How to Measure a 4-20mA Signal Using a Multimeter

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Before the era of electronic circuitry, the entire process was controlled mechanically. Facilities utilized pneumatic control signals where different pressures of compressed air powered controllers. The 3-15psi model became the standard. It is simpler to differentiate a live zero (3psi) signal from a failure in the system (0psi).

In the 1950s, current input became the most preferred and more practical process control signal as electronic systems became less costly. The 4-20mA range later became the norm for similar reasons as 3-15psi did. 

Introducing the 4-20mA Current Loop: What is It? 

The 4-20mA current loop is considered a very highly sought and powerful sensor signaling standard. Did you know that current loops are suitable for data transmission due to their natural insensitivity to electrical noise? 

In a 4-20mA loop, every signaling current flows through every device. All devices in the loop drop voltage because of the signal current that’s flowing through them. Hence, the signaling current isn’t influenced by such voltage plunges as long as the power voltage supply exceeds the sum of the voltage drops within the loop at the maximum signal current of 20mA.  

The current flows through the loop, passing through every load. The voltage that drops at every load could be calculated from Ohm’s law: the voltage decline V1 across R1 is V1 = I X R1.

You see, each component in the loop either has a voltage drop or provides voltage. Nonetheless, the current (I) is the same everywhere within the loop. That’s a crucial principle of the 4-20mA loop. 

Current is identical in every place throughout the loop. That’s the reason why using current as a way of conveying process information is extremely dependable. 

Should You Use a 4-20mA Signal? 

Keep in mind that the 4-20mA current loop is typically sending sensor information in most industrial process-monitoring applications. A sensor is a device utilized to measure physical parameters such as:

  • liquid flow rates
  • speed
  • pressure
  • temperature
  • and more.

Sending sensor information through a current loop is specifically practical when the information has to be delivered to a remote location over extended distances (such as 1,000 feet or more). The good thing about the loop is that its operation is very straightforward. 

The sensor’s output voltage is converted to a proportional current and 4mA, typically signifying the sensor’s zero-level output. Meanwhile, 20mA represents the full-scale output of the sensor.

A receiver at the remote end then converts the 4-20mA current back into the voltage. In turn, that can be further processed through a display module or a computer. 

Pros and Cons of Using 4-20mA Current Loop

You will also find pros and cons to using a 4-20mA current loop. The benefits are that the 4-20mA current loop is a very dominant industry standard. It is better for long distances, easier to connect and configure, utilizes less wiring than other similar systems, and is very straightforward to troubleshoot typical issues such as broken wires.

Like most things in life, it also has its share of disadvantages. Some of this include using a 4-20mA for an active sensor input loop indicating the current loop could only send one process signal. That needs numerous loops where many process variables should be transmitted.

Using numerous loops could result in ground loop issues, especially if the individual loops aren’t isolated.

Another benefit of a 4-20mA loop is safety. Shock is not a risk, even though the power supply is 24 since the current is low (P = VI). Moreover, the 4-20mA current loop is inherently safe for risky areas, including risky levels of vapor dust. That’s because the low power consumption doesn’t trigger combustion if standard operating or fault conditions are in place.

Indeed, the 4-20mA input loop distance of 1,000 meters won’t be as remarkable as in the 1960s, especially if their use of wireless sensors remains to grow. Nevertheless, the old-fashioned 4-20mA sensor loop is naturally immune to hacking through the internet, not to mention it will remain to be more dependable for real-time performance compared to wireless sensors. 

How Do You Measure a 4-20mA Signal Using a Digital Multimeter?

One of the common problems with the 4-20mA signal wire is that the pressure signal isn’t collected, and the value is abnormal. You will find many reasons for that issue, like wiring errors, equipment concerns, or product quality problems. 

To solve that issue, please make sure the transmitter works after power-on. Before you continue measuring, make sure you check the wire first. Check the wiring according to the transmitter housing or manual’s label. 

Did you find the wire is reversed? There’s no need for you to worry about the damage problem. The traditional products feature an anti-reverse design. You can rewire to fix the issue. 

  1. Measure the voltage

Check the wire first. After checking it, adjust your digital multimeter to the voltage to measure the voltage between the negative and positive output of the transmitter. 

The voltage value must be within the transmitter supply voltage range. That could be seen in the product label or your instruction manual. 

You can check if your equipment is normal if the measured voltage value is wrong.

  1. Measure the loop current

Remove the transmitter “output positive” and adjust your digital multimeter to the current position series into your current loop, testing the current loop value. Normally, the non-pressured state is approximately 4mA. 

When doing this test, it would help if you paid attention to the setting of your multimeter’s voltage and current position. 

Final Thoughts 

You are now done doing the steps above. Your digital multimeter normally works for testing. However, what about if the current loop current value is still not about 4mA? Well, it can be determined as a product failure. 

There you have it! We hope you find helpful and useful information in today’s post on how to measure a 4-20mA signal using a multimeter. What about you? Do you think you are ready to do the measuring all by yourself? In case you get lost, feel free to visit this article again. We wish you the best of luck!

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