Pressure and depth

What it's for

Converts between depth and absolute pressure, and between gauge and absolute pressure. Every depth-based function in the library takes the same optional water and surfacePressure options.

Formulas

$$P=P_s+\frac{d}{k_w}$$$$d=(P-P_s)k_w$$$$P_{abs}=P_{gauge}+1.01325$$

• $P$: absolute pressure at depth, ata.
• $P_s$: surface pressure, ata. Defaults to SURFACE_ATA = 1.
• $d$: depth, m.
• $k_w$: metres of water per bar. $k_{salt}=10$ and $k_{fresh}=10.3$ m/bar.
• $P_{gauge}$, $P_{abs}$: gauge and absolute pressure, bar.

Why 1 ata for depth and 1.01325 bar for fills

Depth math follows the dive-table convention: the surface is 1 ata, 10 m of salt water adds 1 ata, so 30 m is exactly 4 ata. That keeps the numbers people expect from tables and computers.

Fill math is different. A cylinder gauge reads pressure above the atmosphere, and gas laws need absolute pressure, so gauge to absolute uses the real standard atmosphere, ATM_BAR = 1.01325. The two constants do different jobs and are never mixed.

Assumptions and limits

• Salt water is 10 m per bar and fresh water is 10.3 m per bar (a fixed ratio, not a density calculation).
• depthAtAta is negative when the pressure is below the surface pressure.
• Negative depths, and non-finite inputs, throw a RangeError.
• To dive at altitude, pass a different surfacePressure (see Altitude).

Sources

• Salt: 10 m per bar, 33 fsw per atm (dive-table convention).
• Fresh: 10.3 m per bar, from 34 ffw vs 33 fsw per atm, so 10 × 34/33.
• ATM_BAR: ISO 2533 / ICAO standard atmosphere (101 325 Pa).

Examples

import { ataAtDepth, depthAtAta } from 'dive-math/pressure'
ataAtDepth(30) // => 4
depthAtAta(2.5, { water: 'fresh' }) // => 15.45

API

See the pressure API reference.