A common misconception: "If my new transmitter is 0.04% accurate, it will stay that way for 5 years." Part 4 testing often reveals that thermal cycling, diaphragm fatigue, or electronic component aging cause gradual decay. Responsible manufacturers use IEC 61298-4 to publish realistic drift specs.
This section defines how to quantify the difference between a device's measured output and its ideal value. iec 612982
