Dry ice shrinks even when nobody uses it; it leaves no liquid, its mass simply goes off as gas. Every bit of heat entering the box sublimates some of it, and because the heat of sublimation is about 570 kJ/kg, the daily loss is heat input (watts) x 86,400 / 570,000:
| Heat entering the box | Loss per day |
|---|---|
| 5 W | about 0.8 kg |
| 10 W | about 1.5 kg |
| 20 W | about 3.0 kg |
These are worked examples with assumed heat inputs; only weighing shows the loss of your own box. Insulation reduces it, and the CGA likewise asks for dry ice to be stored insulated to keep sublimation loss to a minimum.
Four weighings are enough to measure it: the net quantity on the supplier receipt, the weight taken from storage before packing, the loaded quantity after subtracting packaging tare, and whatever remains at delivery. Write the time and lot number against each. The difference between the first two is storage loss, the difference between the last two is consumption on the journey; multiply each by the actual unit price on the purchase record and you have the dry ice cost of the shipment.
