Gauge Pressure — What It Is and Why It Matters
Gauge pressure is pressure measured relative to atmospheric pressure — it reads zero at the surface and only measures the additional pressure from the water column. Your SPG (submersible pressure gauge) reads gauge pressure: when a tank is empty and open to air, the gauge reads zero even though the absolute pressure inside the tank equals atmospheric pressure.
In diving, gauge pressure is primarily used for reading tank pressure. Decompression calculations and gas physics use absolute pressure instead, because dissolved gas behavior depends on total pressure including atmospheric.
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Frequently Asked Questions
Why does my SPG read gauge pressure instead of absolute?
Because what matters for tank management is how much air is available above atmospheric. A reading of 200 bar gauge means you have 200 bar of usable air. When it reaches 0, there is still atmospheric pressure air in the tank — but you cannot inhale it against ambient water pressure at depth.
Is the depth reading on my dive computer gauge or absolute pressure?
Depth is displayed as gauge depth — meters or feet below the surface. However, internally, the computer converts this to absolute pressure for all decompression calculations. Some computers display both depth and absolute pressure in their detailed screens.
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