Dry ice maths for 72 hour lanes

A well packed EPS shipper loses dry ice at roughly 4 to 6 kilograms per cubic metre of internal volume per day. Thicker walls slow it, poor lid seating speeds it up, and a warm dock doubles it for as long as the box sits there.

Take a 60 litre shipper carrying 18 kg of pellets. At 5 kg per day of loss you have 3.6 days of hold time before the last pellet goes. That sounds comfortable against a 72 hour lane. It stops sounding comfortable once you add a 14 hour customs hold, an 8 hour depot wait and a 4 hour final mile in an unrefrigerated van.

Where the margin disappears

• Pellet surface area matters more than mass. Small pellets sublime faster than slabs at the same weight
• Every lid opening at the depot costs about 90 minutes of hold time
• Air freight pressure cycling pushes vapour out of the box, which speeds the loss
• A shipper stored on a sunlit dock loses ice two to three times faster than one in a 20 °C room

Measure the vapour, not the box

Surface probes on the outside of a dry ice shipper tell you about the room. Put the probe in the payload cavity and you get the number that matters, which is the temperature the product experienced.

When the vapour temperature starts climbing off -78 °C, the ice is nearly gone. That climb is the only warning you get, and it arrives four to six hours before the payload crosses -20 °C. On a 72 hour lane with a live probe, four hours is enough time to re-ice at a station. Without a probe, the first evidence arrives when someone opens the box.

A rule that works

Pack for the planned transit plus the longest customs hold recorded on that lane in the last twelve months, then add 24 hours. If the shipper cannot carry that, the lane needs a re-icing stop rather than a bigger box.