How To Measure A 4-20mA Loop Signal Using A Multimeter

The 4-20mA current loop is a common method for transmitting information between sensors, controllers, and transmitters in many industrial process monitoring and safety applications.

Many industrial systems measure physical factors like temperature, pressure, level, etc; these systems use the 4-20mA system to transport that information from the field devices to the control rooms or SCADAs.

How the 4-20 mA loop works

The 4-20 mA loop works this way:  a sensor’s output voltage is first converted to a proportional current, with 4 mA normally representing the sensor’s zero-level output and 20 mA representing the sensor’s full-scale output.

4-20 mA Current Loop

Why must you measure a 4-20 mA loop with a Multimeter?

Verifying the 4-20mA loop is crucial in troubleshooting the process; you need to know what values the sensor sends to the controller or the DCS.

How To Troubleshoot a 4-20 mA Loop

For this reason, I will share how to measure the 4-20 mA loop using a multimeter.

Tools you need to need to measure 4-20 mA loop

A digital voltmeter is essential for troubleshooting the 4-20 mA current loop. The fastest way to verify operation and isolate problems is to use a tool to ensure voltage levels and measure the current through the loop.
how to troubleshoot a 4-20 mA loop

How To Measure A 4-20mA Loop Signal Using A Multimeter

Measuring a 4-20 mA signal with a multimeter is a common task in industrial automation and process control to verify the correct operation of sensors and transmitters. Here’s how you can do it.

Step1: Set Up the Multimeter to the current mode

Select the Current Mode: Turn the multimeter dial to the mA (milliamps) setting. Ensure the multimeter can measure at least 20 mA.

Step 2: Prepare the Circuit

I recommend powering down the system before connecting anything; this is to avoid any possible hazard that might happen.

The multimeter needs to be connected in series with the loop. Disconnect the wire where you want to measure the current and prepare to insert the multimeter at that point.

Step 3: Connect the Multimeter

The main disadvantage of this method is that you will have to disconnect the loop, so the next step will be to identify the 4-20 mA loop. Disconnect one side of the loop where you want to measure the current.

After you disconnect the loop, connect the multimeter’s red probe to the positive side of the loop (the wire coming from the transmitter or sensor).

Connect the multimeter’s black probe to the wire leading to the power supply or receiving device.

Step 4: Measure the Current

Now that everything is properly connected, you can power up the circuit and observe the reading on the multimeter display.

It should be between 4 and 20 mA, corresponding to the sensor’s minimum and maximum values.

Step 5: Interpret the Reading

After connecting everything, read the multimeter. Usually, I will verify at three points: the zero or minimum value, the middle value, and the maximum value.

4 mA

Typically represents the zero or minimum value of the measured parameter (e.g., zero flow, empty tank, zero gas in the environment, etc.).

12 mA

This represents half of the scale; if the sensor detects CO from 0 to 100 ppm, this would be 50 ppm; if the sensor detects temperature from 0 to 150 degrees, this would be at 75 degrees. Most modern transmitters can force mA or simulate it; you can use that option if available.

20 mA

Typically represents the full-scale or maximum value of the measured parameter (e.g., maximum flow, full tank).

Values in Between are Proportional to the range of the parameter being measured.

Step 6. Disconnect and Restore the Circuit

Power Down: Turn off the circuit power before disconnecting the multimeter.

Reconnect the Loop: Remove the multimeter and reconnect the disconnected wire.

Restore Power: Turn the circuit back on to resume normal operation.

Measure the 4-20mA loop signal with the help of a process clamp meter.

It is straightforward to measure the 4-20mA loop signal using a clamp meter; here are the steps.

Step 1: Access the wire connected to the ‘+’ terminal of the transmitter (no need to remove the wire).

Step 2: Put the process clamp meter in ‘Measure’ mode.

Step 3: Clamp the wire connected to the positive terminal of the transmitter and measure the mA signal.

How To Measure A 4-20mA Loop Signal Using A Multimeter when the sensor is not connected to the controller

The procedure I explained above works when there is a communication between the sensor or field device and the controller or transmitter.

There are times that you need to test your sensor or transmitter before you send it to the field, in order to do this you need to identify if your 4-20mA if it is a 2-wire, 3 wire sink or source or 4-wire isolated.

Types Of 4-20 mA Current Loop

There are 4 types of mA output signals:

  • Loop (2-Wire)
  • Source (3-Wire)
  • Sink (3-Wire)
  • Isolated (4-Wire)

Each form uses a different reference path to create the mA signals, dependent on the controller or receiving device (e.g., PLC) to which the field device is connected.

How to measure 4-20mA loop using a multimeter in a 2-wire  loop

types of 4-20 mA loop

The 2-wire device has just 2 wires, the 24V and the signal, here the measuring is simple.You need a load resistance of between 250 to 500 ohms this to simulate the load.

Step 1: Put the multimeter on the mA mode.

Step 2: Connect the load resistance to the + sign.

Step 3: Connect the red terminal of the multimeter to the other side of the resistance.

Strep 4: connect the black terminer of the multimeter to the signal.

How to measure 4-20mA loop using a multimeter in a 3-wire sink.

To measure the 4-20mA loop current using a multimeter on a 3-wire sink, do the following.

Step 1: Put the multimeter on the mA mode.

Step 2: Put the comm wire to the +4-20mA.

Step 3: Put the mA wire to the +24V.

The connection should be as shown in the image below.

How to measure 4-20mA loop using a multimeter in a 3-wire sink.

How to measure 4-20mA loop using a multimeter in a 3-wire source.

To measure the 4-20mA loop current using a multimeter on a 3-wire source, do the following.

Step 1: Put the multimeter on the mA mode.

Step 2: Put the comm wire to the -4-20mA.

Step 3: Put the mA wire to the -24V.

The connection should be as shown in the image below.

soruce

 

How to measure 4-20mA loop using a multimeter in a 4-wire isolated.

To measure the 4-20mA loop current using a multimeter on a 4-wire isolated, you need another power source, remember the isolated need two power sources to work.

Here are the steps.

Step 1: Put the multimeter on the mA mode.

Step 2: connect  the comm wire to the +4-20mA.

Step 3:Connect the other wire of the multimeter to the +Ve of the 24V power supply.

Step 4: Connect he -Ve wire from the power supply to the -4-20mA terminal.

The connection should be as shown in the image below.

Isolated

Safety Tips when measuring A 4-20mA Loop Using A Multimeter

Check Multimeter Rating

Ensure the multimeter’s current rating suits the expected measurement.This will avoid you burning the multimeter.

Proper Probe Insertion

Confirm the multimeter probes correctly inserted into the appropriate ports (usually labeled COM and mA/µA).

Avoid Overloading

Do not exceed the multimeter’s current measurement limit to prevent damage to the multimeter and ensure personal safety.

Key Takeaway: How To Measure A 4-20mA Loop Signal Using A Multimeter

Measuring a 4-20 mA signal with a multimeter involves setting the multimeter to the appropriate mode, connecting it in series with the current loop, and interpreting the readings.

This process helps ensure that sensors and transmitters in industrial control systems are operating correctly.

Always prioritize safety by powering down the circuit before making connections and ensuring the multimeter is properly rated for the measurements being taken.

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