Trident Rubber Tank Valve Cap. This protective cover for scuba cylinder valves fits standard yoke scuba cylinder valves, not DIN scuba valves. The cover helps prevent impact damage to valves. Helps keep dust, dirt, and water out of scuba valves while preventing loss of O-ring. Valve cover is useful for identifying cylinders and is also the standard to let boat and dive operators know if your cylinder is full or empty. Place cap over valve when full and leave off when you need to fill. Cap comes with white string for attaching to tank.