Vossimo — Vacuum Flask Heat Retention | vossimo.shop
Four named paths carry heat out of a flask: the neck and stopper, the two walls below 100 °C, the silvered facing surfaces, and the spacer contacts in large flasks.
The neck and stopper is the dominant path. Most heat transfer happens through the neck and opening, where there is no vacuum; the stopper is the part that matters most. Conduction runs through the stopper material and the air column above the liquid, and a stopper that is not fully seated leaves that channel open.
The inner and outer walls lose heat by conduction and convection through the residual gas in the gap. The gap is partially evacuated of air, which reduces both, and vacuum flasks usually hold contents below 100 °C because radiation across the gap grows with temperature.
The facing surfaces lose heat by thermal radiation. Silvering the facing surfaces reduces thermal radiation, which is why the inner face of the outer shell and the outer face of the inner shell are coated. A silvered flask later held a 1.018 V standard cell to a few parts per million, which shows how well the coating works when it is intact.
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Spacer contacts are the fourth path. Spacers between the shells act as thermal bridges and reduce insulation where they touch, so every contact point is a small deliberate conduction route across the gap. Fewer spacers and smaller contact areas improve insulation.
Two failure paths deserve separate mention. Chips, scratches or cracks are starting points for glass flask failure, so tempering the flask before use is advised. In large cryogenic vessels, liquid nitrogen boils at -196 °C and needs a pressure relief valve so the boiling gas cannot shatter the glass.
Slowing each path is a specific action: seat the stopper fully, keep contents below 100 °C, keep the silvering intact, minimise spacer contacts, temper the glass before use and fit a relief valve on large nitrogen vessels.
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