Chapter 5- Decompression and Recompression Decompression and Recompression 175 Typically, you’ll need your depth gauge primarily for determining decompression information. However, some altitude dive tables require actual depth information. Should you use such tables, you will have to determine actual depth(s) which will require depth gauge correction. How do various gauges differ from true depth when used underwater? When using the capillary depth gauge to determine true depth you simply subtract 3% of the actual reading per 300 m (1000 ft) of elevation at the dive site. This correction is not needed at 300 m (1000 ft) elevation, but it is accurate for use at sites located above 900 m (3000 ft) elevation. For all other types of non-adjustable depth gauges (i.e. not dive computers), you should add 0.3 m per 300 m (1 ft per 1000 ft) of elevation, then add 3% of the gauge reading. While this procedure has a small, inherent error, it is useful for all routine altitude diving calculations. To assist you in calculating true (actual) depths when diving in fresh water at high altitudes, use tables 5-10 and 5-11. SUMMARY There are many different decompression theories and tables, applying to different or highly specific circumstances. It is important to understand the back- PROPER ACTUAL DEPTH OF INDICATED OCEAN STOP DEPTHS Altitude / Elevation Decompression Stop Depths in mfw Meters 3 msw Stop 6 msw Stop 9 msw Stop 0 3.1 6.2 9.3 600 2.9 5.7 8.6 1200 2.7 5.3 8.0 1800 2.5 4.9 7.4 2400 2.3 4.6 6.9 3000 2.1 4.2 6.4 3600 2.0 3.9 5.9 MFW Stop = MSW Stop * ( 10.3 / 10 ) at sea level Altitude Stop = MFW Stop * EXP ( -0.0381 * Altitude / 304.8) TABLE 5-9A. MODIFIED FRESH WATER DECOMPRESSION STOPS CORRESPONDING TO STANDARD OCEAN STOP DEPTHS (METRIC). Altitude / Elevation Depth To Add feet meters* feet meters* 1000 305 0 0 2000 610 1.2 0.4 3000 915 2.4 0.7 4000 1220 3.5 1.1 5000 1525 4.6 1.4 6000 1830 5.7 1.7 7000 2135 6.7 2.0 8000 2440 7.8 2.4 9000 2745 8.7 2.7 10000 3050 9.7 3.0 11000 3355 10.6 3.2 12000 3660 11.5 3.5 13000 3965 12.3 3.7 *approximate conversions from feet to meters True Depth is obtained by increasing the depth as corrected by this table by a further 3% TABLE 5-10. CORRECTION FOR BOURDON (NON-ADJUSTABLE) DEPTH GAUGES. CAPILLARY GAUGE CORRECTIONS feet meters *Conversion Factor 0 0 1.026 1000 305 0.989 2000 610 0.953 3000 915 0.919 4000 1220 0.885 5000 1525 0.853 6000 1830 0.822 7000 2135 0.791 8000 2440 0.762 9000 2745 0.733 10000 3050 0.705 11000 3355 0.678 12000 3660 0.652 *approximate conversions from feet to meters Multiply Guage Reading by the Conversion Factor to Determine Actual Fresh Water Depth TABLE 5-11. CORRECTION FACTOR FOR CAPILLARY GAUGES. PROPER ACTUAL DEPTH OF INDICATED OCEAN STOP DEPTHS Altitude / Elevation Decompression Stop Depths in ffw Feet 10 fsw Stop 20 fsw Stop 30 fsw Stop 0 10.3 20.6 30.9 2000 9.5 19.1 28.6 4000 8.8 17.7 26.5 6000 8.2 16.4 24.6 8000 7.6 15.2 22.8 10000 7.0 14.1 21.1 12000 6.5 13.0 19.6 FFW Stop = FSW Stop * ( 34 / 33 ) at sea level Altitude Stop = FFW Stop * EXP ( -0.0381 * Altitude / 1000) TABLE 5-9B. MODIFIED FRESH WATER DECOMPRESSION STOPS CORRESPONDING TO STANDARD OCEAN STOP DEPTHS (IMPERIAL).
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