Test 2: ¼-lb of Ice per Liter, 12 Hours
For my second test, I pre-treated the coolers with ice so they were the same internal temperature. I then loaded the coolers with a quarter-pound of ice per liter of volume, then left them in the sun for 12 hours, from morning until night. The weather reached a high of 88 degrees that day and the humidity hovered around 60%—fairly pleasant (but still toasty) late-spring weather in the South.
The results were a little more even this time, showing all of the coolers in much more of a dead heat than the first test.
To measure raw insulation power, I simply divided the weight of the remaining ice by the weight of the initial ice. As before, the IceMule still won out, retaining 67% of its original ice versus a low of 44% for the Orca. The OtterBox and Yeti pulled in good numbers with 62% and 57%, respectively. The thin and light HydroFlask put up a respectable 48%.
But because I was specifically testing backpack coolers, I also wanted to measure insulation against portability. To that end, I divided the coolers’ ice retention by their density (dry weight divided by volume), which rewarded lighter, bigger coolers.
Once I balanced weight and insulation, the winner was the svelte, sleek HydroFlask with a 3.51 rating. (It weighs only 3 lbs dry, but holds 22L). The IceMule, OtterBox, and Yeti all came in a close second with numbers averaging around 2.70. The Orca, the smallest and most lightly insulated cooler, came in last with a rating of 1.67.
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