All buried containers shall be completely uncovered for an inspection of all external surfaces at least once every three years. The intervals may be changed at the discretion of the Chief if a test plate is buried adjacent to the container and shows no appreciable corrosion on inspection by the Department after three years of service; provided, however, that every container shall be uncovered at least every 15 years. The test plates shall be of the same material as the shell of the tank and shall be at least 1/4 of the container diameter in length and 6 inches wide, and shall be buried in the water-washed sand at least as deep as the lowest surface of the container and shall not be coated. Test plates shall be provided in accordance with the following:
1. Containers 10 feet or less in length, one test plate.
2. Containers more than 10 feet but less than 20 feet in length, two test plates, one on each side near opposite ends.
3. Containers 20 feet or more in length, one test plate for each 10 feet of length, placed on alternate sides at 10-foot intervals.