Filling the air venturi wingman 74

Stewy,

Yes, that is correct. The tank valve is both input and output. 

A common practice is to attach your compressor out put hose ( also referred to as a line or fill whip). Some whips have a female Foster quick connect on the end. In that case you will need an adapter that would have a male threaded DIN 300 on one side and a male Foster quick connect on the other side. 

Hopefully your compressor has an output gauge that lets you know the pressure in the whip when its connected to your tank , but your tank valve is NOT open yet. Once the whip pressure gauge is pretty much at the same pressure as your partially filled tank, then you open the tank valve to begin topping off your tank. 

If you are filling an empty tank you can open the valve before starting your compressor. 

If you’re filling at a dive shop they will have the proper fill attachment. If filling at an FD, you might need an adapter to convert from DIN to SCBA.