Steel vs Aluminium Backplates

Steel vs. Aluminium Backplates

For the recreational diver, the BCD is a single unit. For the technical diver, the backplate is the chassis upon which the entire life-support system is built.

The choice between Steel and Aluminium is not simply a matter of price; it is a calculation of buoyancy characteristics (trim) versus logistics (travel). In the context of UK diving—thick undergarments, drysuits, and twinsets—this choice defines your comfort in the water.

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1. Stainless Steel: The UK Workhorse

If you are diving a drysuit in the UK, Stainless Steel is the default choice.

• The Ballast Benefit: A standard 3mm stainless steel backplate weighs approximately 2.2kg – 2.5kg (negative in water). This places nearly 3kg of required ballast directly along your spine (your center of gravity).
• The Comfort Factor: By moving this weight to the plate, you can remove 3kg of lead from your waist belt. This reduces lower back strain and prevents the “bruised hips” common with heavy twinset rig diving.
• Stability: The mass of the steel plate acts as a keel, stabilizing heavy twin cylinders (Twin 12s) and preventing the rig from feeling top-heavy.

The downside? Excess baggage fees. Flying with a steel plate eats up 10-15% of your luggage allowance.

2. Aluminium: The Travel Specialist

Aluminium plates are designed for the diver heading to Truk Lagoon or the Red Sea in a wetsuit, or for specific sidemount configurations.

• The Weight: An aluminium plate weighs approximately 0.7kg – 0.9kg. It is barely negative in water.
• The Wetsuit Reality: If you dive a steel plate with a thin wetsuit (e.g., 3mm), you might be over-weighted even with no lead on your belt. Aluminium allows you to shed that weight.
• Durability: Aluminium is softer. Over years of hard wreck diving, the holes for the bolts can elongate (ovalize) if the wing nuts are overtightened, and the anodized coating will eventually scratch off, leading to oxidization (white powder corrosion).

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3. Material Science: The “Marine Grade” Truth

Not all metal is created equal. The UK market is flooded with cheap internet imports made of lower-grade alloys that rust within weeks.

The Spec: AISI 316 vs. 304

• AISI 304 (A2): Common “Kitchen Sink” stainless.¹ It resists fresh water but will rust in saltwater. It is magnetic.
• AISI 316 (A4): The true “Marine Grade.”² It contains Molybdenum, which drastically increases corrosion resistance to chlorides (salt).³ It is non-magnetic.
  • The Test: If a magnet sticks to your backplate or D-ring, it is not 316 grade. It will rust.

Breaking Strength & Deformation

For technical diving, we look at Deformation Load (when it bends) vs. Breaking Load (when it snaps).

• D-Rings: A welded 316 Stainless D-ring (6mm wire gauge) has a breaking strength exceeding 1,000kg.
• Billy/Clip Rings: Cheap, un-welded D-rings can pull open under the load of a stage cylinder (approx. 10kg in air, but high dynamic load if caught on a wreck). Always ensure D-rings are welded.

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Summary Comparison: Which Plate for the UK?

Feature	Stainless Steel (3mm)	Aluminium (Hard Anodized)
Dry Weight	~2.5kg	~0.8kg
Buoyancy	Highly Negative (Sinks)	Neutral / Slightly Negative
Best For	UK Drysuit / Twinset	Travel / Single Tank Wetsuit
Waist Comfort	Excellent (Removes lead from belt)	Poor (Requires more lead on belt)
Durability	Bombproof (Lasts decades)	Medium (Coating scratches, holes wear)
Corrosion	None (if 316 Grade)	Oxidizes if coating breached

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💡 Pro Tip: The “V-Weight” Solution

If you switch between a single tank (summer) and twinset (winter), or if a 6mm steel plate is still not enough weight:

• Don’t buy a heavier plate.
• Buy a V-Weight. This is a lead wedge that bolts into the central channel of the backplate, between the twin cylinders. It adds 2kg–4kg of non-ditchable weight exactly where you need it for trim, and you can remove it in 2 minutes for travel.