Also I look at it like this. If the manufacture is labeling the product with a 400 or so PSI rating, then I imagine that to keep litigation to a minimum then they have tested to that pressure (and I bet 10-15% higher) and have found it safe. It doesn't matter what is creating that pressure (water, air, propane, fart gas)a fluid is a fluid and has the same dynamics (most of the time). Plus if we're talking about strength then why don't car manufactures make parts out titanium? Because steel is just fine under the circumstances and cost significantly less, balancing cost and benefits.
