IT WAS AN AVERAGE MARCH DAY. Sue and Martin entered the water of the lake, snug and warm in their drysuits, hoods and gloves. Only the shock of it, contacting the exposed parts of their faces, reminded them that the water was very cold.
Soon the divers were down at 20m. Everything was routine until Sue disappeared in a violent cloud of bubbles, which freeflowed uncontrollably from her regulator...
These words introduced an article I wrote about regulator freeze-ups and freeflows more than 20 years ago.
Back then, regulator performance was on the up, combined with lightweight plastic second stages. Freeflows in cold fresh water became common, and most divers neither expected it nor knew how to deal with it. There were some unfortunate casualties at the time.
The problem was that the high flow-rates and the dramatic drop in pressure as the air went from the tank through the regulator first stage caused a massive drop in temperature, leading to ice forming and regulators malfunctioning.
On the other hand, I knew some Swiss divers who habitually dived under ice in the winter, yet experienced no such problems.
This was because their regulators were simply less good at delivering air than the latest ones then entering the marketplace.
Today, we see regulators with a work of breathing on the ANSTI test machine of less than half a joule - that is, not very much at all. Designers now put great effort into combating the risk of ice affecting the mechanism.
Then there is good old Stig at Si Tech in Sweden. Dont confuse him with someone who drives fast cars around a track for Jeremy Clarkson. This Stig has been progressing at a steady pace for years. I know he likes to dive with an old Russian military rebreather - thats about as racy as he gets.
Stig designed a regulator many years ago.
It was intended specifically to meet the needs of those who dive in very cold fresh water. The design languished in Stigs drawer for years, until he finally got it into production at the end of 2009. Its the Si Tech Forever S40.

First Stage
The first stage is a solid-looking piece of engineering. Its unusual in that its an upstream piston-type with a venturi-assisted design.
The piston passes what little heat there might be in the water to the very cold air coming from the tank. Being upstream, it wont freeflow, but it might explode if it went wrong, so it has an over-pressure relief valve.
This can be found sitting in one of the four medium-pressure ports, and the user can move it anywhere that makes the routes of other hoses convenient.
Uniquely, tank pressure at the first stage is reduced to only 5.5 bar more than ambient, whereas most commonly encountered first stages work at 8-10 bar more than ambient.
So the pressure of air in the intermediate hose is almost half that of other regulators.

Second Stage
The unfashionably massive second stage employs a venturi-assisted double-action upstream tilt valve, and the exhaust has two mushroom valves and a massive deflector that routes exhaled bubbles well away from the divers face. Its big and bold, and not at all pretty. You will need to buy a matching octopus, because of the unusual intermediate pressure used.
The second stage was easy to disassemble, and the way in which the pressure-sensing diaphragm pushed open the demand valve was worthy of the best Meccano set.
The venturi effect is permanently adjusted by positioning a foil across the airflow. If this is not done adequately, the regulator can feature a permanent yet slight leak of air.

Purge Control
With such a low intermediate pressure I was expecting a very gentle purge effect, but it really worked well. The button is obvious, and easily falls to hand.

I havent seen a second stage this big in a long time. The last bore the Divex brand, and was intended for the sort of divers who use wire brushes and vacuum lifts (they like to call themselves commies but not because they share all their worldly wealth between themselves).
I believe this might well be the same thing. Once you get over the apparent crudeness of such a big lump, it becomes quite normal.
A fresh fall of snow and freezing conditions saw me drive furiously to Wraysbury Lake before the ice on its surface had a chance to melt.
It didnt look very busy there when I arrived. What ice there was lay unbroken at the surface.
Taking pity on someone not relishing the idea of a cold plunge, Mark Bruce, Wraysbury Dive Centres resident diving instructor, kindly offered to give the Forever S40 a whirl on my behalf.
His first impression was that this regulator wasnt sexy. He reported hearing it working as he breathed, though it never faltered.
He used it in conjunction with a DPV we were also trying. The backwash from such things can make a regulator freeflow, by putting pressure on the front diaphragm of the second stage, but the Si Tech regulator was unaffected.
One thing it didnt do was freeze up and freeflow - and thats the point.

Comparable regulators to Consider:

PRICE £499 inc. octopus
FIRST STAGE Upstream piston-type
PORTS 4mp, 2hp
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