What is the correct method to measure a 4-20 mA signal with a digital multimeter in a loop?

What is the correct method to measure a 4-20 mA signal with a digital multimeter in a loop?

• A. Disconnect one wire of the loop and connect the DMM in series
• B. Connect the DMM in parallel across the loop supply
• C. Measure the loop current by placing the DMM across a shunt resistor only
• D. Measure the loop resistance and infer the current

Answer: (A)

In a 4-20 mA loop the signal is carried by current, not by voltage, so you read the current by having the meter carry the same loop current. Break the loop at a convenient point and insert the digital multimeter in series so every milliamp that flows through the loop also flows through the meter. This direct series insertion gives an accurate reading of the actual loop current. Placing the meter in parallel across the loop supply would bypass the current path or create a parallel branch, so it wouldn’t measure the loop current and could disturb or damage the circuit. Measuring across a shunt resistor is possible if you have a known shunt and measure the resulting voltage drop, but that requires converting voltage to current and may introduce additional steps or burden on the loop. Trying to infer current from loop resistance isn’t valid here because the loop current is controlled by the transmitter and power supply, not by a fixed resistance.

In a 4-20 mA loop the signal is carried by current, not by voltage, so you read the current by having the meter carry the same loop current. Break the loop at a convenient point and insert the digital multimeter in series so every milliamp that flows through the loop also flows through the meter. This direct series insertion gives an accurate reading of the actual loop current.

Placing the meter in parallel across the loop supply would bypass the current path or create a parallel branch, so it wouldn’t measure the loop current and could disturb or damage the circuit. Measuring across a shunt resistor is possible if you have a known shunt and measure the resulting voltage drop, but that requires converting voltage to current and may introduce additional steps or burden on the loop. Trying to infer current from loop resistance isn’t valid here because the loop current is controlled by the transmitter and power supply, not by a fixed resistance.