Silly people!
I refer to the sensor manufacturer. On their "data sheet", they say "consult factory for.... comms protocol info".
But that's going to be worth the trouble. After you get it, send them a "suggestion" that they publish it!
Anyway... back to getting your device working...
I'd be tempted to start your work by hooking the sensor directly to a PC, and use a serial monitor on it... Hyperterminal should be okay, if you use Windows.
As for connecting an Arduino to both a PC and a second serial device at the same time, the notes in....
.... may be useful, although in that case, the Arduino is outputting to the serial device, not reading from it. Not a lot of changes needed, though. I'm afraid that the page cited covers more than you are interested in, but within it is some useful-to-you stuff.