You tuned the PID.
It worked.
Then it failed again.

This is not always a tuning problem.

Sometimes the plant is already telling you what is wrong.

STRUCTURAL FAILURE.

• Heater already at 95% but temperature still drops
• Valve fully open but flow is unstable
• Same PID works in winter, fails in summer

These are not just tuning mistakes. They are system-level constraint problems.

Thermal PI Control Training Kit (Excel)

This is not a tuning tool.

This is how you stop tuning the wrong problem.

• Built for real constraints
• Simulate overshoot, saturation, and delay
• No coding required