Deep-Six Underwater Systems, Inc.
"Add Depth to Your Life"

Table of Contents

1 Pressure and Gases
2 The Face Mask
3 The Snorkel
4 The Fins
5 Weight Systems
6 The Knife
7 The Wetsuit
8 Pressure and Water
9 The Ear and Pressure
10 The Sinus and Pressure
11 The Stomach/Intestine and Pressure
12 The Lung and Pressure
13 Barotrauma caused by External Air Spaces
14 The Buoyancy Compesation Device (BCD)
15 The Scuba Cylinder
16 The Scuba Cylinder Valve
17 The Regulator
18 Density and the Diver
19 The 4 Gas Laws
20 Hand Signals
21 Carbon Monoxide Poisoning
22 Hyperventilation
23 Nitrogen Narcosis
24 Diver's Flags
25 Sound Underwater
26 Color Underwater
27 Decompression Sickness
28 Breathing Oxygen
29 Deep Diving
30 Thermoclines
31 Thunderstorms
32 Underwater Life
33 Open Water Dives
34 The Final Examination
35 The Environment
36 Advanced Course

16 - The Scuba Cylinder Valve

     The valve on the top of the scuba cylinder is the weakest link in the system. Most tank "incidents" resulting in leaks of air, damage, tank launches, injuries, and death are the result of tank valve misuse. Letting the tank fall on the valve, unscrewing a valve from a tank that is under pressure, not turning the valve on all the way prior to the dive, and allowing the tank to roll around while being transported are but a few of the reported "incidents."

     The valve consists of a knob to turn the tank on and off, a thread of sufficient length to safely keep it in the pressurized tank, either teflon tape or an O ring to keep the air from leaking around the threads, a dip tube, an over-pressure plug, and a hole surrounded by another O ring to make a seal with the regulator. Some valves have reserves on the side opposite the on-off knob.

     The dip tube is there to prevent any particles in the tank from getting into the regulator when the diver is head down. The snorkel sticks into the tank about 2" allowing some room for avoiding things that should not be in the tank in the first place!

     Prior to putting the regulator on the tank it is a good idea to let a small amount of air escape through the valve. This insures no foreign particles get into the intake of the regulator. The regulator is then put on the valve so the holes line up and a seal is made with the O ring. The valve is turned on all the way prior to diving. Do not turn it back a quarter after opening the valve! In the past, it was thought valves could get stuck in the on position if they were not turned back a quarter of a turn. This does not happen. But, a dangerous thing has happened. When a buddy check was done sometimes the buddy would turn a valve the wrong way (to off) and then back a quarter. This would allow the air to enter the regulator and give the false impression it was fully on. While the tank was full, the high pressure could run by the small opening to supply the diver's needs. When the pressure dropped in the tank, not enough air could pass through the opening, especially at depth where more air is needed, and the diver would "run out of air."

     Divers today keep aware of the amount of air in their tank with the submersible pressure gauge (spg). In the early days of scuba there were no spg's. Divers either judged how much air was left in the tank by experience and guessing or went by time under. There were reserve valves to give a warning when the tank pressure was getting low. A reserve valve restricts the air supply when the tank pressure was 300-500 psi. When the restriction was felt a lever was pulled that moved the reserve valve into the second position. Then the air flowed easily and the diver was warned to begin to surface. The spg is safer. Sometimes the dive took place with the reserve valve already pulled so there was no extra air after the restriction was felt. Surprise! Sometimes they malfunctioned because of improper tank-filling procedures. Today we dive with the reserve lever down so there is no reserve!

     The early US Divers catalog listed tank valves for purchase. Everything was in alphabetical order. "A." might have been a regulator, "B." might have been a face mask, etc. The letter "J." was followed by the reserve valve name and picture. The letter "K." was followed by the valve that had no reserve. Today  the reserve valve is called, "the J Valve," and the non-reserve valve is called, "the K Valve." It's that stupid!

     Prior to removing a gauge, regulator, etc. from the tank valve it is very important to be sure all the air has been removed after turning the tank off. Gauges have a knurled nut to do this. Regulators may be bled of air using the purge button on the front of the 2nd stage. Failure to let all the air out will make turning the knob for removal next to impossible. If the knob is forcefully turned, eventually the O ring making the seal will blow out creating a very loud pop and probably tearing the O ring.

     When cylinders are rated above 3000-3300 psi the standard tank valve may be inadequate because of distortion due to the high pressure. Another type of valve, the DIN valve, is used instead. The regulator is actually screwed into the valve. There is an o ring that makes contact with the valve when it is seated. Because there are threads holding the regulator into the valve there is a stronger contact.   

Copyright Information about this text, DIVING WITH DEEP-SIX is as follows: Copyright 1996 - 2007 by George D. Campbell, III; President. All Rights Reserved. This file may be posted on Electronic Bulletin Boards for download, but may not be modified, printed for distribution, or used for any commercial purpose without the author's written permission. is using this material with the permission of Deep Six. The full version is available at:
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