MK18 - Environmentally sealed or not?

[mahjong]

Hi. I believe I have read most of the archived threads on the MK18 (including DA Aquamaster's terrifically detailed information). That said, I am still unclear as to weather or not--or to what degree--the MK18 is environmentally sealed. I also googled the MK18, and here is one description from Scubastore that claims that the MK18 is "completely sealed":

***
First StageMK 18 Available in 200BAR/3000 PSI Yoke or DIN System
Balanced Diaphragm (This regulator is completely sealed to provide unparalleled performance in challenging environments and provides consistent air delivery regardless of tank pressure)
Delivers air in a consistent manner, regardless of tank pressure.
PASS (Piston Assisted Seat System)
Provides better airflow, even in high demand situations.
Hemispheric Storage Chamber (HSC) (This exclusive SCUBAPRO feature provides a “Venturi” effect that assists in the opening and closing of the high-pressure valve. In addition, the air storage space maintains a reserve supply that stores air between breaths. The result is an extremely fast response to the diver’s demand for air)
Thermal Insulating System (TIS) Patented anti-freeze system.
***

From the Scubaboard threads, I gathered that the MK17 was somehow more environmentally sealed than the MK18--if so, in what way?

Having just bought an MK18UL, I want finally to get the facts straight. Many thanks!

 

[DA Aquamaster]

The Mk 18 uses a TIS (Thermal Insulating System) that use a combination of insulation and heat transfer to keep the parts warm enough to prevent ice formation and/or to limit formation to non critical areas and slow ice progression.

It works very well on the Mk 16 and Mk 18, but uses an ambient chamber that is open to the water.

The Mk 17 in contrast uses a diaphragm sealed dry ambient chamber along with some fins to improve heat transfer. It is currently the most bullet proof cold water first stage vailable.

The Mk 18 is a Mk 16 with a swivel turret for the LP ports. Similarly, the Mk 19 is a Mk 17 with a swivel turret for the LP ports. But like the Mk 18 it is a reg that is not marketed in the US - I suspect to prevent it from competing with the higher priced Mk 25.