www.lesswrong.com/posts/SacB4mg6nJWATKB3Y/a-proposal-for-a-cryogenic-grave-for-c...

NASA’s cryogenic test documentation defines heat leak from liquid-nitrogen boiloff as boiloff rate × latent heat of vaporization, not latent heat plus the enthalpy of warming the boiloff gas to ambient. In parallel, official liquid-nitrogen handling guidance says cryogenic containers are pressure-relieved and normally vent, so that warmed nitrogen gas is not retained as stored cold inventory.

That matters here because the post’s century estimate uses 346 MJ/m³, which comes from counting both:

• liquid nitrogen’s latent heat of vaporization, and
• the heat needed to warm the resulting nitrogen gas by 220 °C.

But for a normal liquid-nitrogen storage vessel, the gas vents away; standard boiloff accounting therefore uses the latent heat term only.

Using NASA’s liquid-nitrogen properties from the cited table:

• density ≈ 0.81 g/cc (= 810 kg/m³)
• latent heat of vaporization ≈ 198 J/g (= 198 kJ/kg)

So the relevant volumetric energy for boiloff is about:

810 kg/m³ × 198 kJ/kg ≈ 1.60 × 10^8 J/m³

not 3.46 × 10^8 J/m³.

Keeping the post’s own heat-leak expression 25 × k × r × ΔT, the resulting time constant is approximately:

(4.2 r³ × 1.60×10^8) / (25 k r × 220) ≈ (1/25,973) × r²/k centuries

So the quoted 1/12,000 × r²/k centuries overstates survival time by about a factor of 2.16.

For example, plugging in the post’s later r = 12 m and k = 0.012 W/m·K gives about 0.46 centuries (~46 years), not 1 century.