How to measure a 4-20mA loop signal


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.

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 transmitter).
  2. Locate the mA signal and zero the mA clamp meter
  3. Verify the mA measurement, 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 multimeter or loop calibrator:

  1. Check with operations before initiating the measurement
  2. Access the signal wires (typically by removing the cover on transmitter).
  3. Select the mA dc measurement function and connect the test leads for 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
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