Atmospheres absolute is abbreviated “ata,” and absolute pressure in pounds per square inch is abbreviated “psia,” “psig” being used to specify gauge pressure. Gauge pressure is converted to absolute pressure by adding the appropriate unit measure for one atmosphere to the gauge reading. In other words, if the gauge is marked in kPa, add 101 kPa to the gauge reading to obtain the absolute pressure; if the gauge is marked in psi, add 14.7 psi; if the gauge is marked in meters (or feet) of seawater, add 10.33 m (or 33 ft). Ambient pressure, or the pressure surrounding you at a specified place, always refers to the absolute pressure. Sample Problems: SI/metric What is the absolute pressure in a fully charged 200 bar cylinder? Answer : Pa = 200 bars + 1.013 bars = 201.013 bars (about 201 bars) U.S./Imperial What is the absolute pressure in a fully charged 3,000 psi cylinder? Answer: Pa = 3,000 psi + 14.7 psi = 3,014.7 psi (about 3,015 psi) This is very likely an example of an instance in which your ability to manipulate numbers exceeds the accuracy of your measuring device (cylinder pressure gauge). So normally, 201 bars and 3,015 psi are sufficient answers. Two possible formulas can be used to find the hydrostatic or water pressure at depth: Hydrostatic Pressure = Depth of water Depth definition of 1 atm Or: Hydrostatic Pressure = Depth of water x Pressure per unit depth For example, in seawater: SI/metric: Pressurewater = Depth 10 msw/atm Or: Pw = Depth x 0.1 atm/msw U.S./Imperial: Chapter 3- Diving Physics Pw = Depth 33 fsw/atm Or: Pw = Depth x 0.445 psi/fsw Having found the pressure in atmospheres (or psi) you can multiply the answer by any appropriate conversion factor to obtain the answer in any other measurement unit. To find the absolute/ambient/total pressure at a given depth, add one atmosphere (or the pressure equivalent to one atmosphere) into your formula. Absolute pressure = Depth of water + 1 atm = Depth definition of 1 atm = Depth of water + Depth of 1 atm Depth of definition of 1 atm Or: Absolute pressure = (Depth of water x Pressure per unit depth) + 1 atm or equivalent. Take care to use the “correct” water. If in seawater, use 10 msw/33 fsw per atmosphere. If in fresh water, use 10.33 mfw/34 ffw per atmosphere. For example, in fresh water: Pabsolute = Depth + 10.33 mfw (SI/metric in atm) 10.33 mfw/atm Or: Pa = (Depth) + 1 atm (U.S./Imperial in atm) (34 ffw) Or: Pa = (Depth x 0.432 psi/ffw) + 14.7 psi (U.S./Imperial in psi) Sample Problems: SI/metric 1. What is the hydrostatic (water, gauge) pressure in atmospheres at 17 meters of fresh water? Answer: Pw = 17 mfw x 1 atm = 1.65 atm 10.33 mfw 2. What is the hydrostatic pressure in kilopascals at 17 meters of fresh water? Answer: Pw = 17 mfw x 1 atm x 101.3 kPa = 166.7 kPa 10.33 mfw 1 atm Diving Physics 83
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