... 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 ... 0.91 transmittance of outer tube4, The outer tube is made of extremely strong transparent quartz sand ... hail up to 25mm in diameter.5, The inner tube is also made of quartz sand, but it ...