Misting DG window and drilling glass: a result!

[The Natural Philosopher]

No, its been used with pretty big gaps before now, but the windows DO bow inwards.

[Chris Hogg]

Anything to make you happy! :-)

[Huge]

I simply don't believe it. An (e.g.) 2ft x 4ft pane is 1152 square inches, which gives a load on the pane of 17,000lbs. Without spacers, the unit will collapse completely flat, or more likely, implode. Have you never looked at an old cathode ray tube from a TV???

[Tim+]

I was told by a DG fitter that this is due to units being sealed when horizontal (causing the glass to sag in the middle). More reputable companies avoid this by sealing the units whilst held vertical. This strikes me as much more likely than units being evacuated.

No argument that vacuum would be better, but given the engineering difficulties (as evidenced by Pilkington using a very narrow gap and having to use "spacers" to prevent distortion ) I just don't believe that any "conventional" DG unit has ever been "vacuum filled".

I think it's a just a conclusion that folk have jumped to when they've seen poorly constructed DG units that are concave.

Tim

[Chris Hogg]

As TNP says, they use a grid of 'micro spacers' at 20 mm spacing to hold the two panes apart. From the Pilkington brochure:

"Pilkington Spacia? vacuum glazing consists of an outer pane of low-emissivity glass and an inner pane of clear float glass, separated by a micro spacer grid of small pillars each measuring just 0.5 mm diameter, set 20 mm apart, which are robotically positioned, with ?intelligent? camera checking. This grid ensures that the two glass panes are kept a fixed distance apart. The edges are welded to achieve a hermetic seal. Air is extracted to create a vacuum via the extraction point, rather than being air or gas filled. The result is an excellent thermal performance from a unit that is only slightly thicker than single glass."

There's also an illustration showing a cut-away section through a DG pane, with little dots all over it, representing the micro spacers. Quite what the panes are like to look through, I don't know.

[news]

Or units that have been "sealed" during a period of low atmospheric pressure (or high temperature) responding to slight changes in atmospheric pressure/temperature. This would probably be enough to distort the panes without completely destroying them.

[Ian Jackson]

Some of my DG glass does to bow slightly inwards. Is it possible that when they are filled with an inert gas, the pressure is deliberately low to allow for the situation when temperatures are high, or the atmospheric pressure is low? The last thing you would want is for the internal pressure to exceed that outside, as this could break the seal.

[dennis]

You can get vacuum panels now. They have to put some sort of spacer in between the panes to keep the gap. The spacer is pretty well invisible AFAIK. The gap is only a millimetre or so.

[Tim+]

Indeed. It's technically possible now, but I think it's hugely unlikely that prior to these Pilkington units that it was ever done before.

[Johny B Good]

The idea of fully evacuated DG panels led me to consider the need for a matrix of supporting spacers to stop the panes being deflected into contact with each other. I didn't think this approach would work without unwanted optical distortion effects other than for 'frosted' glass panels.

It seems the use of 0.5mm diameter glass beads in a 2cm matrix overcomes this problem sufficiently well enough to make vaccuum DG see-through panels a reality. With a vacuum between the panes, you only need to keep them seperated by the smallest of gaps (a few thousands of an inch at the closest points would suffice).

I'd imagine that the glass beads are bonded in some way to just the one surface, allowing the other surface a sliding contact to allow for thermal expansion/contraction effects.

[Harry Bloomfield]

Chris Hogg laid this down on his screen :

Well done Chris and just to add a little to your thread...

Some may be aware that many modern caravan windows are double glazed plastic, sealed by two removable soft plastic bungs in the top two corners of the inner panel. I managed to partially split the inner to outer panel joint along one side, repairing it with the appropriate welding cement quite successfully, but there was a lot of moisture trapped between the panels due to ingress, which needed to be dried out.

I fixed up a 12v computer fan, with a cardboard duct to direct the output, into one of the corner holes. It still took several days to fully dry the moisture out, before the bungs could be refitted on a hot day.