316 stainless, never 304 — the difference shows up in year two

What the molybdenum actually does

Both 304 and 316 rely on a chromium oxide layer a few nanometres thick that forms by itself in air. Chloride ions puncture that layer at random points, and under each puncture the metal dissolves into a pit. The 2.3 per cent molybdenum in 316 lets the oxide layer rebuild faster than chloride can break it. On the pitting resistance index 304 scores 18 and 316 scores 24, and the safe chloride threshold is roughly 200 mg per litre against about 1,000.

Those numbers sound generous until you measure the air in Canggu, Sanur, Bingin or Nongsa. Salt aerosol settles on metal all day, the afternoon shower redissolves it, and the chloride concentration in the thin film of water clinging to a rim can pass 3,000 mg per litre on a windy afternoon. A 304 tub on a site like that starts showing brown freckles at around fourteen months, and it starts in the heat-affected zone beside a weld, where the chromium is poorest.

Welding and passivation: the half of the job you cannot photograph

We TIG-weld under argon on both sides — argon inside the pipe and behind the plate, not only at the torch. Without back-purging, the unseen face of the weld oxidises into blue-black scale, and that scale is where rust begins two years later on a surface you never look at. The finished body is then coated in nitric-hydrofluoric pickling paste for 25 minutes, rinsed in demineralised water and re-passivated. It adds two working days and about Rp 1.9 million per tub, and it is the single thing most often skipped by fabricators quoting lower.

Brushed or electropolished, and when it genuinely matters

A No. 4 brushed finish is right for a house. Its fine directional scratch hides fingerprints and light scale, and it looks like a working object rather than a laboratory. Electropolishing dissolves the microscopic peaks of the surface and takes roughness from 0.5 down to 0.2 micrometres, so there is less for bacteria to hold on to and the tub wipes clean faster. For a recovery clinic or a tub used by dozens of people a day, the extra Rp 4.2 million is repaid in cleaning time. For a family tub, honestly, it is not.

Metal conducts, so the insulation is detailed differently

Stainless conducts heat at roughly 16 watts per metre kelvin, about a thousand times better than polyurethane foam, so every uninsulated centimetre of plate is a thermal bridge. We spray 40 millimetres of polyurethane over the entire outside including the underside and the back of the rim, then close it with a second 316 skin. The 55-millimetre top rim is deliberately hollow with an EPDM thermal break, because a rim welded straight through to the inner wall will sweat and drip onto the floor all day in a humid room — and in an air-conditioned clinic it will do it onto the very floor your patients walk on.

A steel shell is a drum, so we tune it

A stainless body behaves like a drum. Circulation pump vibration that is completely inaudible in a terrazzo tub becomes a low hum travelling through every panel if the pump is bolted to the frame. We hang the pump on rubber mounts chosen for 2,900 rpm, use 40 centimetres of flexible hose on both sides instead of rigid pipe, and bond mass-loaded damping sheet to the three largest outer faces before the foam goes on. Measured result is 34 dB at one metre, about the level of a kitchen refrigerator.

Price and what is inside it

A 316 body from 250 to 900 litres runs Rp 28-45 million, including sprayed insulation, the outer skin panels, the insulated lid, an integrated step where requested and a concealed overflow. For clinic and gym duty, automatic sanitation and temperature logging normally come with it, and that specification lives on the commercial ice bath page. Chillers and every extra are itemised on the pricing page.