Information from Sinn, April 2004
I received a lengthy, precise and exhaustive answer from Sinn concerning the question of the air bubble in the silicone-filled watches (403 Hydro, EZM 2 - both now discontinued - and UX/EZM 2B for Sinn, but also Bell&Ross Hydromax).
I am no technical expert, and some things may well be lost in translation, but I'll try to summarize the reply of their R&D department (I must say that I'll give them great credit for having an admittedly technical question answered by an engineer on their R&D staff). here it is:
- Any oil will absorb dissolved gases.
- When a watch case is filled, there is no gas bubble.
- As the ambient temperature drops, the oil filling contracts more than the volume of the watchcase. This phenomenon is compensated to a degree by the membrane back of the watchcase.
- When the membrane reaches the end of its flexibility, however, a vacuum bubble forms in the oil.
- The vacuum bubble is progressively filled with dissolved gases from the oil, and also with air which continually diffuses into the case (no wristwatch is completely air-/gasproof).
- As the ambient temperature increases again, the pressure inside the watch increases as well. The oil re-absorbs some, but not all of the gases in the gas bubble. This is because due to the diffused air, the total mass of the filling is more than it was before the ambient temperature dropped.
- Especially as this process is repeated, a gas/air bubble can form which is no longer dissolved as the ambient temperature increases.
- To minimize the effect - or to prolong the time until such a bubble forms - the initial oil-filling is currently done at a slight over-pressure (by manipulating the membrane caseback).
- The only way to get rid of the bubble, once it is stable at normal temperature, is to change the oil filling.
- Needless to say, the bubble does not affect the functioning of the watch in the least.
Please keep in mind that I have no technical expertise of my own, but that this is only a summary/translation of what Sinn are saying.