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Len, You wrote: >The simple way to look at it is that each molecule has a mass and therefore >a weight in this gravity well called earth. the high pressure tanks hold >more molecules in the same volume, and this volume of gasses weighs more at >the higher pressure (when full) because of the much higher number of >molecules in the same volume. This higher density produces more change in >weight from full to empty, and it results in more of a change in bouyancy >for the high pressure tanks. Your suggestion that we think of each molecule as having a certain mass and weight (two different things) is excellent but you make an initial false premis; The high pressure tanks that carry the same volumes as their low pressure counterparts (genesis 80) have a smaller tidal volume, the idea of which is to reduce bulk and total weight by using a stronger aloy - otherwise what you say would be true. It is the same number of molecules in a "smaller" volume that makes the pressure higher. As you obviously know, 80 cu ft of air at STP (standard temp & pres) continues to weight the same no matter how much you compress it as long as you stay in the same gravity well and do not introduce any other influential gravity wells such as other planets, black holes, or suns passing by within a few thousand miles or so. The "mass" will never change significantly as long as it is not traveling any more than about 70% of the speed of light no matter where it is or what gravity wells it comes under the influence of. Some scooters and some really anxious divers are able to reach up to 50% the speed of light but never notice the miniscule weight change that results. Tanks heat up when filled because you are concentrating all the heat energy in that 80 cu ft of air into a smaller space and the temperature rises. Heat and temperature are also two different things so that there can be clouds of gases in space that are at thousands of degrees temperature but the molecules are so spread out that they would not (at their present volume) burn you because there is not enough total heat energy transfered by conduction from the fewer molecules that are in contact with your hand. By the same principle you can tolerate a much higher temperature of air on the skin than of water because of the density differrence. More molecules with the same heat energy vibrating against your skin. So ! A tank containing 80 cu ft (measured at 1 ata) carries the added weight of 80 cu ft of air (at 1 ata) no matter how small the tidal volume of the tank is. And compressing the same volume of air into a smaller volume tank will "not generate" more heat but will raise the measured and felt temperature because more molecules with the same unchanged heat energy are banging against the side of the tank. Heat energy resides in matter so that concentrating matter also concentrates the heat it contains. (Yes, I'm sure there will be some "small" amount of energy transfered to the gas by the mechanical action of the compressor) The US divers chart is dead wrong and it is no surprise that so many of this highly critical and observant bunch caught it at first glance. God ! we're good ! Chuck -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send list subscription requests to `techdiver-request@aquanaut.com'.
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