The main purpose of hydrostatic/pneumatic test on pressure retaining parts is to check it for strength.
If you just want to check for leaks (pressure boundary integrity) is a different matter.
For checking strength, the pressure retaining item must be loaded at a pressure (independent of the fluid used) higher than the design one according to the design code. So, for this type of test you can't reduce the pressure. Otherwise you will have no valid information about the overall design/manufacturing process.
Kind regards
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You may want to consider other issues with air pressure testing. 7 bar (101.5 psi) is compressive and dangerous. Hydraulic testing is much safer. I have seen some pretty nasty accidents using pressure testing with air.
If it is leak testing you are looking for you could try inert gas testing using something like Helium.
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As far as hydrostatic pressure testing is concern, the pressure is irrespective of the fluid. Different test standards specify different test pressures with respective to design pressure.
According to ASME it is 1.3 times design pressure (you may correct me if i am not right), thus 24 bar will be test pressure for 18.5 bar design pressure. you have to apply 24 bar only, irrespective of the fluid. If you are using air, you need to take good protective measures for safety of the operator and surrounding. Hydraulic pressure is not that unsafe because of its compressibility. Once some liquid leaks, pressure comes down to zero. Still please remember that hydraulic pressure is also not completely safe. I have experienced hurting power of hydraulic pressure testing. the jet emerging is high energy jet and can harm you, particularly if the jet approaches your eye.
Hydraulic Testing vs. Pneumatic Testing
Re: Hydro test vs pneumatic test
