Watch Terms

Chrono Controls


This is simply a watch that also has a stopwatch built into it, with a flyback chronograph having the ability to 'fly back' to zero without stopping and resetting. Stopwatch minutes and hours are displayed in 2 of the sub dials on the face, with the stopwatch seconds counter being the main (ie the big one) second hand on the watch face. Normal running seconds for the watch are in the third sub dial. Chronographs are all exactly the same to use. The top button is for start and stop, and the bottom button is to reset to zero. A stopwatch that measures better than tenths of a second is a little pointless since the reaction time from brain to finger to click a button is two tenths of a second.


A chronometer is a watch that has been built and tested to a certain time-keeping accuracy, generally taken as 99.990% accurate per day (within 9 seconds). With Swiss manufactured watches, only those certified by COSC, the official body, may use the word 'Chronometer' on them.  The UK has no equivalent certification body.  OFFSHORE Professional 775M2 calibre chronographs are adjusted and tuned to 99.996% accuracy as this is a good limit for a mechanical watch to reach before external influences such as temperature become dominant, and it's twice as good as the COSC standard which is nice.


This is any feature that causes the design of the watch movement to become more complicated than just telling the time. Examples of complications include day & date display, automatic winding, power reserve display, second time zone, chronograph and moon phases. Moon phase is actually quite useful in subsea engineering for predicting the likely strength of tidal flow (as well as the more common use for werewolves).


A mechanical watch is one that uses a wind up machine and mechanical mechanisms instead of a battery and circuit boards. The watch mechanism, the movement, is driven by a mainspring whose stored energy is slowly released through a series of gears to power a balance wheel. The balance wheel is a weighted wheel that oscillates back and forth at a constant rate. An escapement runs from the balance wheel and releases the watch's gears to move forward a small amount with each swing of the balance wheel (it allows them to 'escape' a bit). This in turn moves the watch's hands forward at a constant rate. The escapement makes the ticking sound that is characteristic of all mechanical watches.  The 'how a watch works' page of this website explains things a bit better.

The balance wheel is the key time keeping element as each swing should take precisely the same amount of time. The number of swings per second is called the beat of the watch. High end mechanical watches operate at either 8 or 10 beats per second, which multiplies up to the 28,800 or 36,000 bph (beats per hour) that you often see them advertised as. To ensure consistent timekeeping the balance wheel and main spring should be made from a material with a low coefficient of thermal expansion and lubricated with an oil that maintains its viscosity over a wide range of temperatures and as it ages. The mainspring also needs to operate at a stress range below its lower fatigue level so that it can essentially run for ever without breaking. Finally the balance wheel and mainspring should be made with a minimum or zero ferrous (iron/steel) content to avoid magnetic effects. 

The majority of high end watches use 8 beats per second movements, often made by ETA in Switzerland, although normally with varying levels of in house modifications. The modifications can range from a new self winding rotor with the company name on it and a new 'calibre' name to disguise the ETA origins all the way through to changes in components and rebuild of different parts. ETA build movements in 4 basic grades; standard, elaborated, top and chronometer, although of course with custom specifications and in house modifications the grades get very blurred.  Many companies outside the Swatch group are moving to in-house movements, also normally running at 8 beats per second.  Zenith and Rolex are the main departure from this as they generally build movements that run at 10 beats per second. Another Swiss company called Sellita ( make movements that are almost identical to some of the ETA classics, including their SW500 version of the ETA 7750, and are built to the same rigorous quality, tolerence and material specifications. Oris and some Christopher Ward watches use Sellita movements. There are also several Chinese companies that make mechanical movements, Seagull being a example that has found widespread use.  A chinese 7750/SW500 clone does exist and is generally found in hi-end copy watches or low-end brands as it's considerably cheaper than the Swiss movements.  I can't comment on it's quality, materials, manufacturing tolerences, durability or accuracy though.


When a microscopic crystal of quartz is put in an electric circuit it vibrates and causes the circuit voltage to fluctuate. A digital counter on a separate electric circuit reads the voltage vibrations and moves the watch forward by one second for a set number of vibrations. Quartz vibrates at around 32,000 times a second and will vibrate at the same amount almost regardless of temperature or movement. It’s this high vibration rate that makes a quartz watch at least 10 times more accurate than a mechanical watch, who’s main wheel oscillates in comparison a mere 4 or 5 times a second for a beat rate of 8 or 10 swings per second.

Self charging quartz watches have recently been introduced which avoid the need to replace the battery every 3 or 4 years. In a self charging quartz watch a weighted rotor, identical to that in an automatic watch, turns a tiny electrical generator which charges a rechargable battery or a slow discharge capacitor. Seiko, who introduced this system refer to it as a kinetic movement. Later self charging quartz watches use advanced thin film silicon photovoltaic’s to charge themselves from the sunlight falling on the dial. Rechargeable batteries and photovoltaic film have a finite life though, so you can probably expect around 15 to 20 years use before they're not capable of keeping the mechanism running.


