Just wanna throw it out there, as someone who has studied hydraulics and pneumatics and spend a lot of time working with them, if that small one goes off bang either on tour gun, in your rig or under your bed you are going to die, itlllitterly tear you apart and even if it doesn't, the sudden spike in air pressure will probably kill you. Regardless of if its exempt from hydro tests or not I would not compress it with anything that doesn't have an air drier or cooler and I'd replace it every two years minimum. A lot of people get very seriously injured and many have died as a result if poorly maintained pressure vessels, it's really not worth the risk.
Exemption from hydro testing does not mean the cylinder has not been manufactured to standards which require samples to be tested to destruction.
A Pi marked 3000psi compressed air cylinder will be designed to hold 3000psi (just over 200bar) and to meet that standard must be capable of holding 4500psi (just over 300bar) without failure or expansion beyond specified criteria.
The regulator must be fitted with both high pressure and low pressure burst disks, which act as a safety fuse.
If the pressure exceeds the burst disks designed point of failure then the disk fails and excess pressure is safely released. If / when the burst disk degrades over time due to repeated pressurisation and depressurisation it will safely fail and release pressure.
It will be a brown pants moment with quite a bang, and a 'loose' cylinder will spin around on the ground. 99% of people will run away. I've been there, and among those who moved away.
This will not involve a cylinder explosion
A burst disk will fail due to:
1) Excessive pressure
2) End of life
3) Contamination (Dirty air)
Dirty air includes not using a 'breathing standard' compressors, or nipple cover and ending up with dirt on the nipple then blowing it in when filling etc
HPA cylinder threads are also tapered an designed to vent rather than fire the regulator if you manage to unscrew a pressurised regulator.
Older CO2 cylinders did not have the taper and there was the potential to cause the valve to fire off the cylinder.
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