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 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 Fluorinertwith 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.
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.
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.
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.