A Novel Hydrostatic Watch Modification with Benzyl Benzoate

All images shot with E-M1.2

About a decade ago, a number of watch manufacturers began introducing professional diving watches filled with silicone or fluorinated oil such as the Sinn UX-EZM2B used by the German Commando Frogmen or the Bell & Ross Hydromax and rated for a depth of 12,000 m (although the quartz movement is only rated at 5000 m depth). Neither of these values are realistic since the deepest scuba dive is less than 600 m but it does highlight the incredible pressure resistance of liquids when they occupy the space in a watch formerly inhabited by air. The inclusion of a liquid does confer other important advantages for divers. There is no longer humid air that could condense and fog the dial face as temperatures drop with water depth and the higher index of refraction of the oil improves readability of the dial face at severe angles and also eliminates the onset of total internal reflection which completely obscures dial face visibility.

What’s the deal with dedicated diving watches? Less so today with dive computers but divers need to time their resurfacing carefully to avoid the “bends” where nitrogen gas dissolved in their bloodstream can come bubbling out creating incredible pain and death if divers resurface instantaneously.

The oscillating balance wheel of a mechanical movement cannot operate in the increased viscosity environment of a liquid so only quartz movements are compatible. These watches can also compensate for the thermal expansion of the oil by having a case back that can expand and contract in those minute volumes. Perhaps the definitive application of oil filled watches is that of the astounding (and very expensive) Ressence Type 3 watch. The enhanced dial face visibility gives it the appearance of a smart watch while it remains a truly all analog watch. The oil filled dial compartment is separated from the mechanical ETA movement by a flexible titanium bellows system that compensates for thermal expansion and the dial itself is driven by magnets.

Many watch enthusiasts have started to experiment with adding mineral oil (ie baby oil) to inexpensive quartz watches in an effort to improve the appearance and visibility of the dial face. It’s not clear if the choice of mineral oil is wise since it is a product of petroleum distillation and can dissolve certain kinds of plastics like polycarobonates, polyphenylenes, & thermoplastic polyimides and possibly damage circuit boards. It can apparently also make lumes turn yellow.

What is not in dispute is how the addition of oil enhances the dial face visibility.

The light path when observing a watch under water.  In (a) the watch is flat, facing up and fully visible.  In (b) the watch is rotated 30o and the dial face is still visible.  In (c) the watch is rotated 60o causing the light wave to impact the crystal/air interface at a critical angle resulting in a phenomenon known as total internal reflection.   The dial face turns into a mirror.  In (d) the presence of an oil with a higher index of refraction than the watch crystal at the interface prevents total internal reflection from occurring and the visibility of the dial face is restored.

The critical angle for total internal reflection  =  arcsin (1/n), where n is the index of refraction of the watch crystal.  I’ll be using a watch with a domed hesalite crystal which is made from poly methyl methyacrylate and also widely known as acrylic, perspex, plexiglass and lucite.   The n =  1.5 and the critical angle is 42o.

To determine what angle θ of rotation of the watch will trigger total internal reflection we can use Snell’s Law.

nwater sin θ   =    nacrylic sin α

                     θ   =   arcsin [(1.5 * sin 42o)/1.330]  =  49o

What is not known is the effect of a fluid replacing the air space in the watch with an index of refraction less than that of acrylic (1.5) since total internal reflection only occurs if the adjacent medium is of a less refractive index.  The use of a sapphire glass crystal in many watches (n=1.77) complicates things further since there are few liquids with such a high index of refraction that is also nontoxic and safe to use.   I’ll be using a fluorinated oil known as 3M Fluorinert with an n = 1.261  and benzyl benzoate with an n = 1.5681 to see if there is any difference on the effect of total internal reflection at 49o.

Screen Shot 2021-11-07 at 9.43.41 PM
Some observations on the materials to be used. All liquids are chemically inert, electrically nonconductive and biologically nontoxic.  They also have to be clear, colorless and odorless.  Will the benzyl benzoate remain liquid in cold weather? Will it dissolve polystyrene? It is very inexpensive and has the low viscosity needed to easily penetrate all crevices of the movement and expel air bubbles.  It has been reported that 100 cSt (centi Stokes) mineral oil used in one modification caused the watch to stop functioning after a few weeks due to suspected mechanical failure from component wear working in a viscous environment.  You can buy Fluorinert in smaller 60 mL sizes but not the silicone oil from Sigma Aldrich with the low 5 cSt viscosity.

