What is the system called in which the first stage is screwed into the cylinder valve?

The system in which the first stage is screwed directly into the cylinder valve is known as the DIN system. DIN stands for Deutsches Institut für Normung, which is a German standards organization that developed this design. The DIN connection typically uses a 300-bar thread pattern, allowing for a more secure and higher-pressure connection compared to other systems. This design is advantageous because it reduces the risk of accidental disconnection and provides a more streamlined profile for divers, which is particularly beneficial in technical diving and when using high-pressure systems.

The Yoke system, while also a widely used method for connecting regulators to tanks, involves a clamp mechanism that secures to the valve via a notch rather than threading directly into it. The hydrostatic term refers to testing methods rather than a connection system, and the "K" option does not pertain to standard scuba equipment terminology. Understanding these connections is critical for ensuring proper equipment compatibility and safety in diving practices.