One of Citizen’s vintage divers, the one with two crowns and an internal rotating bezel, is usually referred to as a ‘compressor’, or ‘compressor type’ diver:

I’m not a dyed in the wool diver enthusiast, so to be honest I have adopted this terminology without too much thought… However, I’ve recently had some very helpful comments about this model, from a chap called Hunter. Rather than leave this in the comments thread on the divers page, I’m bringing the key points here – particularly since it is clear to me now that I shouldn’t be referring to these as ‘compressor’ divers at all.

Here’s what Hunter brought to my attention:

“Compressor” watch cases were patented by EPSA and came in three styles. They refer specifically and only to the case sealing method and, as some might correctly assume, the greater the depth underwater, the tighter the case becomes.

One fact needing greater attention is that the third compressor patent (Super Compressor) is usually found on dive watches from the 1960s and early 70s. Dual crowns are used with an inner rotating bezel with the upper crown rotating the bezel. However, Super Compressors come in single crown versions. 

Bottom line is that a watch with twin crowns is exactly that. Unless you find the specific EPSA designed technology (and brevet numbers or at least their diving helmet logo) you do not have an EPSA “Compressor” case.

The Swiss case manufacturer Ervin Piquerez S.A.(EPSA) patented the technology for compressor cases. Hunter provided this link for more EPSA information (and to see some very nice watches too 🙂 ):

http://scubawatch.org/EPSA_SC_FAQ.html

Having now had a closer look at the EPSA designs following Hunter’s comments, it seems to me (although I’m no watch techie) , that Citizen’s so-called ‘compressor’ is not in fact a true compressor diver. To be fair I don’t suppose Citizen called it that, they simply designed a twin crown model in line with the fashion trends of the day.

Since my intention all along with my own collection and this blog is to build reference information on original Citizen watches that is as accurate as possible, I’ll be editing references to the ‘compressor’ diver,  replacing it with ‘twin crown’ diver.

With thanks to Hunter 🙂