SOUTH DEVON
Guest
I remember being told during my open water course to turn the air fully on and then turn it back half a turn, which I always do instinctively but I don't actually know why you do this?
Why do you not turn a tank full on ?
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scubasean
Contributor
I recall someone telling me that some valves occasionally stick open if opened to the stop and left there awhile...
I've never seen this failure mode, but can believe it.
I've never seen this failure mode, but can believe it.
If the valve is turned all the way on your buddy may not know if your tank is on or off during the buddy check. It is common for people to get confused.
believing you forgot to turn on your tank they become helpful and turn it off for you. You get a really really short dive from it.
By turning the valve a 1/4 turn back your buddy knows the tank is on by feel. Making sure it is fully on is left as your responsibility.
Hallmac
believing you forgot to turn on your tank they become helpful and turn it off for you. You get a really really short dive from it.
By turning the valve a 1/4 turn back your buddy knows the tank is on by feel. Making sure it is fully on is left as your responsibility.
Hallmac
Don Burke
Contributor
I've had other valves (of similar design) do that. It really sucks.
I back off just a bit, probably less than a quarter turn. It takes all the challenge out of checking the valve open.
RICHinNC
Contributor
It is a good thing to have old people around every now and then.
In "the old days" the machining process of valves wasnt that great and if you opened a valve to the end of the threads.... there was an excellent chance it would leak like a screen door in a sub...
so...you would open the valve full .....then take it back about 1/2 a turn...and in doing that you would prevent possible damage to the valve and any unnecessary leakage.
In "the old days" the machining process of valves wasnt that great and if you opened a valve to the end of the threads.... there was an excellent chance it would leak like a screen door in a sub...
so...you would open the valve full .....then take it back about 1/2 a turn...and in doing that you would prevent possible damage to the valve and any unnecessary leakage.
RICHinNC
Contributor
dang...thats the thing about being old...sometimes you forget to finish your thought.....
with todays great machining process....it really isnt necessary to turn it back....if you do just so you know its on...thats ok....if you do because some old guy said to....its ok too....
my thing is to remember to have my valve lever in the down position so that i have no reserve air but full access to all air....
know what i am talking about???? know what kind of valves i have??
with todays great machining process....it really isnt necessary to turn it back....if you do just so you know its on...thats ok....if you do because some old guy said to....its ok too....
my thing is to remember to have my valve lever in the down position so that i have no reserve air but full access to all air....
know what i am talking about???? know what kind of valves i have??
PhilD
Guest
I'm new to diving, so may be wrong, but I understood you backed the valve off a quarter turn to prevent wear/damage to the stem seal ?
Karl_in_Calif
Banned
Today we teach students to turn their valves open slowly, then all the way.
No more 1/4 or 1/2 turn back.
Modern valves do not need to be turned back at all.
No more 1/4 or 1/2 turn back.
Modern valves do not need to be turned back at all.
JHAM
Guest
I have to admit, I've been told to do this and do. I however never knew the reason. I really never thought to ask.
Good information folks. Thanks.
Good information folks. Thanks.
FredT
Guest
In part it's good standard engineering practice.
WAAAYYYY back in the dreamtime before sport scuba started Navies were about the only ones who trained divers. A "stuck open" valve on a ship can easily cost the lives of every crew member aboard. It is a lot easier to wiggle a valve loose if you can turn it a bit both ways, hence the "close it a quarter turn" rule applies to all stem type valves on a ship. That the navies carried this practice over to all diving and SCUBA valves is not surprising. The fact that a stuck open scuba tank valve is not treated with an abandon ship order, or the flooded sub hitting the bottom for the last time with all hands even in the worst circumstances doesn't make this a bad practice. It DOES reinforce good shipboard habits too.
In fact this is a good practice on all valves except butterfly and ball valves. These have non-rising stems and generally are a quater turn from full open to shut. I once had to have the water service to most of the NW quadrant of Houston shut down for several hours on New Years Day because someone had turned a 16" isolation gate valve all the way open, after a freeze broke 10" diameter branch pipes in the plant where I worked. That extra quarter turn of play would probably have allowed the valve to be freed, instead of having to drain the main distribution line at N Post Oak and 290 to change the valve.
This same thing is why you don't turn on isolation valves under a sink or feeding the commode or water heater full on. When those things need to be turned off, they generally need it NOW, so keeping a bit of slack in them is a good idea.
FT
WAAAYYYY back in the dreamtime before sport scuba started Navies were about the only ones who trained divers. A "stuck open" valve on a ship can easily cost the lives of every crew member aboard. It is a lot easier to wiggle a valve loose if you can turn it a bit both ways, hence the "close it a quarter turn" rule applies to all stem type valves on a ship. That the navies carried this practice over to all diving and SCUBA valves is not surprising. The fact that a stuck open scuba tank valve is not treated with an abandon ship order, or the flooded sub hitting the bottom for the last time with all hands even in the worst circumstances doesn't make this a bad practice. It DOES reinforce good shipboard habits too.
In fact this is a good practice on all valves except butterfly and ball valves. These have non-rising stems and generally are a quater turn from full open to shut. I once had to have the water service to most of the NW quadrant of Houston shut down for several hours on New Years Day because someone had turned a 16" isolation gate valve all the way open, after a freeze broke 10" diameter branch pipes in the plant where I worked. That extra quarter turn of play would probably have allowed the valve to be freed, instead of having to drain the main distribution line at N Post Oak and 290 to change the valve.
This same thing is why you don't turn on isolation valves under a sink or feeding the commode or water heater full on. When those things need to be turned off, they generally need it NOW, so keeping a bit of slack in them is a good idea.
FT
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