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An Overview of the COSC Certificate and Testing Procedures

By Michael Disher

Some History: Swiss Chronometer Testing and Certification

Chronometer testing in Switzerland took place at observatories in Neuchtel (1866 - 1975) and Geneva (1873 - 1967). 
Each observatory maintained its own testing standards. Generally speaking, the observatories did not test large numbers of movements meant for public sale. Typically, watches tested at the observatories were specially made and regulated pieces. A manufacturer might submit a few watches, then report the best results in advertising. 

The few watches that were actually tested were rarely sold to the public.

Testing of larger numbers of watches intended for public sale was conducted by official testing agencies. The testing 
agencies were originally called "Bureaux officiels de contrle de la marche des montres", abbreviated B.O. These 
agencies were founded between 1877 and 1956, and each operated independently of the others.

In 1973, the official testing agencies were placed under central administration and the name was changed to 
"Contrle Officiel Suisse des Chronomtres" [Official Swiss Chronometer Control], or COSC. The director of the COSC 
was located in La Chaux-de-Fonds, and offices were located in Le Locle, Bienne and Geneva.

Testing Procedures.

By way of summary, watch manufacturers send batches of uncased movements (not watches) to the COSC for testing. 

The COSC charges manufacturers a fee for each movement tested. Movements are fitted with dial blanks and hands, 
and groups of movements are mounted on racks. The test lasts 15 days. During the test, the racks of movements are 
placed in five different positions and kept at three different temperatures. Results are tracked with cameras and lasers 
and reported to a computer. The passing movements receive a Bulletin de marche or watch rate certificate. These 
certificates are usually provided to the ultimate consumer with the watch, though some companies now charge an extra 
fee for the certificate.

Reading and Understanding the Modern COSC Certificate.

For purposes of this article, I am using an actual COSC certificate posted by MJ as TZ Classic # 55. . For convenience, 
I am including the relevant portions of the certificate in this article. We will work our way from the top down. The top of 
the certificate appears below.


The COSC certificate states the results in French, German, Spanish and English. The certificate number appears in the 
upper left portion of the certificate. In the example above, this number is 5523048.

Next to that on the right is the serial number of the movement tested. In the example, 102472. Below the certificate and movement numbers, reading left to right, is a description of the movement by category, functions, diameter and thickness. In the example, the movement is Category I/1, referring to a spring balance oscillator movement that 
is more than 20 mm in diameter and which has a surface area greater than 314 square millimetres.

This categorisation scheme is explained on the back of the certificate (in French). The movement in question is an automatic chronograph, 30.00 millimetres in diameter and 7.55 millimetres thick.

The next section of the certificate - the centre - shows certain of the raw test results, as shown below:

Looking at the column from left to right, the first column is the days, 1 through 15, over which the test was conducted.

The second column shows the temperature in degrees Celsius at which the movement was being tested. Converting to Fahrenheit, 8 = 46.4, 23 = 73.4, and 38 = 100.4.

Column three shows the position in which the movement was tested.

Vertical means the movement is hanging perpendicular to the ground, and horizontal means the movement is lying flat or is parallel with the ground. Dial up and down means the dial is horizontal and either above the movement or below the movement. The references to 3, 6 and 

9 o'clock up in the vertical position mean the number 3, 6 or 9 is highest when looking at the dial straight on (or where 12 o'clock normally would be). 

The next column shows the daily rates. The capital letter "M" with the small subscript numbers 1 - 15 represent the days of the test. The results in this column are not aggregate. In other words, the results are the gain or loss in seconds per day, without regard for the results of the previous day. In this column, we see the absolute gain or loss in seconds 
per day as measured against an accurate, consistent reference such as an atomic radio signal clock.

For example, in the sample certificate shown above, the reading for M8 is -0.7. Reading across all the columns, that means that on 

Day 8 of the test, the watch was in the horizontal, dial down position at 23 degrees Celsius (73.4 F), and over 24 hours 
the rate was -0.7 seconds, or slow by 7/10 of a second compared with the reference.

