A good yoga timer such as the Reiki Timer - Interval Timer with Gong Sound and Reiki Symbol is an absolute "must" for anyone who does meditation. It still amazes me how small things can make such big differences. It follows the 80% / 20% rule, where 20 percent of change makes 80 percent of the difference.
The timer`s buzzer has to be soft, otherwise your meditation session will end abruptly. Thus results of the relaxation will be minimized.
It's a bit like socks, which is the last piece of clothing that comes to mind, but imagine not wearing socks for 1 day? Your feet would start rubbing up against the soles of your shoes, which would heat up the feet, the rising temperature would produce excessive sweat and in a matter of hours you would end up with bloody blisters and would not be able to walk.

The next thing I tried, was to manually check the time, but each time I looked at my watch it would break my concentration and it would be hard to get back on track.

There are plenty of good meditation timers out there. I received several of them as trial gifts and purchase perks. My favourite one was the pyramid meditation timer by Sage Meditation. It's small and compact (width of a DVD) and has a pleasant unobtrusive gong sound (perfect and natural sound to end any meditation session). The timer mechanism is pretty simple, you move the dial to the desired time and turn it on.
The other factor , that I also like is the way it looks. Its housing is made of real wood which looks so much better than plastic.
I once offered one of those timers to a friend of mine, which is a pyramid nut. Literally. She boasted how this timer has the perfect proportions of the Kheops pyramid in Egypt. She said that the pyramid shape creates extra energy in the room and improves the meditation experience. I don`t know about the Kheops proportions, but this timer sure is neat. Ever since I got this meditation timer I was able to achieve the deep meditation stage which never happened before.
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