The original intent of this thread was to querry about some reg's I am looking at. I'm glad that you all are discussing the pro's and con's of light weight, low profile regs. DA Aquamaster, you stated "The "lightweight, low profile regs prevent law fatigue" concept is the biggest lie in dive equipment marketing and was basically invented to convince us that moving to lighter plastic second stages was a good thing - it wasn't." I don't necessarily agree... I am a person who needs a light weight reg due to prior TMJ, so I appreciate your input, however in my case you are incorrect. My doctor actually wanted me to get a full face mask similar to Interspiro's. We came to a compromise if I could find a light enough reg (in and out of the water). BTW I love my low volume mask.
A lighter second stage sounds logical and on the surface the argument even makes some sense. The appearace of truth is what makes popular misconceptions so attractive to anyone who does not take the time to explore the argument in depth and apply some critical thinking skills.
The apparent logic on the surface is however very seductive and I can understand your Dr. taking that line of reasoning and recommending a smaller second stage that would be "lighter".
The problem with him doing that is that regulators are not like cell phones where smaller usually equals lighter. With a second stage that is not usually the case underwater where a larger second stage usually offers more bouyancy to offset the higher weight and actually weigh less when submerged than a smaller second stage.
And again, the biggest force acting on your second stage when underwater is most likely going to be the hose if the lenght is not perfect. The push/pull foirce imparted by the hose is far greater than the weight of the reg in the water.
The irony here is that you bought a Poseidon Xstream Deep. That particular reg is reported by Poseiden to be designed with minimal internal volume to reduce deadspace and reduce carbon dioxide retention (which I think is also largely BS as the percentage decrease is minimal when comapared to the dead space in a diver's mouth and throat, but markering types are seldom prone to stating facts that dispute their claims). What is pretty certain is that less internal voume means less bouyancy with the result that you have a reg that is not very bouyant for it's size and it is going to be heavier in the water than a large, and largely plastic, second stage such as a G250HP.
The further irony is that poseiden hoses are expensive and the selection of lenghts is limited compared to more common hose designs. The OPV fitting on the hose itself is also both large and comparatively heavy.
I am not totally sure how the Extreme compares with the Jet Stream, but the Jet Stream also had a larger than average mouthpiece which adds an additional negative in terms of TMJ and also limits mouthpiece selection. That would be a third strike, if the extreme uses the same oversized mouthpiece design.
Personally, I like to take what my doctor says with a grain of salt and focus on his intent rather than to take his advice verbatim and consequently rely on him not only being an excellent doctor but also knowing the intimate details and implications of my various hobbies and equipment choices. Having a Dr. does not absolve you of some responsibility in interpreting and properly applying what is said to your specific case.
Same thing goes with an LDS owner. I have to say the reg you got is among the last I would have chosen for a customer with TMJ for all of the reasons listed above. Of course being a posieden dealer probably biased him somewhat.
