How to use tachymeter on your watch bezel??

On some of the analog watches or wristwatches from branch such as Omega and Casio, there is a outer scale bezel that inscribed around the rim and named tachymeter (also known as tachymetre or tacheometer). If you flip through the watch’s user guide or manual, you will not likely to find any explanation or definition on what is tachymeter or how to use the tachymeter functionality. Tachymeter is actually a tool used to compute speed based on travel time over a fixed distance travelled (like one mile or one kilometer). Thus, the tachymeter bezel or dial is a logarithmic scale (actually it’s not, as spacings between the marks on the tachymeter dial are proportional to 1/t where t is the elapsed time) that simplifies the computation of speed of an object, by computing the following function:

Tachymeter Dial = 3600 / Elapsed Time In Seconds
 Depending on on the range of the scale printed on the bezel, tachymetre timer normally works and valid for all elapsed times from about 7.2 seconds to 60 seconds, thus only able to measure speed greater than 60 miles per hour or 60 kilometers per hour.

To use tachymeter, simply start the chronograph or stopwatch when the object measured passing the starting line. When the object reaches the next mile or kilometer marker, stop the chronograph or stopwatch. The point on the tachymeter scale that adjacent to the second hand of the watch indicating the speed (in miles per hour or kilometers per hour) of object traveling between the two points.

For example, we want to measure the average speed of a vehicle is moving. Start the chronometer or stopwatch function when the car passes the starting line, and stop the timer after the vehicle traveled exactly 1 mile or 1 kilometer. Then look at where the chronometer hand (the second hand of the watch) is pointing to, and get the reading or value of the corresponding number at the tachymeter bezel or dial. Let’s say the stopwatch stop at 3 o’clock position, meaning 15 seconds had elapsed for the car to travel 1 mile or 1 kilometer. At 3 o’clock position, the tachymetre value shows that number of 240, which means the average speed of the vehicle was 240 mph or km/h.

Due to the limitation and constraint on the tachymeter scale mentioned above which makes tachymetre measurements works and valid only for certain range of speed, so in order to calculate and measure slower speeds or higher speed, user can decrease or increase the unit of measurement (e.g. change to half-miles or half-kilometers, or ten miles or ten kilometres). However, simple calculation is needed in order to get the correct average speed by using this technique. It’s also true if the length of the distance does not exceed a unit of mile or kilometer, or other unit which the speed will be based on.

For example, if we want to measure how fast the 200m athlete is running. Start the chronometer when the race starts, and stop the stopwatch when athlete crosses the finishing line. Let’s say the athlete used 20 seconds to finish 200m running, so according to tachymeter, the speed of the athlete is 180 km/h. However, the athlete only ran one fifth (1/5) of a kilometer, so we should divide 180 with 5 or 1/5 of 180, which mean the actual speed of the runner is 36 km per hour.

The above example can be used for object or thing that moving too slowly too, as when the object travels 1 mile or 1 kilometer, the duration would have exceed 60 seconds, the maximum limit of tachymeter. So the possible workaround is to measure the time taken for the object to move a shorter distance (such as 100m) and divide the tachymeter value with 10 (as 100m is 1/10 of a kilometer).

What if the object travels too fast, such as a rocket or plane. In this case, user can increase the units of the distance covered by the object to measure a longer period of time so that the duration is greater than at least 7.2 seconds (typically the lower limit). We can measure the time taken by the object to cover 10 miles, let’s say it took 30 seconds, so the tachymeter will tell us the the speed is 120 miles per hour. However, 10 miles have been traveled, so that actual speed is 120 times 10, which is 1200 mph.

What does Jewel in Watches means??

Jewel bearings were invented and introduced in watches by Nicolas Fatio (or Facio) de Duillier and Pierre and Jacob Debaufre around 1702 to reduce friction. They did not become widely used until the mid 19th century. Until the 20th century they were ground from tiny pieces of natural gems. Watches often had garnet, quartz, or even glass jewels; only top quality watches used sapphire, ruby, or diamond. In 1902, a process to grow artificial sapphire crystals was invented, making jewels much cheaper. Jewels in modern watches are all synthetic sapphire or (usually) ruby, made of corundum (Al₂O₃), one of the hardest substances known. The only difference between sapphire and ruby is that different impurities have been added to change the color; there is no difference in their properties as a bearing.The advantage of using jewels is that their ultrahard slick surface has a lower coefficient of friction with metal. The static coefficient of friction of steel-on-steel is 0.58, while that of sapphire-on-steel is 0.10-0.15.

Why they are used

Jewels serve two purposes in a watch. First, reduced friction can increase accuracy. Friction in the wheel train bearings and the escapement causes slight variations in the impulses applied to the balance wheel, causing variations in the rate of timekeeping. The low, predictable friction of jewel surfaces reduces these variations. Second, they can increase the life of the bearings. In unjeweled bearings, the pivots of the watch's wheels rotate in holes in the plates supporting the movement. The sideways force applied by the driving gear causes more pressure and friction on one side of the hole. In some of the wheels, the rotating shaft can eventually wear away the hole until it is oval shaped, and the watch stops.

