An ice pack can overcool a box locally. One air reading at the box centre may show the average inside the acceptance band while the product surface beside a pack, a lower corner or the space between two cold sources is colder. For freeze-sensitive goods the lower limit is as real a design condition as the upper limit.
A eutectic plate is not a thermostat
GEBHARDT asks for its eutectic plates to be frozen completely at least 5 °C below their melting point: -8 °C for a -3 °C plate, -26 °C for a -21 °C plate. A plate therefore enters the box below its melting point, fully solid, and until it starts to melt it can pull the package surface next to it below that point. The melting point is the temperature of the phase change, not the temperature at which every point of the product surface will be held.
Where the cold spot is looked for
The area right beside a plate, the package surface touching a pack, lower corners, under the lid and product points between two cold sources are measured separately. A sensor in the centre air does not represent the contact zone, so sensor positions for product, package surface and box air are defined separately on a layout drawing. Separator material, thickness and clearance are tested against the product packaging, plate model and box geometry; cardboard or sponge is not the same barrier in every box.
| Test area | Check | Record |
|---|---|---|
| Product | Freeze sensitivity, starting condition, packaging | Product and lot, starting temperature |
| Cold source | Model, preparation, full solidification | Identity, preparation record, position |
| Load | Minimum and maximum fill | Mass, sequence, contact and clearance |
| Ambient | Hot and cold profile, dwell | Profile, duration, lid openings |
| Sensors | Product-adjacent, remote, surface, air | Identity, position, acceptance band |
| Result | Warmest and coldest representative points | Curve, deviation, decision owner |
Adding packs strengthens protection on the warm side but enlarges the cold-spot risk beside the product; removing packs enlarges the upper-limit risk. So pack count and position are set by testing at minimum and maximum load, in hot and cold ambient profiles and with real lid openings. A new separator, pack count, load or route does not make the old test valid.
WHO's supplement on qualified shipping containers recommends, for pharmaceutical transport, testing minimum and maximum loads and hot and cold profiles, and documenting the coolant conditioning regime and the locations of temperature monitors as part of qualification. It covers pharmaceutical containers; the same logic is adapted to a food box trial (WHO shipping container qualification supplement).
If the lower limit is exceeded, the affected load is set aside and the decision follows the product instructions.
For measurement and records, see proving the cold chain; for contact and separators, see what goes between an ice pack and the product.
