The Power Reserve Indicator
One of the most useful features that a mechanical watch can have is a power reserve indicator. It’s critical that a watch has at least 30% of its mainspring wound for the watch to run at a regular rate. Many watch enthusiasts just pick up their watch from the dresser and shake it a few times, thinking that the watch will run up to standards with this “jump start”. An automatic watch must be worn in motion for about 10 to 15 hours per day to give the watch a full wind. Many people wrongfully think that a watch has stopped during the night due to a malfunction, when really the watch never was worn long enough to assure a 24 hour continuous running. The power reserve indicator is like fuel gauge, it shows the wearer how much energy the mainspring is holding. By glancing at this indicator, the wearer can determine if he or she needs to manually wind the watch to give it more power, wear the watch longer, or check for malfunction of the automatic winding. Or, the owner can simply be assured that the available power is adequate. With a manual wind watch, the wearer should fully wind the watch each day, until it stops winding. This guarantees the owner that the watch has roughly 36 or more hours of continuous running. In an automatic watch, the power reserve shows you when the watch is fully wound. This cannot be determined by the feel of the crown; the mainspring and thus the crown “slips” at full wind. In the above picture the power reserve indicator shows the wearer that the watch has about 45 hours of continuous running before it stops.
When I was doing watchmaking for a living, I had a watch repair shop close by to a convalescent home for the aged. Many of my customers were from this home. The men were usually in their 70’s and 80’s and mostly had automatic watches, which they had purchased in the 1950’s and 1960’s. It drove me crazy for awhile before I finally figured out a watch riddle. I would service these automatic watches, and soon thereafter, the watches would come back in with the complaint that they kept stopping or running several hours slow a day. It finally occurred to me, after re-servicing the watches several times, that the owners might have been wearing the watch all day long, but they didn’t have their wrists in motion enough to swing the automatic winding oscillator, thus keeping the watch perpetually underwound and running less efficient or stopping. Lying in bed with your arm on the mattress, or sitting all day in front of a TV or on a bench with your arm at your sides, doesn’t give enough kinetic power to wind the watches. If the watches had had a power reserve indicator, I could have solved the problem at the onset. How I solved the problem then, was to recommend that they wear quartz watches instead of automatics.
Here’s some examples of power reserve indicators on various watches: