It is a special accessory for measuring the height of oil level in tank and sampling oil. There is one on the top of each tank, mostly near the ladder platform. The diameter of the measuring hole is 468 inch. It is equipped with a sealed hole cover and a tightening bolt. In order to prevent sparks from colliding with iron when the hole cover is closed, an oil-resistant rubber pad is embedded in the sealing groove of the hole cover. Because the steel tape used for measuring is easy to produce sparks by friction at the contact outlet, copper (or aluminium) sleeves are inserted inside the bore pipe or non-ferrous metal guide grooves (ruler grooves) which do not generate sparks are installed outside the fixed measuring points.