[Frank Neuperger]

Update, in desparation we ordered a bladder from Sportogo. They said they had one real FIRST bladder (possibly a sample to compare to thier own) and we got it shipped. I had them weigh the FIRST bladder and the $30 bladder and they indicate that they both weighed the same.... 4 7/8 lbs). We will definitely weigh it when it shows up on Monday. With 1.5 lb cloth skin, this does not add up to 7.2 lbs. Could be a scale calibration issue at Sportogo or the air mass issue mentioned below.
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Garry, good point on the mass of air. Lets see how the numbers shake out. From some experience with inflatable stuctures, gut tells me that it is less pressure than 5 psi. Pressure vessel methodology for calcs.

For such high pressure (5 psi) with that large diameter the tensile forces in the skin would be really high. Hmm .... lets see a one inch wide stripe with a diameter of 40 inches would have an area (area that force acts on in one dimension) of 40 sq inches. Think of this as side view of a barrel hoop. 40 sq inches wth 5 psi is 200 lbs that is supported on 2 sections of the band. So the "band tension" in a one inch strip of material is 100 lbs. This sounds like a lot of tension in that material. Think of the bladder when underinflated by say 1/2 inch and not pressing on the cloth skin. Would a 1 inch strip of that foamed PVC hold 100 lbs? Gut says no. Looking at it another way, it is the same skin tension you would have in a 10 inch diameter ball filled to 20 psi. That is one very very hard volleyball at 20 psi. Gut says 1 psig max. Mass of air is 1.19gm/liter ball is conveniently 1 m dia so 4/3*pi*r^3=vol = 0.524 m^3 =524 liters = 623 gm = 1.37 lbs mass at 1 atm. Assume 15 psi =1 atm. Even at 5 psig (20 psi abs). you have 1/3 of 1.37 lbs = 0.45 lbs net weight. With 1 psig you have 1/15 of 1.37 lbs net weight= .091 lbs net weight.

We will update thread when we get the new bladder.