The stopper is the part that matters most
Heat escapes at the neck
Vossimo is the vacuum-flask guide behind vossimo.shop: where heat escapes from an insulated bottle and how to slow it down. The vacuum flask, also called a Dewar flask or thermos, was invented by James Dewar in 1892 as two flasks placed one within the other with a near-vacuum in the gap, and the Thermos brand later carried it into everyday kitchens. Liquid water holds 4187 J of heat for every kilogram and every 1 °C at 15 °C, so in a modern bottle most of the loss runs through the neck and stopper, where there is no vacuum at all. This page names each heat path, gives the deciding values — including the -196 °C boiling point of liquid nitrogen in laboratory flasks — and runs the check you should do before the first fill.
The stopper is the part that matters most
The gap is partially evacuated
Silvering the facing surfaces
Liquid nitrogen at -196 °C
Vaccines under 10 °C
Chips start failures
Most heat transfer happens through the neck and opening of the flask, where there is no vacuum.
The evacuated gap between the two walls cuts conduction and convection, and silvering the facing surfaces cuts radiation. That is why the body of the flask works well.
The neck and the stopper have no vacuum: heat runs along the stopper material and out through the opening. Seating the stopper fully is the single most effective thing a user can do.
Below 100 °C the flask holds its contents; above that, radiation across the gap grows with temperature and retention falls off.
Specific heat, boiling point and gap temperature set how much heat the drink carries and how fast it leaves.
Liquid water holds 4187 J per kilogram per 1 °C at 15 °C. A 1 kg fill at 60 °C against 15 °C ambient therefore starts with about 188 kJ of usable heat.
Contents are usually kept below 100 °C because radiation across the partial vacuum grows with temperature; the hotter the inner wall, the more the flask has to reject.
Cryogenic use inverts the problem: liquid nitrogen boils at -196 °C, and larger vessels need a pressure relief valve so the boiling gas cannot shatter the glass.
The stopper is the part that matters most, because it is where the vacuum stops.
A stopper bridges the inner and outer shell at the neck. Heat that would otherwise circulate in the gap now has a solid path out.
A loose or partly seated stopper turns the neck into an open chimney: warm air leaves, cold air enters, and convection through the opening replaces the vacuum's job.
Most heat transfer happens through the neck and opening of the flask, so a flask with a worn stopper loses its advantage long before the walls give up.
Six cards, each naming one path or one value.
Inspect, temper, fill below the boiling point, seat the stopper, keep the flask upright.
Chips, scratches or cracks are starting points for glass flask failure, so inspect the vessel before the first fill and temper the flask before use.
Fill below 100 °C: the flask is designed to hold contents under the boiling point, and hotter liquid only raises radiation across the gap.
Seat the stopper fully and keep the flask upright. The ordered procedure with failure modes is on the pre-fill page.
| Value | What it is | Where it applies |
|---|---|---|
| 4187 J | Heat held per kg per 1 °C at 15 °C | Liquid water fill in any flask |
| 100 °C | Boiling point of water | Upper limit for contents; radiation grows above it |
| -196 °C | Boiling point of liquid nitrogen | Laboratory cryogenic flasks |
| 10 °C | Vaccine holding temperature | Arktek medical dewar with eight 1-litre ice blocks |
| 1.018 V | Standard cell voltage | Held in a silvered flask to a few parts per million |
| 1892 | Year of the first vacuum flask | James Dewar's design, never patented |
| 8 × 1 L | Ice blocks in a medical dewar | Keeps vaccines under 10 °C |
Compiled from the stated physical values and historical notes on vacuum flasks; no external citations are added.