What I think I am hearing put on the day cap, get the object focus "off" aka blurry-ish, turn the gain all the way up, and make sure the little sparklies are sharp pin pricks of light and not little blobs (for lack of a better word) while taking breaks from screen time so to speak?
You got it. The random sparkles are called
scintillations, but overall the image in the dark will be grainy, even for the highest resolution tubes, so easy to see individual dots at least when they are at reasonably high contrast.
Usually the term "static" works great, half a dozen people standing there slowly adjusting the position then I say this and they go "ah," then when objective focus "oooh..."
As for purging... I know nothing of the process/advantages but in my AO a good night is 87% humidity. A bad night? Well, it's a good thing the PVS-14 is submersible to 66 feet!
Purge puts a clean, dry gas under slight positive pressure to keep from internal condensation, dust, etc and to avoid external atmosphere intrusion.
However, look at all the seals on the device here, how some are not really truly optimal seals (the lenses threading in and out) and you can guess what is true: the purge will within a matter of months (esp when used regularly) simply go away. Once reduced to 1 atmosphere, external air can get in. So the military has purge units and re-does it every once in a while. Not sure the schedule.
Nitrogen is the standard but that's not magic, it's just air without O2, water, and... that's it. N2 used for purging is not lab grade so has the normal trace elements of the air in it, is mostly just a convenient source of clean-and-dry.
I am very very very not alone in being one who says just use good practice when building/repairing, to keep things dust free (and when not in a cleanroom, "good practice" includes tricks like leaving open parts upside down as dust mostly falls) and use the miracle of modern civilization of air conditioning, do it when the room air is dry-ish and you'l be fine. People repair cameras, lenses, and many devices with no purge, and no problems. Many other devices (say, Nikon lenses) are factory filled with 1 ATM N2; they make most camera lenses in a box filled with nitrogen but do not have a purge valve to pressurize it, for example.
I live in Kansas City, and bop mostly around this area where it gets to condensing humidity sometimes, esp morning in a hollow in the woods.
IF you worry about it, or see consequences like internal condensation, then my suggestion is not to go source a USGI purge kit or make something, but get a wine preserver
like this. Argon is an excellent purge gas (and some optics do use it at the factory), is utterly inert so better in theory for corrosion etc but for our purposes the best bit is that it is notably heavier than air. So, you open the purge screw, face it upward, squirt in a reasonable amount of Ar, put the can down and at your leisure put the purge screw back on. Because it cannot leak out easily, but sits there much like you just filled it with water.
(The wine kits are not just convenient and cheap but low pressure! Do not try to put argon shield gas in, as it's WAY too high pressure so might damage the inside, and will splash out. If clever enough to mess with gasses you need to regulate to like 4 psi and have a flow rate at or under 1 CFM).