 Types

In the escapement, jewels are used for the parts that work by sliding friction:

• Pallets - These are the angled rectangular surfaces on the lever that are pushed against by the teeth of the escape wheel. They are the main source of friction in a watch movement, and were one of the first sites to which jewels were applied.
• Impulse pin - The off center pin on a disk on the balance staff which is pushed by the lever fork, to keep the balance wheel moving.

In bearings two different types are used:

• Hole jewels - These are donut shaped sleeve bearings used to support the arbor (shaft) of most wheels.
• Capstones or cap jewels - When the arbor of a wheel is in the vertical position, the shoulder of the arbor bears against the side of the hole jewel, increasing friction. This causes the rate of the watch to change when it is in different positions. So in bearings where friction is critical, such as the balance wheel pivots, flat capstones are added at each end of the arbor. When the arbor is in a vertical position, its rounded end bears against the surface of the capstone, lowering friction.

Miyota 8215 vs ETA 2824-2

The Miyota 8215 and ETA 2824-2 are two Automatic Date movements that have been around for years. Both are fine watch movements that are dependable and accurate. Both are used by several watch makers world wide.
While it is not publicized, most watch manufactures these days don’t make their own movements. Swiss manufacturers were hit hard in the mid Seventy’s with the advent of cheap disposable quartz movements. Many makers when out of business, most of those who survived now employ "ebauche" (spec) movements, manufactured by ETA ,Valjoux and a few others.

The Swiss ETA 2824-2 25 Jewel Automatic movement is used in the Invicta 9937 a great value at $299.99
The Swiss ETA 2824-2 movement is used in several makes including Auguste Reymond, Cartier, Dugena, Eterna, Festina, Fortis, Gaddy's, Kienzle, Maurice Lacroix, MHR, Motochron, Oris, Schwarz-Etienne, TAG-Heuer, Tissot to name a few.

Almost identical Invicta 8926 with a Miyota 8215 21 Jewel Automatic a great daily wear value at $104.99

The Miyota 8215 is used in such brands as Camel, Citizen, Dugena, Festina and Invicta. Miyota is owned by Citizen in Japan.
The two movements, while similar, have differences that set them apart, both in functionality and application.
Cost - The 25 Jewel ETA 2824-2 Automatic Movement can be purchased from Ofrei (A large watch materials house) for about $78. It is employed in watches costing thousands including the Rolex Tudor Submariner. The 21 Jewel Miyota 8215 costs about $37 and is used in watches priced from $100 and up. 

BPH - Beats per hour. ETA 2824-2 28,800 BPH or 8 beats per second (4 Hz) vs 21,600 or 6 beats per second (3 Hz) for the Miyota 8215. Most mechanical movements run at 18,000 BPH or 5 beats per second (2.5 Hz). Both movements have higher than average BPH a plus.
Accuracy is generally better on movements with higher BPH. The TimeZone has an excellent article on the subject.
Higher end watches employ movements with high beats per second. An easy way to judge movement quality without taking it out of the case is to watch how smoothly the second hand runs. Higher quality movements display a smoother running second hand.
Finish - The finish on the ETA is smoother and more polished than the Miyota. The ETA is available with gold plating as used by the Invica 9937. This does not affect the performance of the movement. Many and many could argue who’s going to look at the movement anyway? It’s is an esthetic issue.
Jewels – The ETA has 25 Jewels and the Miyota has 21 Jewels. Jewels, typically synthetic rubis are used in watch movements, not to add to their value, but to reduce friction and wear on moving parts. Lower end watches may have only 7 to 15 jewels. The standard for quality watches is 17 jewels or more. There is an upper end to the number of jewels necessary, typically, anything over 25 jewels for these types of automatic movements are just for decoration.
Power Reserve - Power reserve is the number of hours a movement will run when fully wound. Most mainsprings will power a watch for 30 hours or more. The ETA has about a 40 hour power reserve, the Miyota has a 45 hour reserve.
Winding Mechanism – Both watches employ a winding rotor that winds the mainspring with the movement of the wrist. The Miyota employs a ball bearing unidirectional winding rotor. The ETA utilizes a ball bearing rotor that winds in both directions. The ETA takes less wrist movement to wind the mainspring.
Second Hand Drive – The Miyota employs an indirect second hand drive and the ETA uses the newer direct second hand drive. The second hand on the Miyota may appear to stop for a moment with various wrist movements, but this does not affect accuracy.
Shock Resistance – The ETA uses Incabloc shock resistance, a method developed in the 1930’s to allow the end jewels on the balance staff to move or ‘float’ so the balance is not damaged when the watch is dropped or bumped. The Miyota uses its own proprietary shock resistance on the balance (Parashock).
Hacking – The ETA’s second hand stops when the stem is pulled for setting the time. A feature called hacking. Handy for synchronizing watches like in the war movies, or setting the watch with a standard clock like WWV. The second hand does not stop on the Miyota 8215 when setting the time. 

Date set – Bot the ETA 2824-2 and the Miyota have the quick set date function.
Conclusion - Both movements have a track record of accuracy, dependability, and longevity.

They are employed by several low, medium and high end makes. The ETA 2824-2 in my opinion is the better of the two. It is Swiss made and has been in service since 1982. With 25 verses 21 jewels, higher beats per minute, omni-directional winding mechanisim, and hacking feature, the 2824-2 comes out ahead.
Based on price, the Miyota 8215 by Citizen is a great value and would be a good choice in an everyday watch or weekend beater.

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