Impact of vacuum in a guide wave radar level measurment

Ironically, I just ran into a GWR unit rated only for atmostpheric or positive process pressure, so apparently it becomes important to know whether the GWR in question is rated for vacuum service or not.

I found this online:
Temperature/Pressure: As GWR probes can be rated from -320 F to 850 F (-196 C to 450 C), and full vacuum to 6250 PSIG (430 bar), there are few process applications that cannot be accommodated.

Full page: a guided wave radar level measurement system,,discontinuity between the air and the process medium.

Propagation of microwaves/micropulses of GWRs are not influenced by vacuum, so the physical principle is suitable for vacuum applications. Probably the limitations found in one model are due to mechanical reasons. If you look at the best GWRs manufacturers you won't find limitations in vacuum
Radar is a form of electromagnetic radiation and utilizes the radio or microwave frequencies and the time of flight principle for measurement. For radar, one of the most important criteria for choosing the technology is the dielectric constant of the medium that it is traveling through and as vacuum and normal atmosphere has almost the same dielectric constant therefore vacuum has no effect on measurement. In other words visible light,infrared,microwave,x-ray,gamma rays etc all can propagate through vacuum as all of them are part of the electromagnetic spectrum.