The MONOFLEX crown

As hinted in our last blog post about the Compressor principle invented and widely used by EPSA for cases of various watch brands of the 1950s and 1960s, also the crowns used in the EPSA cases featured the compressor principle.

Ervin Piquerez (EPSA) Diver Helmet Painted artwork

EPSA called them MONOFLEX crowns and they were an integral part of the whole concept:

When analysing the watches before starting the development, it struck us that the crowns of the old watches never were screw down, yet 200m (20 bars) water tight. This seemed odd, and so we tried to find out how this actually worked in detail. As EPSA went out of business in the 1980s, and until today nothing remained, it was difficult to find out more about this. But sometimes luck is needed, so we found a book about EPSA written in French which challenged our school French quite a bit, and it gave hints for further steps for our research. Eventually we discovered Swiss archives where some documents could be found outlining details about the crowns. The old EPSA patent itself (Brevet 337462) was not sufficient enough – as could be expected. Patents should protect, but not educate too much! But then fortune struck…

The internship report with details about the crowns

Finally, when researching in an archive we found a drawing of the crown and incredibly enough we also found the internship report of EPSA’s technical head’s daughter, Mrs. Rondez. This young woman spent some time inside of EPSA and had to write a detailed report about it. Many of us know how tedious it was to write these kind of reports, and now we were on the receiving end. I can only thank her to have put down her experience! It taught us everything we needed to know.

So, these crowns use a tubular shaped sealing which slides over the tube which comes out of the watch. In normal use, this offers enough water tightness. When the pressure is rising, the tubular structure is pressed with higher force around the tube, resulting in a better sealing performance. The process to manufacture this component was injection moulding.

With that knowledge we went on to redesign the old crown construction. On the old drawing we found the original crown’s producer and were more than happy to see that they were still in business today as a crown maker. Several phone calls and one meeting later, though, we had to find out that all the knowledge was lost and that there was no willingness to reinvent this crown design. Yet, I am again thankful that they were willing to share the old drawings with me, enabling me to have an easier start with the redesign!

My watchmaker / watch construction specialist, Cyrano Devanthey of Bumont GmbH, motivated me to go forward in redesigning the crown to set up my own production. So we updated the materials, changed the design to a more modern production setup and started ordering prototype parts. In Summer 2021 these prototype crowns were tested with the complete watch in air pressure and water pressure. We can now safely say that they withstand 20 bars (which can be equivalent to roughly 200m depth in diving).

The original MONOFLEX compressor crown by EPSA, Ervin Piquerez SA, original illustration from an advertisement
Compressor crown perspective drawing

The evolution of the cross hatch crown design

In old catalogues I found designs of the first MONOFLEX crowns with a very similar cross structure like that for the dive helmet. Only later did this change to the fine cross hatch structure seen on the crowns in the 1950s and 1960s. So I believe the source for the cross hatch design of the crowns rather stems from the dive helmet, not from usability issues.

So, transferring the myth to today’s crowns: of course also the Sherpa MONOFLEX crowns that we manufacture ourselves feature the cross hatch structure on the front surface! And on the case back, we have the beautiful EPSA dive helmet logo to give reverence to these pioneers of water resistance in watches.

The Cross Hatch Myth (TM)

While we are at it, we also wanted to clear up a myth surrounding these crowns. They featured a cross-hatch pattern on the front side of the crown. Basically, this became a trade mark and highly recognisable for consumers of the time and now vintage collectors. Even nowadays, many twin crown, super compressor watches feature crowns with the cross hatch. As if it makes them „compressor crowns“ by this only.

There is a discussion that this cross hatch design was done in order to operate the crown better with gloves. While we cannot really tell if this is true, we found another reason why this design was chosen:

The company logo was a 1950s dive helmet with protection bars in front of the pressure proof window in the helmet. These metal bars had a cross structure.

one of the first drawings of a compressor crown by Ervin Piquerez SA - EPSA