The big differentiator that you'll hear a lot about as you shop for a cooler is ice retention -- specifically, how long a cooler can keep a full load of ice frozen (melted ice, a.k.a. water, isn't as good at keeping drinks cold). The new, expensive options all hang their hat on this test, with roto-molded coolers specifically designed to ace it (and in doing so, to justify their price tags).
That's all well and good, but I worried that a standard ice retention test on its own wouldn't tell us the whole story. Sure, some coolers would probably keep the ice frozen for a lot longer than others, but using the melting point as your metric seems to disregard everything that comes before. I wanted to get a good sense of performance, not just after days but in hours, before any of the ice had even melted.
To do that, I started with a modified version of the ice retention test. Instead of a full load of ice in each cooler, I went with an amount of ice equivalent to 10% of each cooler's total volume. (I already have a precise measurement of each cooler's total volume from the earlier described capacity test.) Less ice meant more of a challenge for the coolers, which would hopefully give us a more granular look at how well they perform relative to one another.
Specifically, I wanted to track the ambient temperature in each cooler, so I spread the ice in each one I tested beneath an elevated jar of propylene glycol solution (watered-down antifreeze) with a temperature probe in it. Why elevated? The temperature down in the ice would have been roughly the same in all of the coolers, leaving retention as the only real variable. Tracking the ambient temperature up above it was much more telling, and it gave us some additional variables to consider.
Oh, and I did all of this in one of our appliance lab's climate-controlled test chambers, and I made sure to let each cooler sit open in the room for several hours beforehand to ensure that they all started at room temperature (about 80 degrees Fahrenheit to emulate a good outdoor summertime temp).
In the end, it turned out to be a fruitful test. After 48 hours (72 hours for the largest coolers), I had a nifty graph showing me the temperature inside each cooler on a minute-by-minute basis -- and the difference from cooler to cooler was striking. To help put this data in perspective, I broke down the coolers into separate size categories after peeling soft-sided coolers into their own category. That left me with small coolers (less than 40 quarts), midsize coolers (40-59 quarts) and large coolers (60 quarts or more). You can find the graphed data for each of those categories below, as well as our performance data on soft coolers (again, you shouldn't expect a whole lot from coolers like those).
Enlarge Image Steve Conaway/CNET
Enlarge Image Steve Conaway/CNET
Enlarge Image Steve Conaway/CNET
Enlarge Image Steve Conaway/CNET
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