calibrated it up against my Rb. freq, last night, it was nearly spot on to start with, reading an steady unchanged 10.000005, and was due for calibration in 2009 apparently.
when I unscrewed the can, was a big old WHOOSH as it equalised pressure. a little tweaking and I put the screw back in, and the sine wave was not moving for two hours and spot on 10mhz perfect!