The fifth and final column shows the difference between the first day measurement and the second day measurement in seconds per day. In the example above, M1 is -0.5 and M2 is -0.4, so the difference reported in column 5 is 0.1.

M1 and M2 are used because they are two 24 hour testing periods under identical circumstances (position and temperature). 

Similarly, M3 is 1.9 and M4 is 2.8, so the difference reported in Column 5 is 0.9.

Just below the chart, on the right, is the date on which the COSC testing was completed. In the sample certificate, that date is July 25, 1997. If you're buying a watch that comes with a COSC certificate, you can compare the date testing ended with the present date to get an idea how long ago the test occurred.


The test completion date won't necessarily tell you how old the watch is or how long its been in the dealer's case, however. As discussed below, manufacturers sometimes place movements in a vault upon return from the COSC, to be cased at a later date.

The important numbers are found at the bottom of the certificate, shown below. These are the Results or Summary. 
This is where the raw numbers are plugged into various formulae and the results of the testing are shown. Note that not all of the figures needed to make the final calculations appear in the centre portion of the certificate.

The Results or Summary reflect seven specific types of error deemed significant by the COSC. Each of the seven results must fall 

within COSC guidelines for the movement to pass. If even one result is outside the guidelines, the movement does not pass. Manufacturers are free to re-submit movements that fail.

The bottom section of our sample certificate is set forth below. A description of the tests and calculations follows.

  1. Mean daily rate in the [5] different positions. The first figure is mean daily rate in the different. This is the average rate over the first ten testing days. Simply add the rates for the first 10 days (in our example, the total is 18), then divide by 10 to get the result (1.8 in the example). To pass, the mean daily rate must be -4 to +6 seconds per day. 

COSC determines the mean daily rates by subtracting the time indicated by the movement 24 hours earlier from the time indicated on the day of observation. The -4 to +6 figures are the most often quoted when someone asks "what are the COSC guidelines".

As we'll see, these two figures are only the tip of the iceberg.

The mean daily rate figure is said by some to be the most important to the owner of the watch. This "daily rate" tends to indicate how the watch will perform on the wrist in actual use. In his book "Wristwatch Chronometers", Fritz von Osterhausen states at page 59: "the lower the daily rate, the more accurate time a watch gives."

  1. Mean Variation, or mean daily rate variation in 5 positions. This test is sometimes referred to as rate deviation.
    The COSC observes the movement's rate in the 5 positions each day over 10 days for a total of 50 rates. To pass, the mean variation in rates can be no more than 2 seconds per day.

Generally speaking, this figure shows if a watch tends to run fast, slow or correct, or perhaps both fast and slow; in other words, the continuous rate of the movement. 

It is said this measurement shows the quality and reliability of a watch - a good quality watch will be consistent, whether it is fast, slow or correct.

The disadvantage of a consistent error is that it accumulates. If the watch is 10 seconds fast every day, it will be over a minute fast at the end of a week. However, a watch that is consistent is more easily regulated, and so the error is more easily eliminated.

  1. Maximum Variation. This is the largest difference between any two day's readings in a single position. In the sample certificate, we see that M9 (9th test day) was 1.3, and M10 (10th test day) was 4.3.

On both of these days, the watch was in the horizontal position, and the difference was 3.0. This was the largest difference or variation between readings in any position. To pass, the greatest variation in any position cannot exceed 5 seconds.

  1. Difference between flat and hanging positions. The figure is calculated by subtracting the average of the rates in the vertical position (first and second days) from the average of the rates in the horizontal position (ninth and tenth days). 

To pass, the difference must be no more than -6 to +8 seconds. Some experts feel this test shows the greatest difference between mass produced and hand adjusted movements. This figure tends to be lower for better quality movements.

  1. Greatest difference between the mean daily rate and any individual rate. This is the largest difference between the mean daily rate (the figure reported in test number 1) and any individual rate during the first 10 days of positional testing. 

