Debasis API gives general guidelines for C.R from short term and long term without specifying how the monitoring is done. You should remember that any code - API/ASME/BS/DIN etc., - can only specify the general guide lines - based on the feed back and the recommendations of the select committee review on a periodic basis from world wide users and practical approaches. As a user, you can specify the monitoring locations - in a planned and phased way - although the exact point or spot may not exactly match in spot locations with possibility of permanent markings erased. As a thumb rule, monitoring spots are done by some users marking a certain distance on horizontal and vertical leg - i.e., a grid profile pattern. Grid profile pattern is specified in ASTM A 435 -98, BS code or DIN Code but that is too large and too laborious for a tank thickness monitoring. In fact, B.S or DIN codes specify more stringent probe dimensions to get more accurate grid profile. A 435 specifies a large probe dia. Essentially using the same principle, you can mark for your tanks - a thumb rule horixontal and vertical grid profile pattern - say at horizontal distance of 0.75 m and vertical distance of 0.5 to 0.75 m for each spot of thickness location monitored. This depends on the tank size (dia and height) if you wish to monitor from external when the tank is in operation. With two or three consistent periodic monitoring using this thumb rule grid profile you will get a reasonable over - all corrosion profile. Peridocity of monitoring is to be decided by the user based on the first profile values and variations noted in plate thickness. If however, you note localized higher external pitting spots at the external areas due to moisture ingress, wind direction being specific, gaps in insulation etc, you can closely monitor those areas to get the more accurate C.R for the particular areas where external pitting is visibly noted. With to-day's data logging thickness gauging instruments, if the points monitored and grid profile distances are marked (rough marking only) you can get a reasonable C.R from short term and long term for a tank. With this guideline, as a user, you can determine your own monitoring profile to get reasonaly prediction C.R for your tanks. No code will give you an exact guideline except the ASTM A 435 which is laborious. ASTM A 435- or B.S or DIN code equivalent to ASTM A 435- was mainly developed for plate thickness monitoring even at the pre-fabrication or post fabrication or in-service monitoring from knowing if the plate supplied had " lamination or inclusions of large sizes" and to determine the lamination or inclusion profile. Trust this answers your querry C.V.Srinivasan Nishi Engineers Pvt Ltd India Oct 22,2005 E-amil:nishi@vsnl.com >API 570 / Section 7 gives some rules for Corrosion rate ( >Short term and long term )calculations. This needs thickness >measurements. But the TML and points in the previous >inspection can not be closely merged with current and future >inspections as all the permanenent markings normally gets >erased by time. How this issue specific to exact point >location monitoring is handled ? Please discuss. >regards >DM