In this review, we are taking a deep dive into the Omega Seamaster Aqua Terra 150m Co-Axial Master Chronometer 41 mm. The Aqua Terra line was introduced 20 years ago but recent updates have given it a powerful resurgence.

The latest version in 41 mm features the Caliber 8900 automatic movement with a 60-hour power reserve, co-axial escapement, and METAS certification and we are going to look at some of the technical features which make that possible.

Let’s start with the 60-hour power reserve. The caliber 8900 has two barrels which makes it possible for the watch to store sufficient power to keep it running for 2½ days. Of course, it is great to have a watch that will stay running for a long time, but for me, I don’t mind winding my watch and I’m active enough to keep an automatic watch wound. I think the biggest advantage of a longer power reserve is improved timekeeping. As the power reserve of a watch winds down the mainspring delivers less torque which can affect the timekeeping, especially at the very end of the power reserve. The extended power reserve means it is less likely that your watch runs at low power, even if you take it off for a day.

The watch also has a free-sprung balance with a silicon hairspring. The free-sprung balance eliminates the need for regulating pins and once again means better timekeeping. When you eliminate the regulating pins, variations in balance amplitude will have less effect on timekeeping. Silicon provides two distinct advantages: first, it is non-magnetic which makes it possible for this watch to meet the stringent requirements for resistance to magnetism that are a part of the METAS certification. Thanks in part to the silicon balance wheel and hairspring, this watch exhibits almost no variation in rates when exposed to large magnetic fields up to 15,000 gausses. The second advantage is that silicon is not malleable which means the hairspring won’t deform when it receives shocks. Watchmakers be aware: no adjustment is necessary (or possible)!

Lastly, of course, is the co-axial escapement. Plenty has been written about the co-axial escapement so I just want to point you to a couple of good resources and give a quick overview. The die-hard fan will want to hear about the co-axial escapement directly from the master himself, George Daniels, and for that, I recommend a 1-hour lecture he gave to the American Watchmakers-Clockmakers Institute which is available on their YouTube channel. For a much shorter explanation, any of the many videos Omega has produced will certainly suffice.

Let’s break down the key points. The Swiss lever escapement found in most mechanical watches is inefficient. The geometry requires that the impulse be delivered to the balance via a sliding action. A considerable amount of energy is lost to friction as the escape wheel tooth slides across the pallet stone and lubrication is required. The co-axial escapement separates the locking and impulse actions which allows the impulse to be delivered with almost no energy lost to sliding and almost entirely eliminates the need for lubrication. In Daniels’ original design he incorporated two escape wheels in order to separate these functions but as he made improvements he incorporated both wheels onto a single axle, hence the name: co-axial escapement.

The increased efficiency and near elimination of lubrication in the escapement means less degradation in timekeeping performance over both the power reserve and service interval of the watch.

We are now many generations removed from George Daniels’ original co-axial designs and from Omega’s first (and failed) attempt at implementing the escapement into mass-produced watches and they have created a watch caliber with a co-axial escapement that is reliable and which performs with excellence.

One thing you won’t see in any of the flashy videos produced by Omega are actual photos or videos of the Co-axial in action. In order to explain the escapement’s function they always use animations and models so I thought I would include some photos of the different components alongside of their counterparts from a traditional Swiss lever escapement.

In the Swiss lever escapement, the pallet fork has two ruby stones each of which has a locking function and an impulse function. In the Co-axial escapement, the locking functions and impulse functions are separated. The pallet fork has 3 stones. The ones on the left and the right only carry out a locking function. The one in the center receives the impulse.

The other impulse is received by an extra impulse jewel located on the roller of the balance wheel.

So, what we all want to know is how it performs. In my workshop, the watch I received for review averaged a gain of just 1 second per day over a 4-day test and demonstrated excellent values on the timing machine with an average daily rate of between 0 and 1 seconds and a delta of 7 seconds across all positions and 2 states of wind.

Verdict

In conclusion, the caliber 8900 performs excellently. If you are looking for a good timekeeper you will find it in the Omega Seamaster Aqua Terra 150m Co-Axial Master Chronometer 41mm. If I had any reservations it would be that you should be aware that the Co-axial movement is more difficult to service than a traditional Swiss lever escapement and the number of qualified watchmakers who can carry out that work is much smaller so you should expect to pay a little more for service, but on the positive side of things, the service interval is longer so you won’t have that bill quite as often and the cost of ownership is probably the same on average. This watchmaker recommends the watch.