Heat transfer with the all metal R109 is superb and the potential for a freeze up in cold water is virtually zero - in general second stage freeze ups in cold water never happened in the era of metal second stages, that is distinctly a plastic second stage phenomenon.
In the same vein, metal second stages recyle exhaled moisture and consequently greatly reduce dry mouth - another mostly plastic second stage phenomenon.
I used to dive the Mk 10 in cold water with the Silicone Protected Environmental Chamber (SPEC) kit. The older ones have small ambient pressure holes designed to hold silicone and the later ones had a groove to accommodate a boot to hold the silicone in a bit better with larger holes underneath to improve water flow and heat transfer and make rinsing easier if you elected not to use silicone. With the SPEC kit cold water performance was superb, without it it could get problematic at temps below about 45-50 degrees if you do not use excellent cold water technique. With the boot installed you coud use Christolube, but it is a bit thin to use on a Mk 10 without the boot. Either way, if you dive a lot, the level of silicone or christolube will need to be topped off once or twice per season.
The Mk 20 and 25 use a Thermal Insulating System (TIS). It is lower maintence but unfortunately is not as effective in water colder than 40-45 degrees. You need very good cold water technique and preferrably low pressure tanks to reliably avoid freeze flow issues at depths below 100' where the flow rate is high.
The early and mid prodction Mk 20's had a swivel cap that woudl accommodate the late production Mk 15/early production Mk 20 SPEC boot. This was the best SPEC boot SP ever made in terms of retaining silicone. Ultimately I ended up using a pair of early Mk 20's with this boot along with a Mk 25 piston and TIS kit to get a reg that was 100% reliable in 35 degree water at 100-150' depths.
When the fully sealed and excellent performing Mk 17 came out, I just simplified my life and bought a pair of them.
In the same vein, metal second stages recyle exhaled moisture and consequently greatly reduce dry mouth - another mostly plastic second stage phenomenon.
I used to dive the Mk 10 in cold water with the Silicone Protected Environmental Chamber (SPEC) kit. The older ones have small ambient pressure holes designed to hold silicone and the later ones had a groove to accommodate a boot to hold the silicone in a bit better with larger holes underneath to improve water flow and heat transfer and make rinsing easier if you elected not to use silicone. With the SPEC kit cold water performance was superb, without it it could get problematic at temps below about 45-50 degrees if you do not use excellent cold water technique. With the boot installed you coud use Christolube, but it is a bit thin to use on a Mk 10 without the boot. Either way, if you dive a lot, the level of silicone or christolube will need to be topped off once or twice per season.
The Mk 20 and 25 use a Thermal Insulating System (TIS). It is lower maintence but unfortunately is not as effective in water colder than 40-45 degrees. You need very good cold water technique and preferrably low pressure tanks to reliably avoid freeze flow issues at depths below 100' where the flow rate is high.
The early and mid prodction Mk 20's had a swivel cap that woudl accommodate the late production Mk 15/early production Mk 20 SPEC boot. This was the best SPEC boot SP ever made in terms of retaining silicone. Ultimately I ended up using a pair of early Mk 20's with this boot along with a Mk 25 piston and TIS kit to get a reg that was 100% reliable in 35 degree water at 100-150' depths.
When the fully sealed and excellent performing Mk 17 came out, I just simplified my life and bought a pair of them.
