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Hi Bob,

>Are there any opinions on whether the mechanically dry sealed first stage
>(Tech 50D) or the silicone sealed first stage (VX10) is preferable?  Are 
>there any advantages or disadvantages to each?  i.e. better at depth or 
>certain temperature ranges. 

With Poseidon, you fit a rubber cap over the spring chamber.  The manual
says you can leave it dry or fill it with an anti-freeze fluid.
Options include Glycerol (sugar) mixed with water, or meths, vodka, 
or silicon oil.  (Diaphragm 1st stage).

From what I remember, for the Posedion system, air is good down to about 40m,
after that, you need fluid.

Whatever you use, you need a flexible interface with the water to transmit
ambient pressure.  If there is air behind it, you will get compression 
problems, and if the rubber is punctured, you're back to water straight away, 
whereas a fluid would take time to dilute enough to be a problem.

If you have a piston 1st stage, I suppose it would be better to have a fluid
with lubricating properties aswell.


Bill.Dyer@mi*.co*.uk*

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