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Air Diving & Decompression

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  1. Air Diving & Decompression

  2. Sources • Joiner, T. (ed.). 2001. NOAA Diving Manual - Diving for Science and Technology, Fourth Edition. Best Publishing Company, Flagstaff, Arizona. • Reference Materials: • In conjunction with this presentation, refer to: • NOAA Diving Manual Chapter 4 • NOAA Diving Manual Appendix IV • NOAA Diving Manual Appendix III

  3. Objectives • After completing this training module you will be able to: • Differentiate between “safe” and “reliable” with regard to dive tables. • Differentiate between a single, repetitive and contingency decompression dive, and plan these dives using US Navy, and NOAA Dive Tables. • Explain the significance of Group Designation Letters.

  4. Objectives • After completing this training module you will be able to: • Differentiate between “safety stops” and “decompression stops”. • List two options for dealing with omitted decompression. • List three advantages of using a dive computer rather than dive tables.

  5. Objectives • After completing this training module you will be able to: • List three basic rules for diving a dive computer. • Differentiate between Dive Tables, Dive Computers, and PC Based Decompression Software.

  6. General • Body tissues absorb additional nitrogen from air breathed during dives and release this excess nitrogen during ascent. • By keeping the amount of nitrogen absorbed and released within acceptable limits, the risk of decompression sickness is reduced.

  7. Decompression Table Development • Physiologist J.S. Haldane developed a hypothetical model for nitrogen uptake and elimination around 1908. • Another notable in dive table development is Swiss cardiologist Prof. AA Buhlmann.

  8. Decompression Table Development • ALL DIVE TABLES ARE HYPOTHETICAL • All decompression modeling are based on theory. • Dive tables do not reflect the actual workings of the human body.

  9. Are Dive Tables Safe? • “Safe” is not a good choice of words when discussing dive tables. • Virtually any hyperbaric exposure imposes an obligation for decompression. • “Safe” implies no risk of DSC. THERE IS ALWAYS A RISK OF DCS, even when diving well within table limits. • A better word to use when describing dive tables is “Reliable”.

  10. Acceptable Risk... • Reliable dive tables are used to define “acceptable risk”. • Individual susceptibility to decompression disorders, environmental and other factors influence your risk of DCS. • You can do “everything right” and can still suffer “a hit”.

  11. Dive Planning Software • The algorithms of Prof. A.A. Buhlmann are the fundamental basis for most dive planning software and many dive computers. • The different programs based on these algorithms manage the decompression models in slightly different ways.

  12. Decompression Planning • Reliable decompression profiles have been produced using dive tables, dive computers, and dive planning software. • But again, no current method of calculating your decompression obligation can guarantee a zero risk of DCS.

  13. U.S. Navy Dive Tables

  14. Basics • A single dive is any dive made more than 12 hours following a previous dive. • A repetitive dive is any dive made less than 12 hours after surfacing from a prior dive.

  15. Basics • US Navy Dive Table abbreviations / acronyms: • NDL (No Decompression Limit) • ABT (Actual Bottom Time) • SIT (Surface Interval Time) • RNT (Residual Nitrogen Time) • ESDT (Equivalent Single Dive Time)

  16. U.S. Navy Dive Table 3

  17. Basics • Depth Range: USN Dive Tables give depth in feet and meters. Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  18. Basics • No-Decompression Limit: (NDL) The theoretical amount of time a diver can remain at a given depth and return directly to the surface. Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  19. Basics • Dive Time is given in minutes. • Actual Bottom Time (ABT) starts when the diver leaves the surface and ends when the diver begins a direct, uninterrupted ascent to the surface at a rate of no more than 30 feet per minute.

  20. Basics • Group Designation Letter: A representation of the amount of nitrogen a diver absorbs during a dive to a given depth for a given period of time. Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  21. Basics • Depth or Time that do not equal an increment available on the dive tables is to be rounded up. For example: A dive with an actual depth and time of 51 feet for 21 minutes would be computed as 60 feet for 25 minutes.

  22. No-Decompression Limits • The NDL for a dive from 61 to 70 feet is 50 minutes. Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  23. Repetitive Dives • A repetitive dive is any dive made less than 12 hours after surfacing from a prior dive. • To compute a repetitive dive schedule, you must determine the Group Designation Letter and take into account the Residual Nitrogen from the previous dive(s).

