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Re: CO2 cylinders

     >Hydro is short for hydrostatic test.  A cylinder/bottle is
     >filled with water and pressurized to a given pressure to see
     >if integrity is still within safe limits.  I do not know
     >what the pressure the bottles are taken to is.  Water is
     >used because it does not compress well, thus if a rupture
     >occurs, the bottle will crack or just leak, not violently
     >explode.  Most privately owned gas cylinders must be tested
     >every 12 months to comply with DOT/state/fed regulations.
     >Many "filling stations" won't fill a bottle that is "out of
     Cylinders are tested by filling them with water and pressurizing them 
     to 5/3 of their rated maximum pressure (the pressure is stamped on the 
     cylinder after the DOT specification number).  For example, a cylinder 
     rated at 3,000 psi would be tested at 5,000 psi.  At the testing 
     pressure, the cylinder expands slightly.  According to DOT 
     regulations, a permanent expansion of 10 percent or more of the total 
     expansion at the testing pressure indicates that the cylinder is 
     unsafe for use and must be destroyed.  Typically, cylinders that fail 
     have several holes drilled into them by the tester so they cannot be 
     used again.
     DOT (Department of Transportation, a Federal agency) requires 
     cylinders to be hydrostatically tested every FIVE years.  Most 
     ruptures occur during refilling, so a wise supplier will not fill a 
     cylinder with an expired hydro.