... 15 days 4.OEM service is provided . 1, Vacuum tube is consists of two layer of glass tubes2, ISO9001, ... 3, 0.91 transmittance of outer tube4, The outer tube is made of extremely strong transparent quartz sand which ... hail up to 25mm in diameter.5, The inner tube is also made of quartz sand, but it is ... absorption and lowest heat loss.6, The air is evacuated from the space between the two glass tubes to ...