Screw it, how hard can it be? The Importance of Correct Tank to Valve Compatibility

SAGE SULARZ   Jul 19, 2023

Understanding your gear is one of the most valuable skills a diver can have. As technology advances, new options come onto the market and older models should be carefully examined to ensure function and compatibility. A recent tragedy involving the death of a European diver resulted from a simple valve-to-cylinder mismatch. Knowing the facts could prevent potentially dangerous situations from arising on your own future dives.

 

Prior to 1958, U.S. SCUBA cylinders were most commonly ½ inch National Gas Tapered (NGT) threads. Over the next decade this slowly transitioned to use of parallel ¾ inch National Pipe Straight Mechanical (NPSM). The next major change occurred in 2006 when International Standards Organization (ISO) cylinders were accepted by the U.S. government. Today a diver is most likely to encounter either the ¾ inch NPSM commonly with 14 threads per inch or the M25x2- ISO with 12.7 threads per inch when diving in the U.S.

 

Why am I giving a history lesson on your dad’s dive gear? The threading style that emerged in the early sixties is, as stated before, still one of the most popular options for cylinders and is often incorrectly compared to its metric cousin. While many threads such as the previous examples might seem interchangeable upon initial attempt, a valve with a metric thread will ultimately require much more force to tighten into a visually similar cylinder. This tank will likely not seal correctly. It might comfortably fit 3-5 threads before starting to bind and using force to continue results in cross-threading. This is the case with crosses between NPSM, ISO, and British Standard valves and cylinders. Improperly sealed tanks can pose an explosion risk.

 

Over tightened and cross-threaded cylinder outlets can have catastrophic and potentially deadly outcomes. Tank setup is done most safely by a certified technician who understands all varieties of cylinder and valve threading. Waterfront Diving Center is available to assist on all your gear needs and questions and perform visual and hydrostatic testing on most compressed gas cylinders.

Mark A. Gresham and Francois Burman. “Mismatched Scuba Valves to Cylinder Outlets.” Divers Alert Network, 7 June 2023, dan.org/alert-diver/article/mismatched-scuba-valves-to-cylinder-outlets/.