1. American Oil Chemists Society, 1960 . Official and tentative methods-Addition and Revision, Cc16-60 . American Oil Chemists Society Chicago (Reapproved 1973).
2. American Society for Testing and Materials, 1968 . D217 (Reapproved 1973).
3. British Standards Institution, 1976 . BS 684: Section 1-11. Determination of penetration values.