The thorough examination meant the receiver was drained down and if possible opened up for an internal examination, though usually this meant just squinting through a little hole while trying to shine a torch inside so you could see what you were looking at. This was usually accompanied by a 'hammer test', which meant hitting it all over with a hammer, listening to the sounds while trying to determine if it sounded 'thin' whilst hoping the hammer didn't go all the way through the shell. If in real doubt a thickness test could be done, which could be a drilled hole in the suspect spot or an ultrasonic thickness test. The fittings especially the relief valve were also stripped down for inspection. The working test was mainly a check that the relief valve worked at the right pressure, the drain valve worked too, and the receiver was clearly marked with the safe working pressure. And that was about it. The main thing was that the receiver HAD to have this examination every 2 years, or more correctly, every 26 months. I very much doubt that all our little workshop air receivers get this much attention. The one saving grace with air as opposed to steam in a boiler is that there is very much less energy in air. In all probability, if an air receiver went it would probably blow a smallish hole and then let all the air escape, still very frightening and dangerous, whilst a steam boiler in the same circumstances would go BANG in a very big way. Very much not something you would be able to walk away from!