An instrument measurement error is the difference between the actual value of the measured variable and:

An instrument measurement error is defined as the difference between the actual value of the measured variable and the indication of the instrument. This measurement error captures how accurately the instrument reflects the true conditions of the variable being measured. If an instrument indicates a certain level, but the true value differs from that indication, the measurement error can provide insights into the instrument's performance, accuracy, or need for recalibration.

For instance, if a pressure gauge shows a reading of 50 psi when the actual pressure is 45 psi, the measurement error would be 5 psi. Understanding this error is essential for diagnosing issues, ensuring quality control, and maintaining the reliability of measurements within a control system.