© Dr. Peter McClean Millar - June 2004




In 1969, Doxa introduced the first diver watch equipped with a Helium Release Valve (HRV) that was available to the general public. It was called the "DOXA SUB 300T Conquistador". The HRV is believed to have been co-developed by Doxa and Rolex, for which Rolex submitted patent number CH-A-492 246 on 6th November 1967. Although Rolex fitted Submariners (model 5513 - non date) with prototypes of this escape valve, which COMEX divers used successfully for 2 years, they didn't actually begin to build the Helium Release Valve into their retail dive watches until 1971. This was when they produced the HRV equipped Sea-Dweller, almost 2 years after Doxa had released the Conquistador. More information about the development of the Conquistador and the Doxa SUB in general can be found in an interview with the chief designer, Mr. Urs Eschle.


The Helium Release Valve (HRV) was really designed for use by professional divers who are doing deep sea "saturation" dives. Saturation diving is a technique developed by the U.S. Navy in the late 1950s that permits divers to remain underwater for weeks or months without having to undergo periodic decompression. It was discovered that when a diver is underwater for a long time (days or weeks) the time needed to decompress reaches a maximum and plateaus out. The diver becomes "saturated" and no longer accumulates additional gas such as nitrogen or helium in their body tissues. Basically it means that the decompression time for a diver who has been underwater for one day should be the same as for a diver who has been down for a week. During deep diving, a diver breathes a mixture of gases (compressed air or a combination of helium and oxygen; or helium, oxygen and nitrogen) at a pressure higher than that of air at the surface. As the diver breathes, a certain amount of nitrogen from compressed air is dissolved in the lungs. From the lungs, the blood carries this additional nitrogen to all the body's tissues. This condition can become potentially dangerous if the ambient pressure decreases too quickly and gas bubbles are allowed to form in the tissues or blood stream. In order to release nitrogen from the body safely, a deep sea diver has to come to the surface slowly, a process known as "decompression." In general, the deeper or longer a diver is underwater, the longer it takes for the diver to decompress safely.


For diving watches, it is helium that causes the problem. The helium molecules are much smaller than oxygen or nitrogen and can penetrate through the case seals in sufficient quantity to push out the crystal at atmospheric pressure levels. This can be avoided by utilizing a Helium Release Valve which allows the helium to escape but prevents water from entering the watch. Before the advent of the Helium Release Valve, divers would unscrew the crown and vent the helium that way. It was not uncommon for crystals to blow off the watches of those who forgot and even Rolex made mention of it in their Sea-Dweller advert shown above.


The following is taken from United States Patent number 5,257,247 (Miche, et al. October 26, 1993 - Safety valve for diver's timepiece) which is for a screwed crown Helium Release Valve. I believe this is the patent for the type of HRV used on the Omega Seamaster. In the patent summary the following information is found (note the reference to the earlier Rolex patent of 1967:-


Titel: Montre étanche
Veröffentlichungsdatum : 1969-11-28
Erfinder : ANDRE ZIBACH (CH)
Originalnummer : CH492246  19691128
Anmeldenummer : CH19670015501  19671106
Prioritätsnummer : CH19670015501 19671106 
IPC Klassification : G04B37/08
Title (eng.) :  
Description(eng.) :  
ECLA-Class 1: G04B37/08
ECLA-Class 2 :  
Family 1:  
Family 2 :  


Effectively, it has been determined that whatever be the sealing qualities of watch cases provided up to the present, when they are subjected during relatively long periods on the order of several hundreds of hours to relatively heavy pressure on the order of several tens of atmospheres and especially when the surrounding atmosphere is formed of a gas having small dimension molecules as is the case with helium for instance which is frequently employed in diving bells, the pressure within the watch case ends up by increasing considerably.


Such conditions of use are not hypothetical but exist in reality, in particular when the watch is employed at great depths underseas in the course of work taking place under a bell. The dive is said to be "in saturation" (reference to the standards annex of ISO standard 6425 for diver's watches).


During return of the watch into an atmosphere at normal pressure, and this in spite of the decompression stages which are necessary for the occupant of the bell, there arises an interior overpressure in the bubbles, which may bring about bursting of the latter, that is to say, ejection of the crystal in particular.


To overcome this difficulty, patent document CH-A-492 246 proposes a valve arranged in a manner to open automatically when the ambient pressure is lower than that prevailing within the case and to close in the inverse case in order to prevent that there occur within the case an interior over pressure susceptible to bring about deterioration of the case during intermittent employment of the watch in an over-pressurized gaseous medium and during passage of the watch from the over-pressurized medium to the medium at ordinary pressure.


I hope you understood all that, because I'll be asking questions later !!!!


Since the 300T Conquistador shares the same movement, dial, hands and bezel as the vintage 300T Professional, I'll briefly describe them so as not to repeat information that can be found in the indepth review I did earlier. One thing that is different to the Conquistador is the name. It breaks with the tradition of the sea used in the other models: Searambler, Sharkhunter, Divingstar and Sea Nymph. It is easy to see why Doxa would call their first orange dialed watch a Professional but it puzzled me as to why Conquistador was chosen in this case. I contacted Rick Marei at DOXA who in turn spoke with Mr. Urs Eschle who said the word conquistador means the conqueror and every new DOXA that was new to the watch industry was called conquistador when it came out. The Helium Release Valve incorporated into the SUB 300T case was certainly new to the watch industry and quite rightly justified the name.


