That is quite an odd request. There isn't any sort of fluid sitting underneath the glass. I'm also curious as to how you propose we measure the gap? Digital calipers have a hard time clamping down on air.
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As far as material underneath the glass, if you're asking about a fluid or sheet, there isn't anything. The air gap could probably be measured by another one of our guide makers; maybe they'll chime in later.
That is quite an odd request. There isn't any sort of fluid sitting underneath the glass. I'm also curious as to how you propose we measure the gap? Digital calipers have a hard time clamping down on air.