Later in his/her career before the 18th month after reporting to the CVN/SSN/SSBN/SSGN, the MMN must qualify his/her senior in-rate watchstation. The following are for NIMITIZ CVN-68 Class (A4W) of Surface MMN's (Chief Reactor Watch for surface MMNs in Reactor Mechanical (RM) Division or Chief Machinery Operator for those in Machinery Division, and Engine Room Supervisor for submariners and FORD CVN-78 Class (A1B) Surface MMN's). The MMN must wait 5 years and be an E-5/E-6 to earn a N25S (surface both CVN Class) or N15S (submarines) NEC. Additionally, some MMNs are designated as Engineering Laboratory Technicians (NEC N26O for surface, N16O for submariners) in Reactor Laboratories (RL) Division following successful completion of an additional 13 weeks of training. ELTs are water chemistry and radiological controls experts for their commands in addition to maintaining other MMN watchstation qualifications. After completion of required watchstation qualification, submarine ELTs may complete an additional senior in-rate watchstation in order to be assigned a supervisory NEC of N16S. For surface ELTs to achieve the supervisory NEC of N26S, they must complete a list of additional requirements. This designation is important as it garners additional pay and consideration during advancement. Also, on submarines, some MMNs are designated as submarine nuclear propulsion plant emergency welders (NEC N51Z). They go through a two-month school learning welding, brazing, and soldering from Hull Maintenance Technician senior enlisted welders. Each submarine is required to have at least one of them to go underway. They are on board to provide emergency repairs to the plant if necessary to get the ship back to port where shipyard welders can complete a certified repair. Various machinist's mate ratings The range of machinist's mate ratings include: MM - Machinist's Mate MM/SS - Machinist's Mate - Submarine Qualified MM/SW - Machinist's Mate - Surface Qualified MM(A) - Machinist's Mate (auxiliary) MM(A)(GE) - Machinist's Mate (A) (bombing) MM(A)(H) - Machinist's Mate (A) (hydrogen) MM(A)(P) - Machinist's Mate (A) (Photographic) MM(CBE) - Machinist's Mate (construction battalion) (equipment operator) MM(MB) - Machinist's Mate (motor boat) MM(N) - Machinist's Mate (nuclear) MM(O) - Machinist's Mate (optician) MM(SRE) - Machinist's Mate (ship repair) (engine operator) MM(SRI) - Machinist's Mate (ship repair) (instrument maker) MM(SRO) - Machinist's Mate (ship repair) (outside machinist) MM(SRS) - Machinist's Mate (ship repair) (inside machinist) MME - Machinist's Mate (engineman) MMG - Machinist's Mate (industrial gas generating) MML - Machinist's Mate (general) MMR - Machinist's Mate (refrigeration mechanic)