At 1512, I pinched off the entire reference pressure tubing so that both Pref and P.0m measured just the reference volume. (It turned out that P.0m's RS232 connector fell off, so we didn't get data from it for any of these tests 
.)
At 1618, I opened the above, but pinched off lines to only connect 6b.
At ~1745, I opened the above connecting the entire tubing system, but pinched off Pref (after reconnecting the P.0m signal).
A first look at the Pref data shows no difference -- the pressure fluctuations are still coming from the volume?