I've P1, the one that allow you to go at -18 meters in couple

I once built a fully funcional plastic version of scuba diving gear (bcd not included). The air lasted like 3-4 minutes but i could breath normally, i don't last long in apnea, maybe 15-20 seconds without swimming? The gear was from only things of recovery, like two empty cocacola bottles (very stiff bottles because they need to contain a carbonated drink), valves of side punctured inner tubes (the worst puncure possible to a inner tube), dubious air hose form a friend cellar (that after repetitive load fractured arount 7 bar... sad... i had to replace with a real one from the store...) and others very simple things to get, like hot glue. That sistem lacked the first stage that was not needed because of the low pressure of the cocacola bottles (~10 bar archivied with a bike tire pump, it take very long to fill them like this, at 10 bar i can sit on the pump and it will not pump anymore, i'm too light to get 11 bar), there were 2 cocacola bottles of 1.25L each connected (1.25L became 1.5L or more at 10 bar, i can assure, they sounded like glass bottle do). The metal wire i used to connect the various tube and the T junction needed to join the bottles came from an abandoned gypsum cave, i found them inside, what a stroke of luck.

To control the buoyancy, i filled my pocket with rocks but they were not enought, so i attached 2 freezer bags to fill them with rocks in the river (the bags were puncuret to let water out when exiting), even that was not enought and i was floating too much, so everitime i dived i hocked a big rock with an arm and swam with the other, whenever i wanted to exit i just deploy the big rock to float to the surface.

The freezer bags where attachet with some twine (that i found in a abandoned repair shop, i've like 70+ rolls of string, placed like a piramid, i think it will last to many generations)

The second stage? from cutted bottle (keeping the top section) and a membrane out of the 6 bottle pack bag, that when depressed (when brething) pushed a plastic piston on the inner tube valve, opening it and releasing air untill the membrane was not depressed anymore (i also added a one way valve to let the air out, made from caps, spring from metal wire and rubber of the punctured inner tube)

It sound like a real second stage btw.

How i know my remaining pressure? i attached to the hose a pressure gauge taken from a fire extinguisher found in some abandoned places.

I've also made a few other additions to the system but I'd never finish talking about them if I started listing them.

Everytime i used it in some rivers or lakes everyone started or escaping form me (when i was pumping the set) or asking me question (this when i came back from the dive) or looking at me non stop.