Silly question - glass CO2 bubble counters

[ingg]

They look like they run inline somehow.... silly question, but exactly how do these things work?

Seems silly to spend twice-four times as much on a plastic one if the glass ones work well and are easy to use...

Like this:

http://cgi.ebay.com/Brand-New-Bubble-Co ... dZViewItem

[JMLenke]

Usually the bubble counters have glycerin in them which slows the bubbles down so you can count how many bubbles per second/minute you are injecting into the tank. I ran my Co2 tank without one the last time i had it filled.