Which of the following is not considered a secondary instrument?

A secondary instrument is typically one that provides additional or supportive measurements in a system, rather than being the primary means of control or measurement.

In this context, while flow meters, pressure gauges, and temperature sensors are primarily used to measure operational parameters critical for the operation of water distribution systems, they often serve as secondary instruments within a framework that may depend on them to inform decisions about the system’s performance and management.

A pH monitor is critical for assessing water quality and maintaining regulatory compliance regarding water treatment processes, thereby serving as a primary tool for direct intervention in maintaining desired water conditions. Its role is more central to the management of water quality than the other instruments listed, which are primarily used for monitoring operational parameters. Therefore, it is appropriate to categorize the pH monitor as a primary instrument, making it not considered a secondary instrument in this context.