There is no such thing as a perfectly accurate watch.
There are extremely accurate timepieces, for example an atomic clock, that keep time by measuring the vibrational frequency of an element, but even these are not perfectly accurate forever.
For watch owners who love a well-built timepiece but want the convenience of one with less maintenance required when it comes to keeping it working, a mechanical watch may be the perfect fit.
Of course, automatic watch accuracy isn’t perfect, but accuracy varies greatly between different models and brands.
Even the most precise watches still lose or gain seconds every day, and an automatic watch is no different.
Let’s look at some things you should consider when you’re ready to purchase your next mechanical watch. First, let’s discuss why there is no such thing as a perfectly accurate watch, not even among the best automatic watches under $500 (or more expensive ones for that matter).
Any device that has moving parts is never going to attain one hundred percent accuracy.
Friction, drag, and the laws of physics limit the ability of any device to operate exactly the same way, day after day, year after year.
This is even more pronounced in the case of watches and other timepieces. These intricate devices usually contain over one hundred different parts, and a very small problem with just one of them can lead to your watch failing to keep accurate time.
The most precise watches will still lose or gain seconds every single day, which means the time on your watch can quickly become off by minutes or even hours.To limit how often you must reset it, you want the most accurate mechanical watch you can get, which means we need to know exactly how accurate a watch can be.
The COSC Standard
The COSC has exactly one function: to test, measure and certify the accuracy of watches. This Swiss non-profit has been testing and certifying watches for decades, and receiving a certificate from the organization is an achievement; for example, only three percent of the watches they test manage to achieve COSC certification.
There are seven different areas the COSC test before they will certify a watch with a mechanical movement and eight areas for a quartz movement. To get a COSC certification, a watch movement must pass all the tests for its movement type; coming up short in just one will result in the watch failing this strict test.
Here’s another thing to note: these are not short, simple tests. Each individual watch movement is tested for fifteen straight days to ensure an accurate reading of its ability to keep time. This is not a certification that is easy to achieve, as you can see. This makes it a great way to gauge just how accurate the new watch you want to purchase is.
Testing Criteria for the COSC Standard
For a mechanical watch movement, there are seven different things that are measured to determine its accuracy:
- Average daily rate of seconds lost or gained
- Mean variation in rates (or how consistent the loss or gain is over a period of days)
- Greatest variation
- Difference in the rate of seconds gained or loss in either vertical or horizontal position
- Largest variations in rates (including in different positions)
- How much variation there is at different temperatures
- Rate resumption (indicates how reliable the watch is over time; it’s calculated by taking the mean daily rate on the last day of testing and subtracting the average mean daily rate of the first two testing days)
To receive an official COSC Chronometer rating, mechanical movements must have the following results on tests (or better):
- Average daily rate: +6 or -4 seconds per day
- Mean variation: 2 seconds per day
- Greatest variation: 5 seconds per day
- Difference of rates in vertical/horizontal position: +8 or -6 seconds per day
- Largest variations: 10 seconds per day
- Thermal variation: plus or minus .6 of a second per degree Celsius
- Rate resumption: plus or minus 5
For a quartz movement watch, the tests are different. The criteria include benchmarks for the average daily rate at three different temperatures (23 degrees, 8 degrees and 38 degrees Celsius), plus rate stability, the dynamic rate, the immediate and lasting effect on accuracy from mechanical shocks, and rate resumption.
For a quartz movement, the marks a watchmaker is aiming for are:
- Average daily rate at 23 degrees Celsius: plus or minus .07
- Average daily rate at 8 degrees Celsius: plus or minus .2
- Average daily rate at 38 degrees Celsius: plus or minus .2
- Rate stability: .05
- Dynamic rate: plus or minus .05
- Mechanical shock effect (temporary): plus or minus .05
- Mechanical shock effect (residual): plus or minus .05
- Rate resumption: plus or minus .05
These measurement standards may make it seem to be an easy thing to find the most accurate automatic watch, but not as much as you might think.
These standards are used to establish if a watch as “chronometer accurate,” which means it is one of the most reliable timepieces available.
However, this doesn’t mean that only a watch with a COSC certification can achieve this level of accuracy; in fact, there are many watches that have no certification that can easily achieve the average daily rate of the COSC standard for mechanical watch accuracy of plus six or minus four seconds per day gained or lost.
This is good news, because the small amount of watches that receive the COSC certification are some of the most expensive watches you can buy. Additionally, there are things that can impact the accuracy of any watch, no matter how expensive or perfectly made.
Factors That Affect Precise Watches
As you can most likely guess from the things tested during COSC certification, there are various factors that can impact how well your watch keeps time.
