The message
from "LSR" contains these words:

Atmospheric pressure in the soil pipe will push water around the U bend to
the level you desire. Then set the vacuum to that pressure. If it's
only an
inch or so out yu should be able to breath OK at the reduced pressure - it
will be no worse than La Paz for example.

You might like to try one of these...
http://www.totallyabsurd.com/toiletsnorkel.htm

--
Skipweasel
Pay no attention to that man behind the curtain.