Condition Monitoring (CM) of offshore Oil and Gas (O&G) process equipment is important as it enables early detection of developing failure conditions. CM can be used to control the process, optimize operation of process equipment and reduce the amount of expensive and pollutive chemicals used to improve the process. It will also allow more time for planning and preparation of remedial actions, and will reduce revenue losses resulting from unnecessary and poorly coordinated maintenance actions. Implementation of Condition Based Maintenance (CBM) will result in fewer unnecessary and expensive maintenance shut-downs and reduced production down-time in relation to maintenance actions. This paper is number three in a series of papers concerning CM, using Non Intrusive Inspection (NII) methods, of three-phase separators for petroleum production. A laboratory separator, equipped with instrumentation tubes to simplify and improve the NII methods, has been used. Test results obtained with gamma transmission and passive acoustic measurements for monitoring of internal separator conditions are presented.