Cheat Sheets · Free PDF

Instrument Loop Troubleshooting Cheat Sheet

A one page fault finding guide for instrument loops: what each symptom usually means, what to measure and the classic flow, level and valve faults.

One page A4Print ready PDFFree, no sign up

Download

A one page A4 sheet to print, pin up in the workshop or keep on your phone. Everything on it is also shown below.

Reading	Likely cause
0 mA	Open loop, no supply, blown fuse
≤ 3.6 mA	Transmitter fault, downscale burnout
≥ 21 mA	Fault upscale, sensor break (TC)
Fixed value	Loop test or fixed current mode, manual DCS
Noisy	Grounding, shield, VFD noise, low damping
Offset	Zero shift, wrong range, wet leg, mounting

Symptom	Check
Reads low	Gas in liquid legs, HP leak, plugged HP tap
Reads high	LP leak, plugged LP tap, liquid in gas legs
Reads at zero flow	Missing low flow cutoff, equaliser open
Double square root	Root in transmitter and DCS

Symptom	Check
DP level wrong	Density change, wet leg loss, seal damage
Radar jumps	False echoes, foam, buildup, mapping
Stuck full	Level in blocking distance, coating

Symptom	Check
Reads ambient	Element not in well, air gap
Moves wrong way	TC polarity reversed
Offset few °C	Copper extension cable, CJC, 2 wire leads
Upscale	Open element or wire

Symptom	Check
Does not move	Air supply, I/P or positioner, seized stem
Hunting	Stiction, positioner gain, loose feedback
Passing	Seat wear, bench set, thrust
Slow	Blocked vent, small solenoid, booster

How do I tell if the fault is in the field or the DCS?

Measure the loop current in the field and compare with the DCS reading; a match points to the process or transmitter.

What causes 21 mA on a temperature loop?

Often an open sensor with upscale burnout.

Why does a DP flow read low?

Gas trapped in liquid impulse lines, or a leak on the HP side, are common causes.

Instrumentation calibration and troubleshooting guide

A practical guide on instrumentationblog.in that explains the theory behind this cheat sheet.