In our sample certificate, we see that on M6 (6th day of testing), the rate was 5.3. The mean daily rate was calculated at 1.8. The difference between these two is 3.5. The figure reported for test 5. To pass, the difference between the greatest daily rate and the mean daily rate can be no more than 10 seconds per day
Variation of Rate per 1 Centigrade. This is the temperature error of the movement, expressed in seconds per degree. 

  1. The COSC subtracts the cold temperature rate (8 degrees) from the hot temperature rate (38 degrees) and divides by 30.

In the sample certificate, the rate at 8 degrees was 7.1, and at 38 degrees it was -3.2. So -3.2 - 7.1 = -10.3 30 = -0.34. 
To pass, the variation cannot exceed 0.6 seconds per day.

  1. Rate Resumption. This figure is obtained by subtracting the average mean daily rate of the first two days of testing from the mean daily rate of the last test day. To pass, the resumption of rate can be no more than 5 seconds.

By way of summary, the COSC passing guidelines for all seven tests for medium and large wristwatches (movements greater than 20 mm in diameter) are currently as follows:

  1. a mean daily rate over 5 positions of a minimum of -4 seconds and a maximum of +6 seconds;
  2. a mean variation between the days in each position of 2 seconds maximum;
  3. a maximum variation between days in each position of 5 seconds;
  4. a difference between horizontal and vertical positions of -6 seconds minimum and +8 seconds maximum;
  5. a difference between mean daily rate and any individual rate of 10 seconds maximum;
  6. a variation rate per degree centigrade or -0.6 seconds minimum to +0.6 seconds maximum
  7. rate resumption of -5 seconds minimum to +5 seconds maximum.

The Value of the COSC Certificate to the Owner

The COSC certificate is not a precise indicator of how well a watch will perform on the owner's wrist. Rather, the test results show how well a given movement performed over a given period of time under certain conditions. Several factors can affect the actual performance the owner observes.

First, The COSC test is only an artificial approximation of actual use. Second, the COSC does not test "watches" but only uncased movements. Following the test (assuming a movement passes), the movement returns to the manufacturer, where it is either cased or placed in a vault for casing at a later date.

Movements can spend significant time in the vault. When they do, they are usually disassembled, cleaned, re-assembled, oiled and regulated, then cased. This can certainly affect the movement's performance, yet re-testing is not required.

Next, the watch may be affected by shipping in the distribution chain, where it might be exposed to shocks or extreme temperatures. Conditions at the dealer's shop also affect performance. If the watch sits unused for many months, if it sits under hot lights or in the window in the sun, if customers drop it while trying it on, this will affect how the watch performs on the owner's wrist.

Finally, the owner's personal habits can affect performance. Is the owner active or sedentary? Is the watch kept near full wind or allowed to run down? Is the watch in one position most of the time? Is the owner in cold or hot temperatures, or moving between the two? Is the watch placed on a winder? How is the winder set? Each of these also affects performance.

Info about COSC testing

The COSC is not used as a way for a manufacturer to do quality control or to test their movements--the tests are much too expensive for that, and their failures become part of the public record. The only reason movements are sent to the COSC is to pass the tests and get a chronometer certificate.

The COSC doesn't rubber-stamp all movements, even though something like 95% of them pass. Since the companies pay a hefty price for the testing (I think it is the equivalent of about $200-$250 per movement), they will have passed very stringent quality control by the manufacturer first--the manufacturer is only going to send movements that they are sure will pass, since they still need to pay for the ones that fail.

There is an early grace period, so if a movement has problems and it is obvious after the first couple of days that it will not pass, it can be withdrawn and only a partial fee must be paid. Watches that fail after the grace period pay the entire fee. The most common reason for failure is that the movement stopped during testing. One can imagine that many problems are the result of all the shipping and handling that these movements would get.

COSC testing doesn't really mean anything significant. It is only an independent test with a certificate awarded for passing. A watch from a decent manufacturer might perform as well as, or better than, than the COSC standards, whether or not is has a certificate. JLC's Master series watches undergo much more stringent tests than the COSC administers, and are tested in their actual cases.

