Double glazed unit failure

You know when you try to clean a window that’s double glazed and you can’t because the moisture is inside it.
This is known as a failed blown or misty unit.
When a unit is made the two panes of glass are cut to size and cleaned in a glass washing machine.
Spacer bar,which is hollow square tubing is also cut to length
The two long bars are filled with silica dessicent crystals.These are often found in a small bag when purchasing electrical or photographic equipment to keep away harmful moisture.corner clips are added and a rectangular shape is made.
The first piece of glass is laid down flat and the Spacer bar laid upon it and then sandwiched with the second piece of glass.
The next stage is to seal around the edges with a hot melted mastic .
The unit is now complete.
The air trapped inside the now sealed unit is holding moisture.This is sucked out by the silica dessicent crystals through small perforations in the Spacer bar wall.So leaving a clear view through the two panes of glass.
Problems start when the unit is constantly expanding and contracting with different temperatures.This constant movement eventually makes the seal between the two panes fail.This in turn allows new air to enter the unit.This new air has moisture contained within it.The crystals can’t hold this new moisture and so the unit steams .
http://www.markcooperglazing.co.uk