ISA-TR91.00.02:2003

Process operating facilities utilize instrumentation to monitor and control processes. The performance of this instrumentation ties directly into product quality and product throughput. In addition, the process may become hazardous to operating facility personnel, the community, and the environment if control is lost.
Some instruments perform essential functions during emergency response activities and a high degree of confidence is required to ensure that instrumentation will function correctly during an emergency.
This guideline is developed to assist engineering, operations, and maintenance personnel with establishing the classification of their instrumentation, thus facilitating all aspects of designing and maintaining reliable operating facility instrumentation.