Quoted:
Double pane windows can be sealed with n2, argon etc but also air. The trick is the seal. You have to get a close fit on the new pane of glass and then don't spare the silicon sealant. Reseal the inside pane inside and out then set the outside pane and "tack" it in place with sealant on the corners. Then fill the inside between the panes with a heavy bead of sealant and wait till it dries, then a heavy bead on the outside of the outside pane. You have to overdo it as any gap causes moist air to flow in and out between the panes of glass and you get condensation etc.
Good luck!
The only problem is that the moisture in the air when you glue up that window will condense out with temperature changes, and you'll get fogging. I'm speaking from experience.
Window manufacturers used dried gas to fill the space between the panes to avoid this.
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