How many hours a thermal bag stays cold depends as much on the mass inside, the starting product temperature, the outside air and the number of zip openings as on wall thickness. WHO guidance on transport containers notes that the load with the lowest thermal mass is the most susceptible to temperature change; a half-full bag warms faster than a full one with the same product.
- Pick the longest route and the hottest hours; minimum and maximum loads are separate trials.
- Place the logger among the products and keep it off the ice packs; a second logger reads the outside air.
- Open the zip at every delivery as often as you really do, and put the bag where it sits in the vehicle every day; if that spot gets sun, the trial runs in the sun. Time runs from the moment the bag is closed to the moment it is opened on arrival.
The hour at which the record leaves the target range is the limit for that pack-out, and the route is planned well below it. When fill, ice-pack layout or season changes, the trial is repeated.
