API St 20H pdf download Heat Treatment Services—Batch Type for Equipment Used in the Petroleum and Natural Gas Industry 4.3Heat Treatment Specification Levels 4.3.1 Heat Treatment Specification Level 1 (HSL-1) 4.3.1.1 The pyrometric requirements for heat treatment equipment in conformance with HSL-1 shall be inaccordance with 8.2. 4.3.1.2A temperature uniformity survey (TUS) shall be performed every 12 months minimum for...