The Feel of Steel: Why Some Divers Insist on All-Metal Second Stages

In an era where “lighter, smaller, and composite” is the mantra for most tech, high-performance scuba regulators often seem to defy the trend. While many excellent regulators use high-tech polymers and plastics, a dedicated class of flagship models, like the SCUBAPRO A700, boasts a precision handcrafted full-metal casing.

The obvious assumption is durability. And it’s true—a metal housing can withstand impacts from tanks, boat ladders, and wreck penetrations far better than plastic. But durability is just a side effect.

The real reasons for choosing metal are far more subtle, rooted in thermodynamics and precision engineering.

Advantage 1: The Regulator as a Heat Exchanger

This is the most critical, yet least understood, benefit of a metal second stage, especially in cold water.

Think of plastic (or polymer) as an insulator. It’s the same material used to make a coffee thermos. It keeps hot things hot and cold things cold. Now, think about what’s happening inside your regulator (as we learned in our article on freeze-ups): the Joule-Thomson effect is making the internal mechanism incredibly cold. A plastic, insulating housing traps that cold inside, making it easier for your moist breath to freeze the components.

Metal, by contrast, is a conductor. It loves to transfer heat.

When you dive with a metal second stage, you create a simple, elegant heat engine:
1. You exhale: Your breath is warm, around 95°F (35°C).
2. Metal Absorbs: The metal casing and valve housing absorb that heat from your breath.
3. Metal Transfers: It conducts that heat through the casing to the critically cold internal components (like the valve and lever).

A metal second stage case construction actively uses the waste heat from your own body to warm its internal mechanism, acting as a powerful deterrent against a second-stage freeze-up. It’s not just a dumb piece of metal; it’s an intelligent thermal management system.

Advantage 2: Precision, Sensitivity, and Size

The second advantage comes from manufacturing. Plastic is typically injection-molded. It’s fast and cheap, but it has limitations in terms of stiffness and precision.

A metal housing can be machined from a solid block of marine-grade brass or stainless steel. This precision allows for a design that is both compact and robust. As the SCUBAPRO data notes, this allows designers to fit a larger diaphragm inside a compact casing.

Why does a larger diaphragm matter? A larger diaphragm captures more of the “signal”—your gentle inhalation—and translates it into a larger, more powerful force on the demand lever. This is what creates that feeling of increasing breathing sensitivity and “effortless” breathing. The metal housing provides the rigid, precise framework needed to make that sensitive system work perfectly, every single time.

The Trade-Offs: Weight and Cost

Of course, metal isn’t perfect. It is significantly heavier than plastic, which can be a factor in travel weight and, for some, in-water jaw fatigue (though this is often managed with modern mouthpiece designs). It is also much more expensive to machine and hand-assemble.

But for divers who push the limits in cold, deep, or demanding environments, the trade-off is clear. The choice for metal isn’t about luxury or looks. It’s a calculated engineering decision for superior thermal performance and mechanical reliability.