Unfortunately you're trying to counter an irrational argument with rational data. It's like trying to explain to someone that has a fear of snakes that snakes aren't lying in wait to attack them every time they step outside. Same with sharks.
I'd be interested to hear about her experience with 200 psi failures in manufacturing. What caused it? Was the equipment inspected? Was the equipment being used correctly and designed for the application it was being used in? Sometimes in manufacturing products haven't completed the testing stage and fail during that testing. Other times there are equipment failures because of improper maintenance or unintended use. Nothing designed and tested for an operation just "fails" without reason. Remove the reason you remove the failure. This includes scuba tanks. Someone with a background in manufacturing should understand that.
Your tanks have passed the testing and inspections. Multiple times I would guess. The fact that they passed should prove them safe from about everything but stupidity. They are safe to be filled, stored, transported, and used as dictated by their design parameters. Again, if she has a background in manufacturing she should understand that.
Honestly, at this point, I'd just fill the tank and store it safely as planned and deal with the repercussions. Either that or get rid of of the tank and accept, and thereby, reinforce her irrational argument.
Bingo. I'm not a clinical psychologist, but I
am a psych prof who studies emotion: this is an anxiety issue.
When we feel anxious, we can reduce that anxiety in a number of ways. But for many people the easiest most obvious way is just to avoid whatever it is that makes us anxious. Anxious about elevators? Take the stairs. Anxious about planes? Don't fly. Anxious about scuba tanks? Don't let your partner bring them in the house/the car/etc. You get the drift...
The problem is that the very WORST thing you can do when anxious is
avoidance. Because avoiding the source of anxiety does work as a sort-of (temporary unsustainable) stop gap - but it doesn't actually make you less anxious. It just prevents you from having to deal with your anxiety. And, in the meantime, because you are avoiding whatever it is that makes you anxious, you have no opportunity have positive or even neutral experiences with it, which means that over time the anxiety gets worse.
This is why exposure therapy is the gold standard treatment for specific anxieties and phobias. I'm
not suggesting scuba tank exposure therapy for your partner - but I do agree with others that facilitating her avoidance (by keeping tanks out of the car, house, etc) is unlikely to reduce her anxiety, and indeed likely to make it worse in the long run. At the same time, arguing facts with her isn't going to do a thing; anxieties like this - by definition - aren't rational.
If there's a way to involve her in the scuba tanks - having her go with you to get fills etc - that would be ideal, as long as those repeated experiences are positive or least neutral. Even just the experience of riding with them half-filled in the car, or sitting in the house with nothing happening. It's not going to happen overnight but repeated "testing" of her anxiety where she can see through personal experience "see the scary tanks were here and nothing bad happened" is ultimately how anxieties like this subside.
