Timekeeping
By Ed Estlow
on 2.4.14

We love our high complications and luxury masterpieces as much as the next watch nerd. But our bias is for timepieces that can “do things”, watches that are each an essential piece of kit. Sure, it’s an instrument for telling time, but it can also be used to time a dive or a racing lap, take a pulse, or calculate remaining fuel or crosswind speed or the distance of thunder or artillery. How, you ask? The answer has little to do with the watch’s movement. It’s all about the bezel, that outer ring of metal (or perhaps, nowadays, ceramic) surrounding your watch’s crystal. The ones we’re talking about have numbers or other markings; they may rotate in one direction or both, or not at all; they may feature some other combination of all that. How each type of bezel works is not complicated per se, but it is deserving of a quick guide.

MORE TIMEKEEPING ESSENTIALS A Complete Guide to Owning a Mechanical Watch | Buying a Vintage Watch | The Chronograph, Deconstructed

Count-Up Bezel With a 0-60 Scale

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Perhaps the most commonly seen bezel markers are on dive watches. These scales go from zero to sixty, indicating minutes in an hour, and are used to keep track of time spent underwater, a critical parameter along with depth and remaining air. The first fifteen (sometimes twenty) minutes are marked in one-minute increments while the rest of the scale is usually marked in five-minute increments. The increased resolution for the first fifteen minutes on the scale allows divers to time decompression stops with relative precision during ascents at the end of a dive. To use a dive bezel, set the zero marker opposite the minute hand; as time passes, you can read off elapsed time on the bezel without having to do any mental calculations.

Countdown Bezel With a 60-0 Scale

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Pretty much the opposite of a count-up scale, a countdown scale is used to set the time remaining before or during an event. Rotate the bezel so the time remaining on your parking meter is opposite the minute hand. When the minute hand reaches zero on the scale you’re in parking ticket territory.

Tachymeter

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The tachymeter bezel is the distinguishing feature on iconic chronographs like the Omega Speedmaster and Rolex Daytona. The logarithmic scale is proportional to “one over elapsed time” (1/elapsed time) and therefore is used to measure units per time increments. Most common of these is speed in miles per hour. However, you can also calculate units per hour on a production line, pitches per hour during a baseball game, or the average rate of any other repeating event. Start the chronograph when one unit passes (mile marker, widget, whatever), stop it when the next unit passes, and read units per hour on the scale.

Pulsometer

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Specialized “medical watches” have a pulsometer at the edge of the dial. This is a specially calibrated tachymeter used to determine heart rate. Start the chronograph timer and count the beats until you get to the number for which the scale is calibrated — usually 15, sometimes 30. Stop the timer and read the heart rate in beats per minute. A related scale, often found on the same watch, is the asthmometer, used to determine a patient’s respiratory rate. The scale is read the same way and is typically calibrated to five respirations.

Telemeter

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This is a scale used to determine the distance from the wearer to an event that can be both seen and heard. Sudden lightning storm move in while you’re on your backcountry trek? Trigger the chronograph timer when you see the flash, stop it when you hear the thunder clap. See if you’re safe from harm by reading the distance in miles or kilometers on the telemeter scale. The speed of sound in air is effectively a function of air temperature (we’ll ignore the minor effects of humidity and altitude), so the scale is usually calibrated at a typical ambient temperature.

GMT

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The term GMT (Greenwich Mean Time) has been abandoned by the scientific community but the label still sticks in the tradition-bound world of timepieces. The bezel on a GMT watch is marked in 24 equal increments, becoming the chapter ring for the watch’s 24-hour GMT hand. This makes the watch a two time zone watch. If a second 24 hour ring is included on the dial, that creates a third time zone. The bezel is also often in two colors, roughly designating day and night. To use this bezel, set the hour marker on the bezel opposite the 24-hour hand for the time zone you want to track. It’s that easy. Just remember that a 24-hour hand only goes around once a day. You’ll get the hang of it.

Compass

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You’re hiking the High Sierra and you lost your compass? Well OK, you’re a watch nerd, not a woodsman. If you’re in the northern hemisphere (hopefully you can tell that much), rotate the compass bezel until the South mark is half way between the hour hand (subtract an hour if you’re on Daylight Saving Time) and 12 o’clock. Point the hour hand at the sun and use the bezel orientation to determine north, south, east and west. Just reset the bezel about once an hour and you’ll find your way home.

Slide Rule

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We’ve saved the coolest bezel for last here; it’s also the most complicated. The slide rule bezel is basically two matching logarithmic scales — one stationary and one on a rotating outer ring. You perform multiplication and division by rotating the outer ring. This is old school math, folks. Cold-War-era engineers will wax nostalgic if you show them this one.

Say you want to multiply 8 x 14. You place the 14 on the outer rotating bezel scale opposite the 10 (a unit index used as a conversion factor) on the inner scale (at around 2:30). Opposite the 8 on the inner scale, read the answer, 112, on the outer scale. Simple huh? OK, we didn’t think so either. We won’t tell anyone if you pull out your iPhone as a backup.

You can use a slide rule bezel to handle all sorts of navigational calculations: airspeed, rate/time of climb or descent, flight time, distance, and fuel consumption, plus kilometer-nautical mile-statute mile and gallon-liter fuel conversions. Unfortunately, those are a little complicated to go into here — refer to your instruction booklet for those (Breitling does a particularly nice job). The slide rule bezel is most commonly found on aviator’s watches; unfortunately, knowing how to use the bezel doesn’t get you any closer to your pilot’s license though. That part is up to you.

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