The choice of watch to modify is also equally important in order to accentuate the visual advantages of using oil. I chose a pair of white and black faced Junkers Bauhaus watches using the Rhoda 515 Swiss quartz movement.  True to its namesake, the dial faces are elegant simplicity with a vintage hesalite crystal in a domed profile not possible with glass or sapphire. Hugo Junkers (yun’kers) was a famed German industrialist whose aeronautical company in the early 20th century was the first to produce an all metal aircraft (1915) and an all metal civilian airliner (1919) and a fully pressurized passenger compartment for high altitude flying (1930). His company is also responsible for the standard cantilevered low wing design we see in almost every modern aircraft today. He was also a leading proponent of the Bauhaus movement of design. This is where form follows function and fine arts meets industrial design and production techniques to produce timeless iconic examples of modern minimalism with soul. Junkers was a socialist and a pacifist so when the Nazis came to power, they forced him to give up control of his aircraft company and patents and placed him under house arrest. He died very shortly thereafter.  As a consequence Junkers is only popularly known for the bomber aircraft it made including the Ju-87 Stuka dive bomber.                                                                                               Today, his great grand-daughter upholds the family name with a new company that manufactures German made mechanical and quartz watches with aeronautical and Bauhaus themes. It is possible to buy modestly priced and well executed watches not made in China. For those of you who don’t share my personal trade embargo against China, do try and stay away from tomatoes, cotton, and some clothing made in China. Those have been proven to to be harvested and made with slave labor and officially embargoed by the US government.

Demonstration of total internal reflection with the watch immersed in water.  I’m shooting perpendicular to the water surface so there is no refraction at the air/water interface and the images seen are the same as those seen by a diver underwater.   Starting at a 40o inclination of the watch face and increasing by 5o intervals.  As predicted, visibility is completely obscured by total internal reflection  by 50o.

Demonstration of total internal reflection in action.


While waiting for the 3M Fluorinert & Benzyl Benzoate to arrive in the mail, I got some SuperLube 100 cSt silicone oil to put into an old quartz Citizen watch I had lying around.

Before (left) and after silicone oil addition (right). The bubble allows the oil to expand with temperature increases without breaching the waterproof seal around the crown.  Others have tried using 2-3mm diameter polystyrene beads that can be compressed by expanding oil volume.

The silicone oil filled Citizen watch shows clear resistance to total internal reflection at 55o and 60o of inclination.  There appears to be no effect on the development of total internal reflection despite the silicone oil having a marginally lower index of refraction than the glass crystal of the watch.

Junkers Bauhaus with 3M Fluorinert oil.   Starting at 50o watch inclination in a water bath and increasing by 5o  increments.  The high domed crystal filled with oil makes the dial face appear to protrude upwards and increasing visibility.       

I put some benzyl benzoate in the refrigerator at 4o C and it remained in a liquid state, surprisingly.

I started to fill the black watch with a syringe and a fine needle and almost immediately realized that it was going to be a disaster.  Benzyl benzoate is apparently a strong solvent and it starting dissolving the plastic date ring leaving the liquid murky with particulate material.   The watch was ruined but the movement ran without problems. Over time the markings on the watch face also began to dissolve.

Clearly, the use of an oil with an index of refraction significantly lower than that of the watch crystal results in some formation of total internal reflection but not enough to completely occlude visibility. The total internal reflection also occurs at a greater critical angle. The ideal oil to try would be the low viscosity silicone oil from Sigma Aldrich but it’s difficult to procure and a little expensive. I had high hopes for benzyl benzoate but it would require a watch with no plastic components to be compatible.

The other Junkers Bauhaus filled with silicone oil and running well past midnight and time for the club’s last call for both alcohol and requests. Watches for under $300 USD with no compromises and made in the free world.  Shot with Olympus E-M1.2 and 20mm f/1.7 Lumix lens.
Screen Shot 2021-08-10 at 9.51.40 PM
                                                                                                          Another similarly priced watch is the Humism Kato from Singapore.  This watch features innovative kinetic art on its dial face and runs on the Seiko NH35A automatic movement.  Time lapse shot with Olympus E-M1.2 and Zuiko 60mm f/2.8 Macro.

And finally a collaboration between Germany and Hong Kong to produce the Ziiiro Jupiter. The ball bearings are moved by conventional hands tipped with magnets beneath the dial face. Inexpensive and impressively innovative, it just goes to show that anybody can afford to wear stylish watches that are not made in China.   BTW, China would have you believe HK and Taiwan belong to China but the two societies developed independently of China for more than the past hundred years into unique sovereign states.   The differences between China and HK & Taiwan are much greater than say those between the US and Canada.                                                                                                             The Ziiiro is sitting on top what may be arguably the best made 4″ apochromatic refracting telescope, the Astro Physics Traveler, entirely handmade in the USA.

A closer look at the silicone oil filled watch.

Time lapse video of Ziiiro Jupiter with inner ball bearing displaying the minutes and the outer ball bearing displaying the hours.  Revolving around my 1980s vintage Celestron 8″ Schmidt Cassegrain Telescope, a legendary instrument made in Torrance, California.  E-M1.2 and Lumix 20mm f/1.7 lens.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s