So... the surface area of the lid inside the seal on a 45qt cooler is going to be about 200 sq. in. Air pressure at 11k ft is about 9.71PSI and air pressure at sea level is about 14.7PSI (difference of 4.99PSI). From the natural gas law, the change in pressure in a sealed container is proportional to the change in temperature in degrees kelvin. So if you opened your cooler at 15.5C (60F), the temperature stabilized, and then the temperature inside dropped to 4.4C (40F) after you sealed it you would have an additional pressure drop of .372PSI. So Going from 11k ft and 60F inside temp to sea level and 40F inside temp results in a pressure differential of about 5.36 PSI from inside the cooler to outside the cooler. Remember that Force equals pressure multiplied by area so that lid would theoretically take close to 1100 pounds to open if pulling straight up from center. You have approximately 2:1 mechanical advantage pulling at the lip so make it 650 pounds applied to the lip of the cooler to open.
Next time just use the drain. The surface area of the drain valve is so small plus you have mechanical advantage from the thread so it will open easy and equalize pressure.
Next time just use the drain. The surface area of the drain valve is so small plus you have mechanical advantage from the thread so it will open easy and equalize pressure.