Dry ice goes in to absorb the heat that comes through the container walls; cooling the product is not its job. So the kilos follow from the container's hourly consumption and the worst-case duration of the trip, not from the weight of the product: the amount needed is hourly consumption times hours.
Hourly consumption is found once, by weighing your own container:
- Prepare it with the real payload, add a weighed amount of dry ice and close the lid.
- Weigh it again every few hours; grams lost divided by hours elapsed is that container's hourly consumption.
- Summer and winter, block and pellet are measured separately; pellets expose more surface and run out faster.
A worked example
In the FAA's 2006 measurement, pelletised dry ice in a foam box of about 3.3 litres inside with 38 mm walls lost around 45 g per hour on average at 22-29 °C. If delivery is planned for 18 hours and the worst case with traffic and dock waiting is 26 hours, the requirement is 26 × 45 g, roughly 1.2 kg. On a multi-drop route lid openings raise consumption, so the weighing is repeated on that route.
Container volume enters the calculation through surface area, because heat passes through the walls. When the same insulated box doubles in volume, its surface and consumption rise about 1.6 times; doubling wall thickness roughly halves the heat coming through. These ratios are a first estimate; the scale gives the exact figure.
