There are a lot of brands out there that use Seiko’s unbranded SII NH35A movement in “dive watches” that don’t say “Diver’s XXXm” on the dial face. So I looked up the magnetic resistance rating of the Seiko (SII) NH35A movement (which powers brands like Invicta, Vostok, Minus 8 Layer, etc). But first, lets look at the standards for “Diver’s” which a watch manufacturer must meet to include the word “Diver’s” on the dial.
The International Standards Organization (ISO) has a set standard for Dive Watches which includes a level of antimagnetic resistence. In short, to meet the ISO standard a watch movement must remain unaffected by a DC field of 4,800 A/m. There are also shock resistance and depth resistance standards, but those are a bit easier to explain.
There are many ways to express magnetic force, but essentially the magnetic strength of 4,800 A/m translates to just over 60 Oersted or Gauss. A standard refrigerator magnet is about 50 Gauss, and a standard Iron magnet about 100 Gauss, and neodynium magnet can be up to 18 times stronger than a standard magnet. So you shouldn’t be laying your Dive watch on top of a rare earth magnet any time soon.
The ISO shock resistance requirement is that the watch maintain time and operation after surviving a one meter drop onto a wood floor. The depth rating is simple, it needs to be able to remain water tight at a certain pressure for a set time.
The rating on the NH35A movement is greater than or equal to 4,800 A/m according to the Seiko fact sheet about the movement. So even if Invicta or Vostok doesn’t include “Diver’s” on the face plate (probably because Rolex doesn’t, and Orient doesn’t bother with the Mako or Ray offerings) the movement itself meets the ISO dive watch standard for anti-magnetic requirements.
The NH35A documentation doesn’t describe the movement’s impact resistance in those terms, but does describe that it has a shock absorb device for the balancer shaft, indicating that it would meet the ISO standards for shock resistance especially since the action or it’s on brand equivalent is used in ISO certified diver’s watches. Other reviews say that it uses the “Diashock” impact resistance spring setup that Seiko has used for decades. Whether Diashock is better than Incablock is more a matter of opinion than fact, as the fact is that both work quite well at producing shock resistant watch movements.
In the realm of anti-magnetic watches, you have some premium watches like the Rolex Milgauss and the Omega >15,000 Gauss watch, both of which will set you back a pretty penny. These two watches use two different approaches, the Rolex creates a “magnetic shielding” in the watch case using “dead soft iron” and the Omega replaces anything in the watch movement that could be affected by magnetism with something that is magnetically transparent/inert. But, either way these watches are much more magnetically resistant than the minimum ISO standard of 60 Gauss. And cheap made in Malaysia or Russia watches using the SII NH35A aren’t going to bother testing those watches to find out exactly how magnetic resistant they are or are not based on case construction.
Now all of this is “interesting” but not very useful to anyone other than budding watch geeks who might still be enthusiastic about picking up an Invicta 8296OB for cheap powered by the SII NH35A.
The 21,600 beats per hour of the NH35A (and many other Japanese movements like the Miyota 8215) isn’t “high beat” by any stretch of the imagination. However it does offer at least as smooth a second hand sweep as early Rolex Submariner watches which were powered by 18,000 bph and 19,800 with the 1530 and 1520 movements. When comparing a watch with an older ETA 1256 (18,000 bph) and an ETA 2824, the NH35A doesn’t come with the same “Swiss” pedigree, but it does come with the solid lineage of Seiko, which got into the dive watch game in the 1950s shortly after the Swiss did.
The 24 Jewels in the NH35A is more than enough, as quite honestly 17 jewels is the minimum for a “precision” or chronograph grade movement. The fact that the NH35A is at a price point (essentially 30 US Dollars last time I checked as of May 2018) where it would cost more to get chronograph certification than manufacturing the entire watch cost. I think one of the nicer features of the NH35A movement is how easy it is to adjust so that you can get very close to +/- single digit seconds per day. There are plenty of guides on how to do this, youtube videos too, but it really boils down to just keep adjusting once every day or two days until you hit the right spot then stop.
The 41 hours of power reserve the NH35A has is nothing special but still highly functional as if you wear the watch every day you will keep it going. The Seiko 6R15 is essentially the on brand upgrade to the 4R35 with a Spron 510 mainspring which increases the power reserve to 50 hours, or just over two days. I honestly don’t believe that the additional 9 hours of power reserve is meaningful to most watch owners but to collectors the 6R15 movement is definitely considered more desirable. For comparison the ETA 2824-2 movement (and Chinese clones made by Seagull, Peacock, etc) used in many more expensive dive watches is advertised with 38 to 40 hours of power reserve. The Chinese clone movement watches (such as Tauchmeister) can be found in similar price ranges to SII powered watches, but I don’t have any deep familiarity with them to recommend them or not.
I hope this has been a fun look for someone, although still a cheap solar powered quartz watch will keep better time and be much more maintenance free over the long haul, but then again on “Prime Day” you can pick up an Invicta powered by the NH35A with a depth rating of 200 meters if you want to get a mechanical watch that won’t make you cry if it gets lost or stolen. Automatic watches are a bit of a fun piece of technology even in the world of Apple and Android smart watches.
UPDATE: Since I wrote this post some of the search results have been for “NH35A upgrade” and related sort of sentiments. The the outer case diameter for the NH35A is 27.4mm with a height of 5.32mm, the same as the NE15 with the upgraded SPRON mainspring if someone wants to have a jeweler or watchmaker swap out the NH35A for an NE15 that would get an extra nine hours of power reserve.
At some point in the future Seiko may decide to put a 4 hertz escapement to make a 28,800 high beat movement on the basic 6R framework (6R15, 35, 36) which would make an easy upgrade path for folks with an NH35A based watch, the same way the NH35A was a minor fitting issue for upgrading a 7S26 (diameter 27.4mm, height 4.90mm). But from a functional user perspective, there is no real benefit going from 21,000 to 28,800 bph in terms of accuracy or reliability. I have a sneaking suspicion that the 6 beat per second is 25% less wear and tear on the action compared to an 8 beat per second movement, which may contribute to Seiko’s near legendary reputation for making watches that get used and abused for many years, sometimes decades, between services.
However, if someone is really interested in a “high beat” upgrade, the outer case dimension for a Sellita 200 (same as an ETA 2824, or Chinese clone) is 25.6mm with a height of 4.60mm, so it is possible to fit an ETA 2824 into a watch case currently holding a NH35A, however it isn’t a swap that I would recommend as unless you are doing the customizing work yourself, the cost of paying someone to do it would be about the same as buying a decent Swiss watch. A Seiko 4S15 movement with 28,800 bph has an outer case movement diameter of 26.0mm and a height of 4.17mm, so it could be “made to fit” as well, but you run into the same cost problem if you are paying someone else to do the work. A Miyota 9015 is 26.0mm and 3.9mm, and I would recommend just buying a watch on the used market with a 9015 movement rather than trying to upgrade a watch with a NH35A movement.
In my opinion the NH35A is a great movement, highly reliable, designed to be rugged, and not fussed with every two or three years like a Swiss movement. It is much like the Toyota engine in that it is put in a pickup to go to horrible environments, be used by people who are hard on their gear, and serve them reliably, but can also be cross branded into a Lexus sedan and give a smooth, comfortable drive. I also think that there are so many ETA 2824-2 or Miyota 9015 movement based watches in the secondary market that anyone wanting a “wealth status symbol” watch can really get a good one for not too much money if the extra smoothness on the second hand sweep is a desired feature.