This standard is applicable to pressurized light water reactors. Experience has shown that in a pressurized light water reactor (PWR) system, under certain abnormal conditions, two phases (water and steam) and sometimes two levels can exist simultaneously in different parts of the reactor coolant system. This may appear as one level in the reactor vessel and a different level in the pressurizer. This was not considered fully when PWR instrumentation was originally specified. The objective of this standard is to define requirements for additional instrumentation to measure coolant parameters, which are of interest when abnormal conditions arise with either one or two phases of coolant or with gas included in the reactor vessel. PWR users can acquire this instrumentation to present information on coolant conditions, in order to assist the operator to decide on actions needed to maintain adequate core cooling.