... years' gurantee 3. OEM service offered solar vacuum tube1, Three-target Solar vacuum tube for solar water heater2, ISO900:2000, CE certificate3 ... , 0.91 transmittance of the outer tube Specifications: 1, Vacuum tube is consists of two layer of glass tubes2, ISO900 ... The inner tube is also made of quartz sand, but it is coated with a special selective coating with strong heat absorption ...