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The quarter-turn is no myth; it is a standard industrial and marine = practice as already pointed out by Jim Cobb. As Jim mentioned, some = valves were designed with a "backseat" (not like a car) which sealed the = packing materials from the system pressure and fluid. In this case the = valve was opened all the way against the basckseat. These types of = valves had their own special set of problems because of this, one of = which was sticking. A valve opened fully generally has the back of the = valve "disk" against the housing. This can cause the valve to stick in = this position if there is thermal differences within the valve and = external to the valve (think steam) or due to the valve materials. The = materials of construction of the valve stem and housing are generally = different and have different coefficients of thermal expansion (i.e. one = material would expand more than the other at the same temperature = change). A valve turned back 1/4 of a turn eliminates this problem. Thermal mismatch generally would not cause a scuba valve to stick open. = It would be more than likely due to the physical interaction of the = parts in the valve being squished together. Think about it, the valve = mechanism is nothing more than a threaded nut and bolt arrangement, = although it may not be readily obvious. When you crank the valve all the = way open you are stretching the stem (same as preloading a nut and bolt) = as a result of the applied axial force and modulus (stiffness) of the = stem material. It may not be as easily to separate the parts when you = try to close the valve due to surface friction between the two parts = (remember the dynamic friction coefficient is generally lower than the = static friction coefficient). 'May also have a bit of galling between = the parts. A valve turned back 1/4 turn will eliminate this problem. It is standard practice to check a valve position by turning in the = closed direction rather than the open direction. If the valve is = closed, then checking it in the closed direction will do nothing (not = let any fluid pass). However if you check it in the open direction, = then you will momentarily permit some fluid throught the valve. Big no = no! Valves do stick when they are cranked all the way open; its a fact = of life. If one properly checks a valve open or closed by turning in the = closed direction and it does not move because it is stuck, then he or = she may be mislead into believing the valve is closed. At this point he = or she attempts to open the valve which is already open and sticks it = harder or breaks off the stem. I can tell you a good story about this = situation on a nuclear submarine just out of the Guam (did not involve = the reactor). A 1/4 turn from fully open eliminates this problem. Some scuba valves (crap like the Catalina for example) have the stem = o-ring "exposed" so that when you crank the valve open it "smashes" the = o-ring. Do this enough times and your tank will leak (while I would not = own one of these, I have repaired countless numbers). If this type of = valve is fully opened and someone checks it by cranking open, well it = will wear out quicker. Gently opening your valve fully and closing 1/4 = turn will eliminate this problem. And last, if your dive partner is too stupid to negotiate a valve = position check involving checking in the closed direction, back fully, = and then 1/4 turn closed again, then follow rule one, don't dive with = the idiot. 'But then again there are a lot of idiots out there. I always = tell my partners if they think my valves are not right when they check = them then tell me before they make any command decisions on valve = manipulations. I want to be part of the process. I have yet to have = someone turn off my gas. 'But then again my partners don't get confused = so easily. I also check all regulators after the valve checks but prior = to submergence. In addition the 1/4 turn closed does not (for all = practical purposes) affect the flow coefficient of the valve. The = argument that it reduces the flow from the tank is senseless, has no = practical basis because the valve is permitting alot more flow than the = regulator. As far as being confused as to the proper direction of closure in an = emergency if it was 1/4 turn off of fully open, give up diving if you = can't figure that one out. Why worry about it because you're dead = anyways. The people who tell you the 1/4 turn closed valve check is = bullshit will also tell you the world rotates in the wrong direction. = They would also tell you that teaching buddy breathing is not necessary = since everyone has an octopus. So much for diver skill. =20 Of course this is just my opinion, Doug Chapman
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