If you test it I would stay at a safe distance. Personally I would get
an old propane tank and use that, they come in many sizes and are more
than up to the pressure. Remove the valve and clean it out and either
refit the valve and use a POL to BSP adapter or organise something to go
into the tank thread. My local welding supplier has many old tanks that
they get given by the council when people drop them off at the tip,
they're usually quite happy to get rid of them as they're invariably
tanks from out of the area suppliers and can't easily be used.
I've used PET bottles in this way before and never had one fail at up
to 8 bar. I understand they are tested to about 10 bar before filling
at the factory. My only concerns would be the effects of UV light
over time (not sure if PET is affected) and the possibility of fatigue
or embrittlement, particularly of the screw on lid if that is where
you intend to connect. Perhaps you could screw cut a bigger "lid"
from a block of nylon (or aluminium alloy?) and use an O ring seal.
I'm not sure what the thread is though.
Have a look at water bottle rockets online and you'll find that people
use these bottles in many ways, even cutting and joining two together,
then filling to high(ish) pressure.
Experience with using plastic fizzy drink bottles for water rockets is
that the burst pressure is typically 120 psi plus.
When they fail the shrapnel is less dangerous than for example a steel
container bursting as the fragments are lighter. I still wouldn't
want it to fail right next to me though.
The difficulty may be with how long it will stand these pressures -
bottle rockets are pressurised for short periods and end up getting
lost on roofs. If you only pressurised it while you were working and
kept it away from delicate items (inc you) I expect you'd be alright.
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