Let's clear up some of the information drift in a few areas.
Test pressures
3AA and 3AL tanks are hydro tested to a pressure equal to 5/3rds the service pressure.
So for an AL 80 with a 3000 psi service pressure, the test pressure will be 5000 psi.
Some other common 3AL and 3AA service and test pressures:
3,300 psi (super 80, AL 100, MP 72, etc) service pressure = 5,500 psi test pressure
1,800 psi (some older steel 45s) service pressure = 3,000 psi test pressure.
2,015 psi ( other older 45s, plus the Catalina AL 14) service pressure = 3,358 psi test pressure
2.250psi (steel 72) service pressure = 3,750 psi test pressure
2,400 psi (low pressure 85, 95, 104, etc) service pressure = 4,000 psi test pressure
3,180 psi (PST 65, 80, 100 and 120) service pressure = 5,300 psi test pressure
Exempt or Specials Permit tanks are made to a different design standard and they are tested to 3/2 the service pressure (1.5x the service pressure).
3,442 psi (most E7 and E8 series tanks) service pressure = 5,163 psi test pressure
3,500 psi (Genesis HP tanks) service pressure = 5,833 psi test pressure
The test pressure for these Exempt and Special Permit task is thus slightly less relative to the service pressure - 10/6 of the service pressure for 3AA and 3AL tanks compared to 9/6 of the service pressure for E and SP tanks.
Burst disc pressures
Burst discs are required by law to fail at between 90% and 100% of the tank's test pressure, regardless of whether it is a steel, aluminum, or composite tank.
That however is the burst pressure when it is new. Burst discs flex and they fatigue with fill cycles. General speaking you need to replace the burst disc every time you send the tank in for a hydro test, which is every 5 years for steel and aluminum tanks and every 3 years for composite tanks. That ensures the burst disc will get retired before it fails.
Generally speaking, if a burst disc fails during the course of a normal fill, or shortly after a fill (the two times they are most likely to fail) it's because the burst disc is way past its prime.
That 90% to 100% window is handy as it means some burst disc assemblies can accommodate different tanks.
For example the 5000 psi Test Pressure burst disc assembly used for an AL 80 is one of the most versatile assemblies around as it must rupture between 4500 and 5000 psi. Thus if a burst disc ruptures at 5000 psi, it will work for these service pressures:
3,000;
3,180;
3,300; and
3,442 psi.
The 5,250 Test Pressure burst disc is also hand to have as the minimum burst pressure is 4,725 psi and it will work in:
3,180;
3,300;
3,500; and it is a close match for
3,442 psi tanks.
Both the 5000 and 5250 burst disc assemblies work fine for a 2400 psi tank cave filled to 3600 to 4000 psi.
Fatigue limits matter
Aluminum fatigues with every fill. The good news is that Luxfer tests its aluminum tank designs to the full test pressure for 10,000 cycles. That pretty much ensures that the fatigue life at the service pressure is not going to be exceeded during the useful life of the tank - and despite what some shops claim, that useful life is far longer than 20 years.
But...overfilling a steel tank is frowned upon as it does in fact have a fatigue life.
Steel has:
- an elastic limit - the point to which it can be deformed and return to it's original shape;
- a fatigue limit - the point at which it will return to its original shape, but start to fatigue; and
- a plastic limit - the point at which it will suffer permanent deformation.
For a steel tank, the three limits occur in that same order, although the separation between the elastic limit and the fatigue limit varies based on the steel used and the tank's design. That's important when comparing 3AA and E or SP series tanks.
3AA steel tanks are over engineered and they have the advantage that they effectively have no fatigue limit as the forces they experience at the test pressure are still below the fatigue limit. That's why there's a 20 plus year history of 2400 psi tanks being filled to 3600 psi in cave country with no issues with tanks failing or even failing hydro tests every five years.
What this means for a 3AA steel tank is that provided it isn't rusted to the point of having pits or thinning of the metal, or damage to the valve threads, a 100 year old 3AA steel tank is just as sound today as it was the day it was made. I seen numerous WWI and pre-WWI era welding tanks meet the criteria for and receive a + rating despite being 100 year old tanks.
As an aside, the plus rating allowing the 10% overfill of 3AA steel tanks was a WWII invention to offset a shortage of welding tanks needed for war production. Thus most of the first tanks that were plus rated were al ready well past their first hydro test dates and there is no truth to the claim that a + rating can only be done on the first hydro test. It does however require an additional wall stress calculation and it requires documentation of the Rejection Elastic Expansion (REE) limit. Still, you'll find many RINs who refuse to plus rate tanks or claim it can't be done. In some cases it really can't if the RIN doesn't have factory documentation of the REE, but in most cases the RIN either doesn't know how to do it, or is too lazy to do it.
Now...with cave fills creeping up to 3800-4000 psi the last few years its possible that this extra 200-400 psi in current cave fill is starting to cause fatigue in these tanks, but it's not all that likely. Still....I don't want to see a fill pressure past the test pressure, even if it cools back to 4000 psi. I'm happy with a 3600-3800 psi fill when cool.
E and SP tanks are a different story as they are much less conservatively designed - which is why they are lighter for any given internal volume. Less metal = less weight, but also less strength and perhaps little or no margin between the elastic limit and fatigue limits. I don't know the specifics, but the safe assumption is that the test pressure on an E or SP tank will take a cycle off its fatigue life, so no one filled them much past the service pressure (3600 is pretty common for a 3442 psi tank in cave country and most shops everywhere else will round up to 3500 psi).
Cave fills and burst discs?
There are a couple schools of thought.
One school holds that a burst disc should be used and should reflect the fill pressure, and thus it should have a 5,000 or 5,250 psi test pressure burst disc installed.
The other school holds that a burst disc is a failure point and they advocate using a stainless steel disc that is not designed to rupture.
Then there are the mis guided souls in the middle who think using doubled burst discs is a good idea. It isn't. A burst disc plug in a valve is designed to very tight parameters and when you double the disc you're putting yourself squarely in crash test dummy territory as the two copper discs can interfere with each other and fail at a lower pressure than would a single disc.
The respective schools of thoughts are based on safety in a fire, where a burst disc (even a 5250 psi burst disc in a 2400 psi tank) can release the pressure in the tank before it ruptures, and safety in the water, where the theory is that a burst disc could fail and cause catastrophic gas loss.
In 30 plus years of diving and 20 plus years of technical or cave diving, I'd had exactly now burst disc leak in the water, and it was a very slow leak with a very small bubble every few seconds. I've also had exactly one burst disc fail, and it was a new disc that failed the day it was installed about 30 minutes after a cave fill. It was properly installed and torqued and I suspect it was just a defective disc.
Thus I'm pretty firmly in the "use a burst disc" camp as the in water risk is essentially non existent provided you change them out every 5 years or so.
