Pressure measurement because atmospheric pressure is always changing and the reference in this case is fixed at 1 bar. To produce an absolute pressure sensor, the manufacturer seals a high vacuum behind the sensing diaphragm. If the process-pressure connection of an absolute-pressure transmitter is open to the air, it will read the actual barometric pressure. The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m or kg·m·s). This special name for the unit was added in 1971; before that, pressure in SI was expressed in units such as N·m. When indicated, the zero reference is
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