The 4-20 mA current loop has been the standard for signal transmission and electronic control in control systems since the 1950’s.

The 4 mA to 20 mA current loop is a common method of transmitting sensor information in many industrial process monitoring applications—typically in systems monitoring pressure, temperature, pH, flow, or other physical factors. These systems employ a two-wire, 4 mA to 20 mA current loop, in which a single twisted-pair cable supplies power to a transmitter and also carries the output signal.

In a current loop, the current signal is drawn from a dc power supply, flows through the transmitter, into the controller, and then back to the power supply in a series circuit. The advantage is that the current value does not degrade over long distances, so the current signal remains constant through all components in the loop. As a result, the accuracy of the signal is not affected by a voltage drop in the interconnecting wiring.

Voltage drop

Voltage signals sent over a long distance will, however, degrade in accuracy and develop a voltage drop (using Ohm’s Law) proportional to the length of the cable. Accuracy loss of the voltage signal would equal the mA signal value multiplied by the resistance of the wire.

The use of 4 mA as a “Live Zero” enhances the signal-to-noise-ratio at low levels. This Live Zero also makes a loop failure more apparent. A nonfunctioning current loop with an open termination or connection has zero current flow, which is outside the valid 4 to 20 mA signal range.

Verifying the 4 to 20 mA loop

Milliamp loop calibrators are typically used to test loops that measure pH, pressure, temperature, level, and flow. Verifying the 4 to 20 mA loop is a crucial step in any instrument calibration. Full loop verification includes testing the output of the transmitter, the wiring, input to the control system as well as the control system input card and the return wiring back to the transmitter.

The loop’s operation is straightforward: 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. A reading of 20 mA means that a direct-acting valve, for example, is 100% open, and a reading of 4 mA means that it is closed. (The opposite is true for a reverse-acting valve). Readings between the maximum and minimum values indicate that the circuit is controlling the valve.

Verifying a 4-20 mA loop is a crucial step in both troubleshooting and calibrating process systems. Full verification includes testing the output of the transmitter, the wiring, input to the control system and control system input card, and the return wiring back to the transmitter.

The functions of an advanced loop calibrator allow technicians to troubleshoot on the spot without disconnecting wires or “breaking the loop.” Multifunction process calibrators can also be used to test 4-20mA loops as well as digital controls.

To measure a 4-20 mA loop signal with a process clamp meter:

1. Access the signal wires (typically by removing the cover on the transmitter).
2. Locate the mA signal and zero the mA clamp meter
3. Verify the mA measurement, which should be between 4 and 20 mA
4. This measurement technique does not interrupt (break) the loop to measure the 4 to 20 mA signal

To measure a 4-20 mA loop signal with a multimeter or loop calibrator:

1. Check with operations before initiating the measurement
2. Access the signal wires (typically by removing the cover on the transmitter).
3. Select the mA dc measurement function and connect the test leads for the mA measure
4. Locate the mA signal wire, disconnect one lead of the signal wire and put the meter in series with the mA signal wire, and view the mA measurement
5. This measurement technique interrupts (breaks) the loop to make the 4 to 20 mA measurement