A deeper look at the SII NH35A watch movement

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.

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 or a Vostok Amphibia for cheap which are powered by the SII NH35A.

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.


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3 Responses to A deeper look at the SII NH35A watch movement

  1. Pingback: Watch Review, Invicta Pro Diver Automatic | Wandering Through The Night

  2. Pingback: Spinnaker Medium Cahill Diver style watch initial review | Wandering Through The Night

  3. makais says:

    helpful info on this movement, thank you


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