Been wanting to build this for a while.
Its simply 2 mini digital panel meters with calibration pots hooked up to a CR123 3.7v battery.

Housing is made from 24mm ID 30mm OD acrylic tube as is end cap.
Sealing is achieved with 3 O rings.

With a 880ma cell I calculate run time is about 52 days so I have done away with a power switch. Will simply put in a fresh cell and run it for a few weeks at a time. I guess it will need calibrating more often than a fresh battery.
It also has the option of back lighting too but I have left that off for now.

It now needs a cable fitted that will go directly to a O2 sensor ( or 2) for analizing nitrox.
But my real aim is to build a sensor housing that will ft inside the counterlung of my IDA 71 rebreather. I will make one that holds 3 O2 cells and have the 3rd cell monitored of a VR3 computer.
Having some issues with my lathe at the moment but once that is resolved I will start that part of the project.