There are two US standards for determining the quality of any thread seal tape. MIL-T-27730A (an obsolete military specification still commonly used in industry in the US) requires a minimum thickness of 3.5 mils and a minimum PTFE purity of 99%. The second standard, A-A-58092, is a commercial grade which maintains the thickness requirement of MIL-T-27730A and adds a minimum density of 1.2 g/cm3. Relevant standards may vary between industries; tape for gas fittings (to UK gas regulations) is required to be thicker than that for water. Although PTFE itself is suitable for use with high-pressure oxygen, the grade of tape must also be known to be free from grease.