I'm doing some research from the Rubicon Research Repository on decompression meters. I found this article by Carl Edwards on Automatic Decompression Meters from about 38 years ago. I'm continuing to look, but be sure to see the cartoon at the end of this article.
In further research I found this patent which explains the theories behind one of the first decompression meters (but I don't know which one yet). It seems to use air, but has provisions for duplicating a He/ox breathing mixture not with the gases, but with a different diffusion gradient (look for yourself, as I don't completely understand what was described yet). www.google.com/patents/US3886801
Be sure to go to the bottom of the page and click on the links to other decompression meters too.
Well, I've looked in my ScubaPro catalog for 1976, and they have an "Automatic Decompression Meter" which looks identical to the SOS meter. On its label is the patents which cover it, and it is "U.S.A. N' 3,121,333." I looked up this number (see the link in the previous sentence), and it states:
...Referring to FIGURE 1 the improved instrument comprises a deformable chamber of a bellows type externally subject to the hydrostatic pressure from the liquid into which the instrument is immersed. The chamber 10 connects through a conduit 11 with a non-deformable chamber 12, the chambers 10, 112 being filled with a comprissible fluid such as air. A throttle 13 is interposed in the conduit 11 to delay the flow of the fluid from either chamber to the other, and a pressure gauge 14 is provided for measuring the instantaneous pressure in the nondeformable chamber 12 with respect to an initial reference pressure, such as the air pressure at the surface of the liquid in which dive is effected. The throttle 13 may consist of a porous air-permeable partition... (emphasis added, jcr)
My conclusion is that it is air inside that flexible membrane, from what this patent says. Also, the other patents above are interesting, but I believe this is the patent that covers the SOS Decompression Meter.
PUH-EEEEEEEEEEEEEEEEW! I got the lens off of mine and it smells like old fashioned burn ointment: sorry, that's the only thing I know that smells like that! It took some torque to get off too, p'rolly 25-30 lbs!
Yes, the interior of my meter smelled pretty foul when I opened it up. I wonder if the smell is from the "bag" or the mysterious gas used to fill it. ;-) Did your hands itch after handling the internal part of your meter?
Jaybird, Have you ever had your "Decom Meter" in the water? I have purchased a second "Decom Meter" that appears to be functional. I would not use it to manage a dive. But it would be interesting to see what it would indicate.
I'm using the name "Decom Meter" because that was how it was nicknamed in a 1970 Skin Diver article "Getting to Know Your Decom Meter". The writer was very upbeat about the device. He even commented that "further evidence of its national acceptance and adoption is the Americanization of the name - it is called the Decom Meter."
The other choices, "Automatic Decompression Meter" is so formal, "SOS Meter" definitely doesn't instill a sense of confidences and "Bend-O-Matic" is a name that is not well deserved, in my opinion. From what I understand from my reading, I wonder if many of the incidences of the "Bends" were more due to the misuse of the meter than the inherent design of the device. There are accounts of the meter's use during hundreds of dives without incidences of the "Bends".
I wonder why it wasn't nicknamed the "Deco Meter" in place of "Decom Meter"?