How to measure a 4-20mA current loop?

Process industries require monitoring of pressure, temperature, pH level, etc. for smooth operation.

These transmitters send information to PLCs or controllers on a 4 mA to 20mA loop.

Often engineers troubleshoot these transmitters by checking 4 mA to 20 mA loop signal.

Here, I am going to show you two different methods on ‘how to measure a 4-20mA current loop’ with illustrations.

How to measure a 4-20mA current loop?

Method #1: Measure the 4-20mA loop signal with the help of a multimeter

Step 1: Put the multimeter knob in mA mode.

Step 2: Remove the wire of the ‘+’ terminal of the transmitter.

Step 3: Connect one end of the red lead of the digital multimeter to the ampere port and the other end to the transmitter positive terminal.

Step 4: Connect one end of the black lead of the digital multimeter to COM and the other end to the wire (which you have taken out from the ‘+’ terminal of the transmitter).

In this way, you have connected the multimeter in series.

You can note down the reading shown by the multimeter.

Advantage of this method

• The digital multimeter which can measure mA is less costly than the process clamp meter.

Disadvantages of this method

• This method takes more time and requires a bit of alertness to have a measurement.
• The biggest disadvantage of this method is that you need to break the loop to measure the current.

Method #2: Measure the 4-20mA loop signal with the help of a process clamp meter.

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 as shown below.

You can use Fluke’s 771 or 772 or 773 process clamp meters to measure mA without breaking the loop.

Advantage of this method:

• You need not to break the loop to measure the mA signal. This is the biggest advantage.

Disadvantage of this method:

• Process clamp meters are expensive.