  24. Repetitive Dives • Use this simple format as a worksheet for computing repetitive dive profiles. Surface Interval Time (SIT) Group Letter Group Letter Group Letter Depth Depth ABT Residual Nitrogen Time (RNT) + ABT ESDT

  25. Determine a Group Designation • A dive to 80 feet for 20 minutes produces a Group Designation of E. Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  26. Determine a Group Designation • A dive to 80 feet for 20 minutes produces a Group Designation of E. E 80’ 20

  27. USN Table 4 Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  28. USN Table 4 • Table 4 is a combination of two tables. • The upper portion is Surface Interval Time (SIT). The time ranges are in hours and minutes. • The lower portion provides Residual Nitrogen Time. RNT is necessary to properly compute a repetitive dive. Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  29. New Group Designation • 80’/20 produces a Letter Group of E. After a 2:00 SIT the New Group Designation is C. Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  30. New Group Designation • 80’/20 produces a Letter Group of E. After a 2:00 SIT the New Group Designation is C. 2:00 E C 80’

  31. Repetitive Dive • As a “C” diver, you want to plan a no-stop repetitive dive back to 80’ for 20 minutes. • To determine the Residual Nitrogen from the first dive use the lower portion of USN Table 4. • You will also need to know the NDL for 80’ (Refer to the No-decompression Limit column of Table 3.)

  32. Determine the RNT • Trace down the column below C and across from 80’. Where these points intersect is the RNT to be used to compute the next dive. Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  33. Determine the Adjusted NDL • The NDL for 80’ is 40 minutes. • Since this is a Repetitive Dive you must adjust for the RNT and determine the Adjusted NDL. • NDL – RNT = Adjusted NDL Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  34. Determine the Adjusted NDL • The Adjusted NDL gives you the maximum bottom time for a return to 80’ as a C diver, without incurring required decompression. NDL 40 -RNT-13 Adjusted NDL 27

  35. Repetitive Dive • You now know a repetitive dive to 80’ for 20 minutes can be conducted within the No-Decompression Limits of the Navy Dive Tables. • To complete the Repetitive Dive planning process, determine the Equivalent Single Dive Time (ESDT).

  36. Equivalent Single Dive Time • The RNT for a C diver to return to 80’ is 13 minutes. • The Actual Bottom Time (ABT) planned is 20 minutes. RNT 13 + ABT+ 20 ESDT 33

  37. Determine a Final Letter Group • Take the ESDT back to Table 3 to determine the Group Designation Letter at the end of the repetitive dive. Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  38. Repetitive Dive Profile 2:00 (SIT) E C H 80’ 80’ 20 RNT 13 + ABT 20 ESDT 33

  39. Exceptions to Normal Repetitive Dive Planning • Computing a dive to 60’ for 30 minutes with a 30 minute SIT followed a dive to 60’ indicates an RNT of 36. • How can this be? The NDL for 60’ is 60 minutes. The diver should have 30 minutes of unused NDL prior to the SIT. • This seems to show the diver on-gassing nitrogen during the surface interval.

  40. Exceptions to Normal Repetitive Dive Planning • This is an exception to the tables. • The diver can be conservative and use the indicated RNT to compute the dive profile • OR: The diver can ignore the indicated RNT and use the remaining NDL from dive one to compute the dive profile.

  41. Surface Intervals Less Than Ten Minutes • Table 4 does not allow for a SIT of less than ten minutes. • Dives with a surface interval of less than ten minutes are considered ONE DIVE. • Add the bottom times together and use the deepest depth reached to calculate the dive profile.

  42. Decompression • Technically, decompression is something that happens on every dive. • However, for the purposes of this discussion decompression means the diver is required to follow a specific time, depth, and breathing gas profile.

  43. Decompression • A decompression profile, or table, is designed to allow the diver to ascend to the surface without DCS symptoms. • A decompression profile may involve stops, or only require a specific ascent rate without stops.

  44. Precautionary Decompression Stops • Commonly know as “safety stops”. • While not required by US Navy Dive Tables, safety stops are recommended for all “no-stop” dives conducted 60 fsw or deeper, plus all repetitive dives. • Safety stops should be performed for three to five minutes in the 10 to 20 fsw depth range.

  45. Decompression Dives • Unlike a safety stop, a mandatory decompression stop is required by the dive tables. • Decompression dives have substantially greater logistical requirements, and are generally believed to increase the risk DCS.

  46. Decompression Dives • For the purpose of this presentation decompression dive calculation is presented for contingency planning purposes only. Additional training is necessary prior to engaging in dives involving required or mandatory decompression stops.

  47. U.S. Navy Table 5 Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ * See No-Decompression Table 3 for Repetitive Groups ** Repetitive Dives may not follow Exceptional Exposure Dives

  48. U.S. Navy Table 5 • A dive to 60’ for 65 minutes results in a required decompression stop at 10’ for 2 minutes and a Repetitive Group of K. Credit: Permission granted by Best Publishing Company (NOAA Diving Manual 4th Ed.) Flagstaff, AZ

  49. Decompression Profile • A dive to 60’ for 65 minutes results in a required decompression stop at 10’ for 2 minutes and a Repetitive Group of K. K 10’ 2 60’ 65

  50. Omitted Decompression 1 • A diver realizes he has exceeded his planned dive schedule and does not have contingency tables. • He should ascend at a proper rate and stop at 10 to 15 fsw for a minimum of 15 minutes or until cylinder pressure reaches 300 psi, whichever comes first.