several times the question raised is how to get accurate 4–20 mA output readings for 4–20 mA output devices. What is most helpful for customers is an understanding of the basics of 4–20 mA current loops and how it integrates into their process.

This article describes the 4–20 mA communications protocol, step-by-step, which will dispel the confusion and complications around the protocol and help customers get set up and running.

There is a need to determine the flow rate (or temperature, or pressure) without observing the display of the meter. The following are a few ways to do this:

1. Utilizing the RS-232 serial commands

2. Utilizing the 4–20 mA support within the meter

3. Utilizing HART, Modbus, or other supported digital communications protocols

A 4–20 mA current loop is a means to pass a “value” like flow using an established electrical current (the loop) as the carrier of this information.

The following are the five discrete parts to the “loop” (see the image represented above):

1. Sensor: Provides the “value”

2. Transmitter: Takes the “value” and converts it to a meaningful 4–20 mA value

3. Power Source: The physical [DC] power that the transmitter uses

4. Loop: The physical “loop” (such as wires)

5. Receiver: Reads the 4–20 mA value from the “loop” to determine the “value”

Typically, the 4 mA depicts the minimum/low value (like 0 for flow); and the 20 mA depicts the maximum/high value, or full scale, of the value that is sent. In the example provided in the article, this value is for “flow.”

Most people tend to overthink the math for 4–20 mA in their application. This is similar to using calculus to solve a simple math problem. It is not overly complicated once the values used in the equation are understood.

As described above, the 20 mA represents the full scale, and the 4 mA represents “zero” [flow].

So, if the flow setup is 0 to 1000 SLPH, the 4 mA means “0” flow, and the 20 mA means “1000”.

The flow range is represented using the following notations:

mAValueLow = 0 (value that represents “low”)

mAValueHigh = 1000 (value that represents “high “or Fullscale)

Now if 500 is flowed, the following notation is used:

Since the users know that the value they want to pass on the 4–20 mA loop is really a ratio between mALow to mAHigh, the following formula is applied to determine the correct milliamps (mA) that are needed.

mA_Value = ((mAHigh – mALow) * ((currentValue – mAValueLow)/(mAValueHigh – mAValueLow))) + mALow

mA_Value = ((20 – 4) * ((500 – 0)/(1000 – 0))) + 4

So, in the current 4–20 mA example, flowing 500 is repesented as 12 mA on the 4–20 mA loop. If it was zero flowing, (currentValue=0), it would be calculated as: