There are a few options to choose from for filling up your tires on the trail.  Portable air compressors and CO2 are two of the most popular.  However, if you are a scuba diver or have the ability to fill a scuba tank (ie. certified),  you might want to consider using your scuba cylinder instead of CO2.  Although similar to a CO2 bottle in weight and size, a scuba tank has a few differences.  Unlike CO2, Scuba uses compressed air, that is 78% Nitrogen, 21% Oxygen and 1% other gasses.  Because air remains a gas when compressed it has a few advantages over liquid CO2. First, you do not have to mount your tank vertically.  You can have the tank flat inside your rig and not have to remove it and stand it up to fill up.  Second, you can keep your tank in a hot cab and not have to worry about blowing the burst valve.  Above 100 degree F. even a slightly overfilled CO2 tank is very likely to burst the safety valve.  Thirdly, scuba tanks are filled with very dry air.  So, you can fill at very high rates without freeze clogging the regulator.

What about cost?  The price of  a standard 80 cubic foot aluminum scuba cylinder is comparable  to a 20 pound Co2 bottle.  However, a scuba air fill is about half the price of a CO2 fill.  I pay $5 at Sports Chalet.  CO2 is typically $1 a pound.

As far as capacity,  an 80 cubic foot, 3000 psi scuba tank holds about the same volume of gas as a 10 pound bottle of CO2.  To be exact, 1 pound of liquid CO2 equates to 8.8 cubic feet of gas.  If you want more capacity, Scuba tanks go as high and 130 cubic feet but cost significantly more.

After the tank you’ll need a first stage regulator.  Good used ones typically sell for around $30.  And they are typically adjustable from 100 to 200 psi.  Then use a 3/8″ to 1/4 Npt adpater,  and you can hook up any standard air compressor hose and tire chuck.

3/8" male to 1/4" female NPT adapter

Alternatively, if you want to use your BC inflator hose, most dive shops carry a tire chuck that will plug right into it.

BC inflator tire chuck