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4–20 mA Current Loop

Change the process pressure and watch the loop current, the voltage at the I/O card and the DCS reading. Then break the wire or fail the sensor to see how the DCS detects faults.

4–20 mA current loop animation A pressure transmitter on a pipe is wired to a 24 V power supply and a 250 ohm resistor in an I/O cabinet. The I/O card measures the voltage and the DCS shows the pressure. 4–20 mA current loop instrumentationhub.org Process pipe Wire broken 12.00 mA 50.0 % PT-101 Range 0–10 bar + − I/O cabinet 24 V DC Loop supply 250 Ω AI card 3.00 V DCS faceplate PT-101 Line pressure 5.00 bar 100 % 0 % 50.0 % NORMAL I = 4 + 16 × (P − 0) / (10 − 0) = 12.00 mA Start your screen recorder then press Space or click to play the demo (about 50 seconds)

Loop condition

Video mode (for YouTube)

Loop current
12.00 mA
Voltage at AI card
3.00 V
% of range
50.0 %
DCS reading
5.00 bar
Signal status
Normal

Status follows NAMUR NE 43: ≤ 3.6 mA or ≥ 21 mA is a fault; 3.8–20.5 mA is the valid measuring range.

What you are seeing

  • The 24 V supply, the transmitter and the 250 Ω resistor are in series, so the same current flows through all of them. The moving dots show that current.
  • The transmitter sets the current: 4 mA at 0 % of range and 20 mA at 100 %. The I/O card measures the voltage across the 250 Ω resistor: 1 V to 5 V.
  • Because the bottom of the range is 4 mA, not 0 mA (a "live zero"), a broken wire (0 mA) can't be mistaken for a real zero reading.
  • A healthy transmitter that detects an internal fault drives its output to ≤ 3.6 mA or ≥ 21 mA (NAMUR NE 43), so the DCS can flag the reading as bad.

Work out your own values with the Analog Signal Scaler, or read Transmitters & Signals.