This is the most common factor that can affect your watch’s ability to keep time, due to changes the temperature creates inside your watch that are invisible to the naked eye. As you know, heat will cause metal to expand, while cold will cause it to contract, or shrink.
A well-built watch consists of over a hundred moving parts, all machined with an exacting level of precision. Some of these parts are extremely small, which means that a very small change in size of any of those parts can become magnified as the watch movement works.
The extremes tested by COSC, 8 degrees Celsius (46.4 degrees Fahrenheit) and 38 degrees Celsius (100.4 degrees Fahrenheit), are the temperatures at which you’ll start to see an effect on watch movements. If you wear your watch in conditions that are warmer or colder than those extremes, or even close to those temperatures, you will see a decrease in your watch’s accuracy.
Another obvious threat to your watch’s ability to keep time is shock from any kind of impact.
The larger the impact your watch suffers, the larger the chance that its delicate internal mechanisms will become damaged or misaligned, which will decrease its accuracy.
Many of those tiny, precision-machined parts inside your watch are made from ferrous metals, which means they are affected by any magnetic field they’re exposed to.
Spending time in contact with a magnetic field will slowly degrade your watch’s accuracy over time, and if it spends enough time in a strong magnetic field, some of the pieces of your watch movement could become magnetized themselves.
How to Check Automatic Watch Accuracy
All this information does you no good if you don’t have a simple method of checking the accuracy of your watch.
Luckily, there are two relatively easy methods you can use to check the accuracy of your watch, one that’s been in use for decades and another that’s truly a product of the 21st century.
Smartphone App to Check Watch Accuracy
There are many different accuracy checking applications available on the app markets for both iOS and Android devices. Most of these apps match the time of your automatic watch to a standard time, usually kept by your cell phone provider and regularly updated through their network.
While these apps do the math for you, converting the data on your watch into an easily-understood rate of deviation per day, this also means you’ll have to manually input the data on your watch to get a true reading.
You will have to track things such as the temperature, whether the watch was in a vertical or horizontal position, and the difference in time as compared to the network’s time.
Once you have input this information for several days, the app will calculate your automatic watch accuracy with no cost but the time it takes. However, there is an easier method of checking the accuracy of your watch.
As you can imagine, watchmakers and watch repair professionals don’t have the time (so to speak) to track a watch’s measurements over a period of several days in order to determine if a watch needs to be serviced or repaired. The professional’s tool for doing this is called a timegrapher.
The timegrapher is a very specialized device that can measure the accuracy rate of a watch almost immediately. The tool has a small pedestal to hold your watch; once your watch is in place and the device is activated, it will tell you the accuracy of the movement inside the watch in seconds instead of days.
The advantages of using a timegrapher are obvious: nearly instantaneous, highly accurate measurement of your watch’s accuracy, anytime you wish to check it. The only advantage an app has over this device is the cost; many of the watch accuracy apps are free to use, while purchasing a timegrapher may be more than you want to spend to keep a check on your watch’s accuracy.
The Most Accurate Automatic Watch
While a seemingly simple question, there is no one correct answer to "What is the most accurate automatic watch?"
Common wisdom would lead you to believe that the cost of a watch has a direct relationship to how accurate it is, but that’s not always the case, even though there is some truth to this line of thinking.
The 3% of Swiss watches that achieve COSC certification are among the most expensive watches available, from Rolex to Omega, Tag Heuer, and Breitling, and these are indeed some of the most accurate watches available.
However, just because a watch does not have a COSC serial number stamped into the case does not mean it is necessarily less accurate than one that does. Seiko, Citizen and other watch manufacturers make watches that easily hit the sec/day accuracy standard for certification.
Any watch that is built to chronometer standards is going to be extremely accurate, but remember, even the most accurate watches are still subject to things that can reduce their accuracy, like temperature and shock, and even whether it’s in an upright or horizontal position. Small things can make a large difference in your watch’s accuracy.
A watch with a mechanical movement is always going to suffer from some amount of variation in its ability to keep accurate time. Automatic watches in particular require special care to maintain accuracy; if you’re going to have the watch off your wrist for more than a day, make sure to store it on a watch winder to keep it ticking.
Quartz movements inside watches are much more accurate; you may have noticed that the COSC certification standards for quartz movements are much stricter. This is because a quartz movement is much less prone to losing or gaining seconds as it operates.
There’s just something about a well-made precision mechanical movement watch that makes it special to many, though. For a lot of people, the accuracy of the watch isn’t as important as other factors; wearing a fine watch is a feeling in and of itself, and for most it’s worth the adjustments needed every few days to keep it set properly.