The idiot was testing it live.... lol. Only one way to test any pressure vessel and thats with water! because if it fails you will only get wet, use air and you have a potential bomb on your hands. Oh and send the safety valve off to a spec firm for a service and calibration. And as already mentioned drain the water regular (even better, fit a auto drain valve).
FYI....
Pressure Systems Safety Regulations
The Pressure Systems Safety Regulations 2000 require that where the pressure x volume of a pressure vessel is greater than 250 bar litres a written scheme and inspection certificate will need to be issued to comply with the legislation.
The calculation pressure x volume takes the pressure rated in bars and this is multiplied by the capacity of the tank in litres. This is commonly found on a plate on the receiver.
e.g. MWP 11 bar 50 litres capacity
In this example the maximum working pressure (MWP) is 11 bar multiplied by the 50 litre capacity, which gives a rating of 550 bar litres. 1 bar is equivalent to approximately 15 psi. As a general rule air receivers with a diameter in excess of 12 inches operating at 150psi will probably need to comply with the legislation.
The other components of the air compressor set (i.e. compressor and motor) do not need a statutory inspection. If an air compressor has no receiver it does not need an inspection. Hydrovane manufacture compressors with no receivers and so are exempt from the legislation.
This posting may have been helpfull, on the other hand probably not... Either way, I may have had drink when posting it in which case it's probably not as helpfull as it could have been
Work safe, and remember all technical problems can be resolved with a hammer, the trick is knowing where to hit it and how big a hammer to use (I often find it may also help to have a spare bit in the shed to replace the bit you have just mullered

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