Topic
4-20 mA
13 guides across the knowledge base.
Analog Signal Problems
The controller side of a bad analog value: raw counts against the module range, channel diagnostics, scaling that does not match the transmitter, wiring by device type, noise and filtering, resolution, and a symptom table that points at the check.
8 min readUpdated Sep 5, 2026
4-20 mA Signals from the Instrument Side
How a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
9 min readUpdated Sep 5, 2026
HART
Digital data riding on the 4-20 mA loop: how HART signaling works, what a handheld or a HART-enabled input can read, multidrop and burst modes, and the wiring conditions that make HART fail while the current loop keeps working.
10 min readUpdated Sep 5, 2026
Loop Checks
Proving every I/O point end to end before a system goes live: what a loop check is and is not, the three-point analog check, discrete and output checks, the two-person method with the sheet that records it, what to do with a point that fails, and why a loop check is not a calibration.
9 min readUpdated Sep 5, 2026
Signal Isolation
Isolating analog signals: loop isolators and their variants, splitters that feed two receivers, converters, the common-mode problem isolation solves, ratings, accuracy and response costs, and when an isolated input module makes the isolator unnecessary.
8 min readUpdated Sep 5, 2026
How to Calibrate a Pressure Transmitter
A five-point calibration with a pressure source and a reference: isolate and vent, record as-found, decide whether to adjust, trim the sensor and the output separately, verify as-left, and record it so the next calibration means something.
9 min readUpdated Sep 5, 2026
Transmitter Reads Wrong Value
The reading is steady, plausible, and wrong. How to prove it against a reference, then separate a scaling mismatch, a range change, a zero shift, an installation effect, and a genuine transmitter fault, with the order of checks that finds it fastest.
9 min readUpdated Sep 5, 2026
Signal Pegged High or Low
A 4-20 mA signal stuck at a rail or a fault current: a transmitter in fault, a process truly out of range, an open or shorted loop, a wrong range, a wiring error, an input channel fault, or a scaling clamp. How the current itself tells you which.
8 min readUpdated Sep 5, 2026
Loop Powers Up but Reads Zero
The transmitter is lit and the controller shows zero, so loop current is not reaching the input: a loop wired for power but not signal, an active and passive mismatch, a missing jumper, an unconfigured channel, a blown fuse, or a transmitter sending nothing.
7 min readUpdated Sep 5, 2026
When the Transmitter and the Gauge Disagree
The SCADA screen says 62 psi and the gauge on the pipe says 58, and someone has to decide which one to believe. Why two readings of the same pressure differ, and a procedure for settling the argument with a reference instead of an opinion.
9 min readUpdated Sep 5, 2026
4-20 mA Current Loops
Why the industry standardized on current instead of voltage, how a two-wire loop is powered, what the live zero buys you, and how to read a loop with a meter.
12 min readUpdated Aug 29, 2026
4-20 mA Signal Is Unstable
An analog reading that will not settle. How to separate a genuine process swing from induced noise, a ground loop, or a loop running out of voltage.
8 min readUpdated Aug 23, 2026
How to Test a 4-20 mA Loop
Prove a current loop end to end with a meter and a calibrator, without interrupting a process you were not authorized to interrupt.
8 min readUpdated Aug 6, 2026
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