Yoke Expansion and O-Ring Service Life

Research has shown that a yoke will move slightly away (expand) from the o-ring seal on the face of a scuba valve when pressurized.  This is normal and expected.  However, if the amount of expansion increases beyond a normal amount, it can reach a point where the service life of the o-ring seal can be severely reduced or a potentially life threatening situation can occur due to o-ring blowout.  The amount that a yoke will expand under pressure is based on a number of factors including:

Maximum pressure rating of yoke

Yoke design

Stress corrosion cracking or fatigue fracture on a yoke

Service pressure of the valve/cylinder

The following operational guidelines should be adhered to avoid excessive yoke expansion:

• Only use yokes that are rated to or above the maximum service pressure of the valve/cylinder.  Thermo K type valves have a maximum service pressure of 3300 psi and Thermo PRO type valves have a maximum service pressure of 230 Bar or 3442 psi @ 70° Fahrenheit.  Some yokes have a maximum service pressure of 3000 psi and cannot be used above 3000 psi.

• Always inspect fill and regulator yokes for damage, scratches, corrosion, etc. on every use.  Yokes should be checked regularly for signs of stress corrosion cracking or fatigue fracture.  The first sign of stress cracking may be seen by chrome plating becoming damaged.  Check yoke knob screw threads for excessive play.

• Never fill a cylinder or use a valve above the maximum service pressure of the yoke!

• Always fully tighten a yoke to the valve.  If a yoke is only partially tightened, there is already some expansion because of the loose connection and this could then increase to a dangerous amount under pressure.

• On Thermo K type valves, never exceed 3300 psi during the fill cycle.

• On Thermo PRO type valves, never exceed 3442 psi @ 70° Fahrenheit during the fill cycle.

• Always open cylinder valve and fill valve slowly.

• Inspect valve o-ring for damage on every use.

• Only use manufacturer's recommended parts.

Remember that it is normal for a yoke to expand away from the valve under pressure.  When valves are used at the top end of their service pressure ranges (3300 psi for Thermo K type valves and 3442 psi for Thermo PRO type valves), the amount of expansion is also at it's most.  Following the above guidelines become more critical as the pressure increases.

Thermo Valves Corporation,  Santa Rosa , California