“Scuba tanks are tested (in not america) for twice their pressure limit (nb, not until they explode) and that is quite arbitrary from a perspective.“
It is more complicated than that. Tanks are periodically tested to 2x, but they are designed to withstand much more, probably 4x, and tested to that at the factory. When I designed hydraulics, they were tested to 2x and designed to 4x.
For these lines, the weight rating is where it is designed to break with a static load. I looked up the specification for 500lb paracord and it requires it to withstand 90% of 500lb when tested at the factory i.e. if it breaks at over 450lbs, it passes. As soon as it leaves the factory, it degrades due to wear, up, knots, moisture, etc. Dynamic loads are multiples of static loads.
And then a business needs to add an extra safety margin because some customers will abuse your product and then complain when it fails. That is why making your own gear is so satisfying - you can make it to your needs and if it starts to wear prematurely, the next iteration will be better.
(I confess that I have made a suspension with dynaglide as an experiment and have not had a problem.)
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