Does dry ice melt?
At normal pressure there is no liquid phase: liquid carbon dioxide forms only above 5.185 bar. Dry ice does not melt; at -78.5 °C it turns straight into gas (sublimes) and leaves no water behind. When people ask in how many hours dry ice melts, they mean how long its mass takes to turn to gas and run out.
How many hours does dry ice last?
How long dry ice lasts depends on five things: the container's insulation, the quantity, piece size, outside temperature and lid openings. In the FAA's measurement of pellets in a foam box the loss averaged 2 per cent of the starting mass per hour; the time is roughly the starting quantity divided by your container's hourly consumption. Figures for 1 kg are in how long 1 kg of dry ice lasts.
Judging the remainder mid-route. When the lid is opened the cold gas escapes and warm air comes in, so what is left is read from three signs instead:
- The temperature curve: while ice remains it runs flat; a slight climb means the ice is running low, a steep rise that never comes back means it has run out.
- The number of lid openings: more than planned means less time left than planned.
- The outside of the container: if condensation and frost around the base have faded, the inside has started to warm.
Once the curve starts to turn, the lid stays shut and the remaining stops are re-sequenced so the most sensitive product is delivered first; adding dry ice on the road is rarely possible.
The true measure of what is left is the weighing back at the depot: departure and return weights from several runs on the same route give the hourly consumption and the kilos for the next run.