An automatic or self-winding watch is a mechanical watch whose mainspring is wound automatically by the natural motion of the wearer's arm, making it unnecessary to manually wind the watch. Self winding is achieved by having a weight on a wheel that moves around with the motion of the watch, winding up the spring. When the spring is fully wound, the connection between the weighted wheel and the spring will slip to ensure it is not over wound. The rotor of an automatic watch can be clearly seen on a glass backed case and is often engraved or coloured to customise the movement.

Automatic watches have two big advantages over manually wound watches; three if you include the obvious 'forgot to wind it up' advantage. Because an automatic watch spends much of its time fully or nearly fully wound, the torque from the mainspring is constant and so time keeping is better. Secondly, since you don't have to turn the crown 20 or 30 times each evening there is much less wear on the crown seals and so water and dust resistance are also far better.

Power reserve

A power reserve indicator shows the tension, or stored energy, on the mainspring and provides a good estimate of how long the watch will run without any winding or movement. 42 or 48 hours is the normal power reserve. Watches that don't have a power reserve indication actually have the same reserve, they just don't display how much of it is left. A self winding watch will be at 100% power reserve within about 2 to 4 hours of putting it on.


Jewelled bearings are used to reduce friction and wear within a watch movement. The jewels in all watch movements made since the 1920’s are synthetic sapphires or rubies, both made from corundum (an aluminium oxide). The difference between sapphire and ruby is simply the impurities added to the corundum to change its colour. The jewels themselves are manufactured with a very small hole down the centre. This hole acts as a lubricant storage with the oil staying in place by surface tension alone. Like a car, the lubricant should be changed every once in a while to ensure it is clean and working efficiently. Once every 5 to 8 years is generally sufficient for this.

The only bearings that really need to be jeweled in a watch are those that transmit energy from the mainspring to the balance wheel. These are the bearings that are constantly loaded by the mainspring and are in continuous movement. The gears that turn the hands, the calendar wheels and any other complications are so lightly loaded and rotate so slowly that they don’t really warrant jeweled bearings.

With a basic 15 or 17 jewel watch, every bearing from the balance wheel to the mainspring is jeweled. If jeweled bearings are added to the lever and escape wheel the number increases to 21. If the mainspring barrel arbor is jeweled as well then the total becomes 23. With a self winding watch some of the wheels in this mechanism are normally jewelled, bringing the total count up to 25. As noted, the first watches to use synthetic corundum jewels appeared in the 1920's. Well cared for examples are still running (like clockwork) almost 100 years later, and there's no reason to think your 2010's vintage watch won't be running just as well when your great grandchildren inherit it.

Any watch boasting more that 27 to 31 jewels should be looked upon with a little suspicion as the manufacturer may be adding completely pointless jewels just to increase the total count. A too high jewel count is a good indication of a watch being designed for form over function, or a high complication watch being based on a standard movement with additional modules fitted.


The case, of course, is the housing for the watch mechanism. It’s the case, along with the glass and seals that determines how robust and how water resistant a watch is. The general rule for cases is pretty simple, with heavier and bulkier providing better protection. If you intend to use your watch out in the field then it’s always worth looking for a solid case that has a smooth and snag free profile to it. The general rule doesn't always run true though as a number of seemingly solid watches actually turn out to be all show and no substance.  In particular, my experience is that if the sapphire front glass projects above the bezel or watch case, it becomes quite susceptable to knocks that can shatter it.  Sapphire is very hard and scratch resistant, but with these two material properties also comes low toughness and ability to absorb impact.  Case materials are covered on the materials page.

The size of a case is the distance measured across it from the 10 o’clock to 4 o’clock position. Anything less than 34mm is generally a ladies or childs size, 34 to 36 mm is known as mid-size, 38 to 40mm represents full-size and is common on unisex watches, 42 to 45mm is referred to as slightly oversized and generally used for mens watches, and 48mm plus is a silly-size used when maximum bling is dominant.  The Field Engineer is 44 mm.


The lugs are the 'arms' of the watch case at the top and bottom that attach it to the bracelet or strap. The actual connection is typically made by spring loaded pins in the strap/bracelet lug pins that engage into small holes in the lugs.  These spring loaded lugs pins are actually very important as low quality or poor material choices can cause them to corrode or deteriorate and allow the watch to fall off your wrist.  OFFSHORE Professional use slightly wider lug pins than normal (1.8 mm diameter versus 1.5 mm), with high chrome content springs, so that the spring is thicker than standard and offers better corrosion resistance.

