EASA/GCAA Part-66 · Module 2 — Physics · Fluid Dynamics

Pressure & Depth

Objective: the pressure in a fluid rises steadily with depth — P = ρ·g·h. It depends on the depth and the fluid's density, but not on the shape or width of the container. Lower the probe and watch the gauge climb.

Tank & depth probe

Pressure vs depth

Probe depth h 3.0 m

Gravity g 9.81 m/s²

Depth h

Gauge pressure ρgh

0kPa

Absolute pressure

0kPa

Density ρ

0kg/m³

Gauge pressure: P = ρ·g·h · Absolute: P_abs = P_atm + ρ·g·h

Pressure at a point in a fluid is the weight of the fluid column above it pressing down, spread over an area — which works out to ρ·g·h. Go twice as deep and the pressure doubles; use a denser fluid and it rises in proportion. Crucially the pressure depends only on the depth, not on the shape or cross-section of the container (the hydrostatic paradox), and it acts equally in all directions. The absolute pressure also includes the atmosphere pushing down on the surface (about 101 kPa at sea level); a gauge reads the difference above that.

Try this: Lower the probe — the gauge and the pressure–depth line both climb straight and steady.
Try this: Switch to mercury (13 600 kg/m³) — the same depth gives a far higher pressure.
Try this: Compare oil and water at the same depth — the denser fluid reads higher.
Try this: Lower gravity and watch the pressure fall for the same depth.