I hear you & you're right but that boils down to what you have in the esky ..... cans will retain the cold much better than an esky full of salad ...... I'm simply measuring the air temperature inside the esky as this esky will be used to hold fresh food items like Ham , eggs , salads , vegetables etc .... or for thawing meat that was frozen.
So when I ran the 4 day test the other day - the temp stayed between 4.2 - 4.7 deg C - which I was pretty happy with .
Chris