Some watches forgo the lug pin in favour of a through bolt.  This is a bolt that, as the name inplies, runs through both lugs and the strap or bracelet.  Much debate can be found on watch forums over its superiority or not compared to lug pins.  It can certainly be stronger that a lug pin connection which most people take as being better, but my opinion is that this isn't actually a good thing.  As with a military strap (see below), if the strap to case connection is too strong, then if you snag or catch the strap on something it'll potentially break your wrist.  Under these circumstances I think I'd prefer it if my watch dropped to the ground.


The crown is the bit that sticks out at the 3 o’clock position on the dial. It’s used to set the time and date and to wind up an automatic watch to get it running. A good crown should either have a supporting tube or screw down onto the case since Sod’s first law of gravity states that you’ll drop a watch on its crown. Because a 3 o’clock crown can be quite a vulnerable position for snagging, some manufacturers place it at the 4 o’clock position, or even around the other side of the case at the 9 o’clock position.


The bezel is the part that surrounds the front glass on most chronometers and on all divers watches. It can carry a variety of markings which can be useful or not so useful.

The Breitling Navitimer and several other pilots watches have slide ruler markings that allow complex sums to be calculated. This was useful in the pre-calculator era (doing complex mathematics with logarithmic tables for those that remember them), but altogether less so now. The Rolex Daytona and many other chronographs have a base 60 tachymeter scale, which is a very simple method of calculating your speed. As you pass a fixed point you set the chronograph second hand running and then stop it when you pass another fixed point exactly 1 mile away. The second hand will be pointing at your average speed. A modification on this is the doctors tachymeter which is base 30 or 50 and in which you count 30 (or 50) beats of a patients heart and the bezel gives you their beats per minute. With a range finder bezel you start the second hand when you see a gun flash and stop it when you hear the gun sound. You can then read off the distance to the target.

The bezel on a drivers watch is altogether more useful, and easy to use as you simply set it at the number of minutes air supply or time at depth you have. It’s also handy for number of minutes parking if you live or drive into any town or city. An important feature on a divers bezel is that it should only be able to move one way so that if it’s knocked it won’t accidentally show you having more air left than you really have.


Put simply a bracelet is a metal strap around your wrist, whereas a normal strap is made of leather or rubber. Bracelets don’t degrade or wear out and are impervious to the humidity, sunlight and wrist sweat that can degrade and split the material of a strap. The disadvantages are that a polished metal bracelet will pick up scratches over time, needs to be cleaned out every once in while, can also have tendency to pull the little hairs out of your wrist if not designed properly and might/or might not be less comfortable than a strap. Bracelet cleaning can be done with a soft toothbrush and washing up liquid or by buying a special bracelet cleaning kit (which consists of a soft toothbrush and some washing up liquid).  A childs toothbrush is ideal for strap cleaning (but if it's a real childs toothbrush them remember to thoroughly clean out the washing up liquid before giving it back to them or you'll be in trouble).


As noted, the straps on high end watches are either leather, rubber, exotic skin and are fastened by a deployment buckle or a clasp. Straps are more comfortable than bracelets and can be easily let out a notch when your wrist expands in hot weather. However, even with the best watch manufacturers you shouldn’t expect a strap to last much more than three or four years before the first split or tatty edge will appear. The cost of replacing a strap with a new one from a high end watch manufacturer can come as an unexpected shock. OFFSHORE Professional straps are switching to an exotic Teju skin with internal layering and a Teju lining, which will hopefully make them far more durable than any one elses. The strap is also the only part of an OFFSHORE Professional watch that can be changed for off-the-shelf alternatives as many 22mm wide curved end straps will fit.  Straight end 22mm straps will also fit, but personally I don't like the little gap you get to the case that can collect dust and is difficult to clean out.  Another job for the kids toothbrush I suspect.

Military Strap

The military strap doesn’t stop at the lugs like a normal strap or bracelet, but instead runs along the back of the watch case and forms a continuous strong fastening completely around the wrist. The intent is to avoid the traditional weak points where a strap or bracelet connects to the case. Be warned though, that by using a military strap you have also lost the natural break point that makes your wrist stronger than the strap. Get a normal strap or bracelet caught on something and the lug pins will break and the watch fall to the floor. Catch a military strap watch and you could well break your wrist. Catch it on a fast rotating piece of machinery like a drill bit and it’ll strip or tear your hand off. It’s for this reason that military strap watches (and wedding rings incidentally) are banned from all oil & gas installations, and for this reason that I never wear one. Much as I like my watches, there is a limit to how far I’ll go to keep them in one piece.

Tempus fugit, but sometimes it's the only thing that does - stuck at Cabinda with nothing allowed to fly