:Dave
Martins comments are about right, but i would add this.......if the watch isn't worn a great deal and doesn't get fully wound and unwound regularly then the watch oil may get alittle sticky and the main spring isn't being used like it was designed to be and may get a memory effect. Also has the watch been subjected to any extremely cold temps since it was service.....old springs don't like extreme cold or heat?.

Also try laying the watch in differring positions at night on your bedside table and observe if the problem chances. My Cal 11 siffert runs for about 38hrs before the time keeping slips, but the Caliber 11 is quite fragile and a common fault with these is a date change problem associated with the watch stopping........does it stop shortly after a date change ? or there abouts. If it does, then chances are either the date wheel or hour wheel{parts 2558 and 2556} have a bent or broken tooth. Commonly this is due to the click over of the date at 12,so... to check this, if your watch stops, advance the time by an hour or two then shake the watch to start the movement without hand winding it, and see how long it goes runs for. If it stops again before the date change is due then its probobly the hour wheel,or a weak main spring, if it doesn't stop till the date change then it could be the date wheel{more common}.
Hanz is unlikely to have got something wrong in his service, but if it was 18mths ago then there has been alot of ticking going on inside the watch during that time, if you see what i mean.
Its probobly wise to get its serviced again.

Hope this is of help.
Bryan

What should be the power reserve time for the
: cal 11 siffert. If the reserve is approx. 24
: hours and keeps great time should I worry.

: What causes a low power reserve and are there
: secondary symptoms? Watch was serviced by
: Hanz 18 months ago.

: thanks