Section 183.516 - Cellular plastic used to encase fuel tanks(a) Cellular plastic used to encase metallic fuel tanks must: (1) Not change volume by more than five percent or dissolve after being immersed in any of the following liquids for 24 hours at 29 °C:(i) Reference fuel B ASTM D 471 (incorporated by reference, see § 183.5 ).(ii) No. 2 reference oil of ASTM D 471 (incorporated by reference, see § 183.5 ).(iii) Five percent solution of trisodium phosphate in water; and(2) Not absorb more than 0.12 pound of water per square foot of cut surface, measure under Military Specification MIL P-21929B.(b) Non-polyurethane cellular plastic used to encase metallic fuel tanks must have a compressive strength of at least 60 pounds per square inch at ten percent deflection measured under ASTM D 1621 (incorporated by reference, see § 183.5 ), "Compressive Strength of Rigid Cellular Plastics".(c) Polyurethane cellular plastic used to encase metallic fuel tanks must have a density of at least 2.0 pounds per cubic foot, measured under ASTM D 1622 (incorporated by reference, see § 183.5 ), "Apparent Density of Rigid Cellular Plastics."CGD 74-209, 42 FR 5950, Jan. 31, 1977, as amended by CGD 77-98, 42 FR 36253, July 14, 1977; CGD 81-092, 48 FR 55737, Dec. 15, 1983; USCG-2000-7223, 65 FR 40059 , June 29, 2000