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This detector is well worth what it cost to build it, if you have a consuming interest in it, or a significant need for it. For a learner, and experimenter, the parts bill is pretty high, due to the single unit price on the PIN diode detectors. The BPW-34 you mention, is inexpensive, but not very sensitive. Is there a "middle ground" device, with better performance, but produced in commodity volume, so the price is more affordable?
I think your "compressor converted into a vacuum pump" is a good solution. My only problem is the noise and vibration from those little pumps is annoying. Do you think a 12Vdc "Windshield Washer Fluid" pump would take too much time? I do not mean pumping the fluid through the pump (although it is rated for alcohol). I mean using the ability of the pump to lift fluid on its intake side (by vacuum). Most would have 50-60 (cm of H2O) lift ability on the intake side.