Movements that go to the COSC are mounted in temporary cases, with hands and dials, for testing purposes. After they come back from testing, they are put into their real cases. Watches with no second hand are sent for testing with a temporary second hand fitted.

 There is a picture in the Chronometer Watches and Their Testing book which shows chronometer movements under test being wound with a small motor. All watches under test get wound by their crowns, even automatics. The winding device is smallish and looks like a dentist's drill or a Dremmel motor tool. The business end is a cup that fits over the crown, like a socket wrench.

The movements cases are threaded one-after-the-other on long straps, about 10 to a strap. There are about 10 of these straps mounted in a rack or frame, so that if you look at the frame from the front, you see 100 watch faces equally spaced in rows and columns.

During testing, the entire rack, with cased movements attached, is placed in the various positions, and moved into the hot and cold testing areas. (Very old chronometer certificates refer to the "oven" and the "refrigerator.")

Instead of trying to read the time on each face when the daily test results are recorded, in the past they took a photograph of each rack, including a reference timepiece in the photo. This would record the position of each second hand in the rack, at the same instant, and would make it possible to read the indicated time on each face to a fraction of a second, as well as documenting the correct time on the reference timepiece.

 The photographic negative would be read under magnification, and the test sheet for each movement would be filled out.

Today, a laser scans across the rack and reads the position of each second hand, and the test results are stored as a computer file.

 Automatic watches are wound once a day, by their crowns. They are happier in real life, where they are constantly wound during the day by their rotors, and their mainspring tension stays fairly constant. Because of this, they tend to perform slightly less well in testing than they do in real life--they run a little slow.

 The opposite is true for hand-winds, which do slightly better in testing than they do in real life. Some companies regulate their autos slightly fast before they go to the COSC, to compensate for the once-a-day winding, then regulate them slower again when they come back.

Rather than absolute accuracy, which can be regulated easily if the watch runs slightly fast or slow, the most significant aim of COSC testing is to test for consistency in different positions and temperatures. This is primarily a test of the balance and hairspring.

 The balance and hairspring are part of a module which is not taken apart even when the watch is serviced, so a watch that performs well in testing should do so as long as this is not tampered with.

 In the Chronometer-book, the author reports on a large number of vintage chronometers, mostly from the 1950s and 60s, which he has repaired and serviced and then subjected to simulations of their original COSC tests. Most of them could still pass, once they were returned to good running condition.

 One thing that the book impressed upon is the extreme difficulty of making a mechanical watch perform well in all positions and at different temperatures.

 The COSC standards are compromised from an ideal, so that the standards fall within ranges that can be met realistically in mass production. The standards have been influenced by the companies, and were loosened slightly during the early years of the COSC, because they were too difficult for the movements of the time, and too many watches failed.

There were once "passed" and "passed with distinction" ratings, but today movements either pass or fail. The standards have gotten tighter over the years.

 In some instances, the methods of calculating and recording a watch's performance, rather than the actual test itself, can be questioned.

For instance, if the watch runs -5 seconds in one position, and +8 in another, this falls within the test range because the deviation is measured from 0.

 However, the range of variation is 13 seconds. The deviations caused by temperature change are listed not by total variation, but by variation per degree of temperature change.

 A .5 second change per degree looks pretty good, but if you multiply it by 20 degrees, 10 seconds doesn't look so good. The results are not fudged, but the way they are reported makes most watches sound better than they are.

 Just to repeat, the book explains how difficult it is to build a mechanical watch, in large quantities, which will perform consistently under changes of position and temperature.

 Seeing the issue from both sides, I personally don't have any problem with the test standards. However, I also don't think the certificate is as significant as some people do.

 You can be smart and well-educated, and yet not have a college degree, but some people are more likely to trust what you say if you have diplomas and certificates on your wall. If your watch has a COSC certificate, it means that it was able to pass a bunch of tests under artificial conditions. Someone with a diploma was also able to pass a bunch of tests under artificial conditions. In both cases, your real-world results may differ.