Intermediate · Diagnose equipment
Is the pressure wrong, or its measurement?
Trace a 4–20 mA loop, collect useful evidence and verify two operating points.
Compare the reference gauge, current, available voltage and indication. Record two points without confusing 0 bar with an open loop.
IDEAL MODELSelect a device
Measurements
- Loop current
- —mA
- Transmitter voltage
- —V
- Voltage across 250 Ω
- —V
- Indicated pressure
- —bar
A · remembered
B · current
Checks with the current configuration
Recorded evidence
- Choose an instrument to keep a reading.
Installation, principle and limits
Ideal two-wire transmitter: I = 4 + 16 × pressure / span in mA, limited here to 0–22 mA. Receiver: 250 Ω; exercise transmitter minimum: 12 V. With insufficient headroom, current is capped at (Vs − 12 V) / (Rline + 250 Ω). This is a teaching simplification, not a product characteristic. Open-loop floating terminals show no fabricated voltage: “—”. Indication is unavailable outside 3.6–21 mA. Noise, HART, precision, internal faults and PT100 are excluded.
Texas Instruments · two-wire transmitters and compliance voltage ↗