Oh dear, the resident armchair specialist and Google hero couldn't resist a burp.
Testing at 500 mbar.
The armchair specialist is quite right.
0.5 bar is ISTR the maximum pressure recommended by the HVCA and HSE for an initial pneumatic leak test, which is not to be confused with a pressure test. The safety precautions even for that are quite onerous. Their experts deem that pressure to be hazardous, but Dick Puller thinks he knows better, of course.
Young persons can hear some of the leaks at that pressure. I can't, 'cos my ears are knackered, so I use an ultrasonic leak detector.
The hydraulic pressure test, on a leak free system, is done to prove it will withstand the pressure. 1.5 times working pressure is recommended for water services (Water Regulations), 2 times working pressure is common on heating systems, 10 bar is specified by most plastic pipe manufacturers.
The persistence of Puller and others in clamouring for pneumatic tests is because they have no confidence in their own workmanship.