We test our own SV's here. The test bench and lap room are in one part of a large workshop - with separation from the general workshop activities.
Given that it's a large workshop, it's cold when it's cold, and hot when it's hot. There are space heaters.
I think the key is to have a relatively clean area of sufficient space and adequate test equipment.
I've been to several PRV overhaul organisations in our area and none operate a "clean room" situation - and they all workshops that do a mix of activities.
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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."
Unless you have special service PRVs, i.e. Oxygen Service, a clean room is not neccessary. If you do have O2 Service PRVs, you need special Test Equipemnt (Clean), probably SS, and clean test medium (Nitrogen).
Climate conditions vary. I have worked in Air Conditioned Shops and others which were not so much. No standard calls for such conditions. Offifial NBBI Certified PRV capacities for ASME PRVs are in SCFM, GPM or PPH which are corrected for Lab Conditions. Therefore, Test Room conditions do not necessarily need to meet any specific conditions re Temp and Atmospheric Pressure. That being said, high altitude, does affect set pressure to some degree.