What are instruments that provide permanent records of changes in a process variable known as?

Instruments that provide permanent records of changes in a process variable are known as recorders. Recorders are designed to continuously monitor a specific process variable, such as pressure, temperature, or flow, and produce a time-based graphical or tabular representation of that data. This is essential in many industrial applications, as it allows for the analysis of trends over time and can help identify issues or validate performance.

Recorders differ from indicators, which simply display the current value of a process variable without any historical record. Transducers convert one form of energy into another and are an integral part of the measurement process but do not serve the purpose of recording. Controllers, on the other hand, are devices that regulate the operation of a process to maintain specified conditions but also do not provide a permanent record of variables. Thus, recorders are the instruments specifically designed for documenting and preserving data over time.