By Dovydas Kersys · Chapter 53 of 58 in Machine Commissioning.
Before you begin: Follow the machine documentation, risk assessment, site procedures and lockout/tagout requirements. Commissioning can expose personnel to electrical energy, stored pressure and unexpected motion. Never defeat a safety function merely to make a test easier. Examples and diagrams are generic and must be checked against the actual machine.
Symptom
A pressure transmitter shows about 6 bar locally while the HMI shows 38 bar.
Diagnostic approach
Because the value changes smoothly, technicians initially suspect the transmitter. The correct method is to measure the actual loop current and compare it with PLC raw counts and scaling.
What the team found
The measured current corresponds to approximately 6 bar, and the PLC raw count is also correct. The engineering scaling was copied from a 0-100 bar transmitter while the installed unit is 0-16 bar. Scaling is corrected and tested at low, mid and high values.
Commissioning lesson
A plausible changing number can still be wrong. Always separate signal correctness from scaling correctness.
Use the same layered isolation method even when the symptom seems to point at software.
FIELD RULE: Preserve evidence before making changes: values, states, diagnostics, timestamps and the exact machine condition.
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Adapted from Machine Commissioning for Maintenance and Automation Technicians, by Dovydas Kersys. © 2026 Dovydas Kersys. Featured cover image is an AI-generated editorial illustration.