It is just a small tank for a grill.
The one I saw in the catalog actually screwed into the stem that attaches to the gas line to the grill. I have since found one like that in stores, and am currently using it, so I will see if it continues to work.
Once you put the gauge on, then you hook the line to the gauge.
http://www.nextag.com/propane-tank-gauge/search-html
The second gauge down on the page is the one I found at a local hardware store. Has anyone used this one? Does it last?
NOVA Pros