TomV is right about the cartridges, but you can't beat them for compact roadside tire inflation. A few decades ago, however, I blew up a tire with a CO2 inflator that had only an ON_OFF trigger. You got the full blast - or nothing. It took a light touch to use safely. Now I have a PDW Shiny Object that works really well. Instead of a trigger, it has a control knob that lets you regulate the flow of CO2. 

I have had two manual inflators, but they take a long, long, long, long time to get a tire up to pressure. But if you are thinking in terms of just being able to get home or to a place with a better pump, they work OK.

My .02

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