GM wrote:

Saw an interesting device in the home store today. Replaces the need
for a vent on a drain line. Essentially a 6" nozzle with a one-way
valve to let air in but not out. Probably been around for a while, but
first time I've seen one.

Has anyone tried one of these? I daren't ask whether it meets code.....
Presumably would only be workable in an accessible situation such as a
sink or basin.

Lots of info he
http://www.studor.com/index2.htm

Yes, it must remain accessible.

Code acceptable? Depends.
We don't have a "National" Code (US) and
big differences exist between regions of the
country.

Won't replace the stack vent, but can be used
on individual fixture traps.

Jim