Initially I believed that the Conquistador uses the automatic 17 Jewel ETA 2852. However, I discovered what looks to be the letters DOXA 118 (could be 116) on the actual movement body. From what I can gather this reference number signifies an ETA 2472 which was first used in the 1967 Sub 300T Searambler. It is very difficult to find any information on the movements used in the vintage SUBs and what information there is can be confusing. Although I have seen the rotors of both Doxas and Hamiltons which displayed the wording 'seventeen jewels', many other references to the ETA 2472 claim that it is a 25 jewel movement. The Conquistador uses a non signed screwed crown which is unique in the Doxa SUB 300T series because it is decoupled. The thing about a decoupling mechanism built into the crown is that when you screw it down or unscrew it, it is not connected to the barrel and thus you're not winding the watch at the same time. There is a noticeable change in resistance between bolting the crown and winding the watch. It also is a little unnerving the first time you use a decoupled crown because when it is unscrewed it feels really "wobbly" and you think it is about to fall off. Once you remove the winding stem from the mechanism, you can see just how articulated the crown actually is.


Removing the movement gave me a chance to inspect the inside of the case and the release valve. It sure is a small piece of engineering and basically consists of a non return valve mounted at the 9 o'clock position.


The dial is similar to the US Diver SUB 300T Professional except it has the word Conquistador. Most people put a premium on having one of the original SUB 300Ts with the US Divers logo and there have been many calls for Doxa to include it on the new SUBs. The logo was used when the US Divers Co. was the official distributor of the DOXA SUB diver watch in the USA in the seventies. Also the Non - decompression dive table incorporated into the Doxa SUB models was developed in co-operation with the US divers Co. It is also worth noting that Jacques Cousteau was Chairman of the Board of US Divers from 1957 until his death in 1996. Another thing to note on the above photo is the bezel design. Although they look the same, closer inspection shows a difference in the inner ring markings, with the Conquistador bezel looking very similar to the one on the new SUB 600T.


Even though the casebacks if the thinest and thickest vintage SUBs used the Doxa ship logo, there is a subtle difference in them. The Conquistador's logo looks like a medal has been inserted into the caseback, whereas the Sharkhunter's (seen in the above photo) is stamped. Doxa returned to the early look for the new SUB 600T by using a medal insert of the Jenny family fish logo in the caseback. One feature of the Conquistador which is not noticeable until the winding crown is removed and can be seen in the above photo is the diameter of the stem tube. It is a much bigger bore in the Conquistador.


The Conquistador uses the thinest of the Doxa SUB cases, which, incidentally, is the case which the new SUB 600T was designed on. Even though there are basically 3 different thicknesses of SUB 300T cases, all retain the classic Tonneau shape.


There really is something about this SUB that is different from the rest. Sure it looks like most other vintage, orange dial SUB 300Ts, except it is in wonderful condition. The watch was definitely well looked after. It's not just the fact that it has a HRV built into it. My Breitling Superocean has one as well, and it's not the fact that the watch is pretty unique. For me it is what it represents and the fact that it sort of came right out of the blue. It always makes me wonder just what I'll find next in the Doxa world. Just when you think you have seen it all something else wondrous turns up. It is a pity that so much of Doxa's history has been all but erased with the passing of time and the loss of detailed records. We really have to rely on the memories of people who were involved with the company during their heyday, unfortunately, old father time is catching up with them and when they are gone much of the information and knowledge they have will be lost forever as well. Maybe that's why the SUB 300T Conquistador is so important to me. It really is a time capsule back to the days when people like Jacques Cousteau, T. Walker Lloyd, watchmakers from Doxa, Rolex and divers from Comex were pushing the envelope of commercial and sports diving and dive watch technology.


When it comes to Doxa SUB watches there are a few that would be considered the Holy Grail of ones to own. Dr. Clive Cussler's original 300T Professional, which was imortalized by Dirk Pitt in his many adventures, has to be top of the list, followed by the SUB 200 T.Graph owned by Astronaut Gene Cernan, the last man to walk on the moon. Pictures of that watch can be found here and here. To that esteemed company I would have to add this SUB 300T Conquistador. The reason being that until a few months ago few people outside of Doxa even knew such a watch existed. I had certainly never seen or even heard of one. Up until quite recently the history of the Helium Release Valve centered on the research done by Rolex and Comex. No mention was ever made of Doxa. Now, here you have a watch that predates the general release of the HRV equipped Rolex Sea-Dweller by almost 2 years. It is not known how many Conquistadors were produced. I would say that it wasn't very many and there are unlikely to be many left, especially in as good a condition as this one. Anyone who is lucky enough to own one, surely has a unique piece of dive watch history.


A Flying Doctor Production
© Dr. Peter McClean Millar