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The Sinn 903 St II is a black-dial mechanical navigation chronograph with an automatic column-wheel movement and a logarithmic slide-rule bezel. Sinn says the bezel can help calculate times, distances, and fuel consumption; that function makes the watch aviation-inspired, but does not establish that it is certified flight-navigation equipment. The specifications below are Sinn’s, not the results of an independent hands-on test.
What the Sinn 903 St II is
Sinn Spezialuhren describes the 903 St II as “The navigation chronograph.” It is the black-dial member of the current Series 903, which comprises two models. The St II has a black electroplated dial with sunburst decoration, a stainless-steel case, and a mechanical chronograph. Its defining combination is a column-wheel chronograph and a rotating external bezel marked with a logarithmic slide-rule scale. Sinn’s November 2025 903 St II brochure supplies the model description.
Calling it an aviation chronograph describes its design and intended calculation feature; the available manufacturer material does not establish aviation certification, approval for flight use, or suitability as a replacement for dedicated navigation instruments.
What the slide-rule bezel does
A logarithmic slide rule lets the wearer align scales to perform certain arithmetic operations. Sinn says the 903 St II’s bezel can be used for calculations involving time, distance, and fuel consumption. The scale is intended as a compact analog calculation aid, not a digital calculator or flight-navigation system. Sinn describes it as operable from the watch’s outer diameter; that description alone does not establish how easy it is to grip or how precisely it operates in practice.
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The brochure does not provide a validated, step-by-step procedure for a particular calculation. Treat the listed uses as the manufacturer’s description of the function, rather than assuming a particular calculation method or real-world accuracy.
Specifications Sinn publishes
| Specification | Sinn’s stated detail |
|---|---|
| Movement | LJP L110, self-winding automatic |
| Movement details | 26 jewels; 28,800 semi-oscillations per hour; seconds-stop function; anti-magnetism according to DIN 8309 |
| Functions | Hours, minutes, subsidiary seconds, date, chronograph, and logarithmic slide-rule bezel |
| Chronograph construction | Column wheel |
| Case | Stainless steel, polished and satinized |
| Crystals | Front sapphire crystal, anti-reflective on both sides; transparent sapphire case back, anti-reflective on the interior |
| Crown and case back | Screw-down crown and screw-down case back |
| Pressure resistance | 20 bar, with low-pressure resistance also stated |
| Band-lug width | 22 mm, as listed in Sinn’s November 2025 Series 903 brochure |
| Case diameter | Not established: Sinn’s 2025–2026 catalogue excerpt includes conflicting 41 mm and 43 mm captions |
The movement and case figures in the table are manufacturer specifications from Sinn’s December 2025 technical sheet; the lug-width and diameter information comes from the November 2025 Series 903 brochure and the 2025–2026 Sinn catalogue. The source material available for this review does not resolve the case-diameter conflict, so neither figure should be treated as definitive. Confirm the current product specification with Sinn before buying.
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How to assess the design and performance claims
The published specification gives a useful picture of the mechanism and construction, but it is not a performance review. The available sources do not establish an independently measured rate, chronograph accuracy, dial legibility, lume performance, comfort, durability, or water performance in use. The presence of a 20-bar pressure-resistance specification is not an independent water test.
Sinn’s November 2025 brochure describes hybrid ceramic luminous elements. That identifies the material category in the manufacturer’s product description, but it does not provide a measured brightness or duration result. Likewise, the anti-magnetic statement is specifically “according to DIN 8309”; it should not be expanded into an unsupported claim about resistance in every magnetic environment.
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Choosing between the two Series 903 models
Sinn’s current Series 903 information identifies two models. The black 903 St II has a black electroplated sunburst dial; the blue 903 St B E II is described with a blue dial and luminous ivory coating. They share the series’ central proposition: an automatic LJP movement, column-wheel chronograph, slide-rule function, sapphire front and display back, and stated 20-bar pressure resistance. The available material does not establish a performance advantage for either dial option.
Check the current listing for the exact strap or bracelet included with the configuration you are considering. Do not assume that a pictured or previously listed accessory is supplied with every stock item.
Rank #4
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Fit, straps, and purchase checks
The November 2025 Series 903 brochure gives a nominal 22 mm band-lug width. That is a starting point for strap searches, not a guarantee that every 22 mm strap or bracelet will fit: end shape, clearance, and attachment design still matter. Confirm compatibility for the exact St II and accessory before ordering.
Price, taxes, delivery charges, stock, warranty terms, service costs, and authorized-dealer availability depend on market and date; the cited Sinn documents do not establish current local terms. Sinn also notes in its December 2025 technical sheet that specifications reflect its current production situation and that partner stock can differ after technical changes. Confirm the details of the specific watch being offered before purchase.
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Verdict
The Sinn 903 St II is a mechanically specified navigation chronograph whose strongest point of distinction is the pairing of a column-wheel chronograph with an external logarithmic slide-rule bezel. Sinn states the bezel’s intended uses and publishes substantial movement and construction details. The evidence supports those specifications, not claims about accuracy, wearing comfort, or real-world calculation performance. The unresolved case-diameter discrepancy is a practical reason to verify fit directly before committing to a purchase.
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