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Made out primarily of glass. This glass must be tempered and reinforced. It should be of high perspective.
From a lateral view, it is better if the glass looks white.
If it looks greenish the perspective rate is lower.
Normally only one sheet of glass is enough, but in cold areas or where higher water temperatures are required double pane glass or glass with film is needed to prevent rapid heat loss.

 The middle layer of the pressure vessel is made of PU forming insulating materials. The outside layer is stainless steel or zinc plated painted steel. The tank must be rust proof, with good insulating materials. While the inside tank must resist high water pressure.