Hydros are designed to test the tank's modulus of elasticity. All materials react to stress with either elastic or plastic deformation (strain). Elastic deformation is temporary while plastic deformation is permanent. Work hardening, caused by abuse and/or deterioration of the wall, affects the modulus of elasticity, usually resulting in a tank that has plastic deformation which is not good.
Tanks are filled with water (hence the name), placed into a graduated vessel (also filled with water) and most are pressurized to 5/3s of their rated pressure. The initial, as well as the increased volume of the tank, is noted and then the pressure is released and the end volume is compared to the initial volume. The tank is condemned if the tank volume increases too much, not enough or does not return close enough to the original volume. There are special procedures for some tanks, but overall this is it.
Sure, there are other things that may condemn a tank and some of these may not appreciably affect the MoE, but the safety of the tank rests in its ability to demonstrate a predictable strain when stressed as well as its subsequent recovery after the strain is released. Any tank that passes a hydro is golden. It's not "feels", it's science. Science. Science that trumps feels.
Those who don't understand the science are prone to overreacting and being a slave to their feels. They attempt to justify their responses with an appeal to fear and somehow think that their standards are much higher and even reasonable.
Unfortunately, most of the divingverse only know that hydros exist and not the science behind them. They run on feels rather than knowledge. They would rather prove themselves right rather than spend the time to discover what is right. Dive shops are particularly prone to this and well, that's their right, just as it's mine to find a shop a bit more reasonable and able to trust the science.
I'm glad the OP found a reasonable shop.
