... gurantee 3. OEM service offered solar vacuum tube1, Three-target Solar vacuum tube for solar water heater2, ISO900:2000 ... , 0.91 transmittance of the outer tube Specifications: 1, Vacuum tube is consists of two layer of glass ... 91 transmittance of outer tube4, The outer tube is made of extremely strong transparent quartz sand ... between the two glass tubes to form a vacuum, so it eliminates heat loss.7, ...