Within the specified working factors, the ability of the instrument to maintain certain performance over time is called stability (degree). Because the environment of the consuming instrument is very different, the temperature and pressure of the measured medium are also very different. When the instrument is consumed in this environment, the ability of certain parts of the instrument to remain unchanged over time will be reduced, and the instrument’s Stability will fall back. There is no quantitative value for the stability or characterization of the instrument. The chemical industry group usually uses the zero drift of the instrument to measure the stability of the instrument.

Instrument reliability is another major performance index pursued by instrument engineers. Reliability and instrument maintenance are in opposition to each other. High instrument reliability means less instrument maintenance. Conversely, if instrument reliability is poor, instrument maintenance is greater. For chemical group inspection and process restriction instruments, most of them are installed on process pipelines, various towers, kettles, tanks, and vessels, and the continuity of chemical production, most of the toxic, flammable and explosive environment, these harsh factors give the instrument maintenance A lot of problems have been increased. One is to consider the safety of chemical production, and the other is related to the personal safety of instrument maintenance personnel. Therefore, the staff consumption inspection and process limitation are required to maintain the instrument as small as possible, that is, the reliability of the instrument is required to be as high as possible.