Ambient pressure concept of pressure becomes irrelevant, and it is undefined at a gravitational singularity. The SI unit of pressure is the pascal (Pa), which is a very small unit relative to atmospheric pressure on Earth, so kilopascals (kPa) are more commonly used in this context. The ambient atmospheric pressure at sea level is not constant: it varies with the weather, but averages around 100 kPa. In fields such as meteorology and underwater diving, it is common to see ambient pressure expressed in bar or millibar. One bar is 100 kPa or approximately ambient pressure at sea level. Ambient pressure may in
Equivalent air depth depth is governed by Pascal's law: Using the SI system with pressures expressed in pascal, we have: Expressing the pressures in atmospheres yields a convenient formula (1 atm ≡ 101325 Pa): To simplify the algebra we will define formula_12. Combining the general formula and Pascal's law, we have: so that Since formula_15, the equivalent formula for the imperial system becomes Substituting R again, and noting that formula_17, we have the concrete formulas: Although not all dive tables are recommended for use in this way, the Bühlmann tables are suitable for use with these kind of calculations. At 27 metres the