Mxsmanic wrote:
I am curious to know how many GA pilots always have oxygen on board,
even if they don't normally fly at high altitudes.

I carry it and will tend to use it if I know I'll spend time above
10K.


It seems like it
would be good to have around, but I don't know how much it costs.

The initial cost is the biggest impediment. Topping off the bottle
isn't that abd.

Do
oxygen systems have to be frequently topped off or inspected or
measured?

You refill them as they get used. They need inspection every
5 or 10 years depending on construction or the fillers won't
refill them.

Do they just consist of masks for passengers and pilots to
wear?

Masks or nasal canuals and then some kind of flow regulator.
In the simplest form these are just needle valves with either
some sort of flow guage or just preset (by altitude) flow.
The fancier ones that use less O2 for a given effect, work
like scuba regulators in that they flow more while you are
inhaling and throttle back when you are not.