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Air Diving & Decompression

Air Diving & Decompression. Sources. Joiner, J.T. (ed.). 2001. NOAA Diving Manual - Diving for Science and Technology, Fourth Edition. Best Publishing Company, Flagstaff, AZ. Reference Materials: In conjunction with this presentation, refer to: NOAA Diving Manual Chapter 4

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Air Diving & Decompression

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  1. Air Diving & Decompression

  2. Sources • Joiner, J.T. (ed.). 2001. NOAA Diving Manual - Diving for Science and Technology, Fourth Edition. Best Publishing Company, Flagstaff, AZ. • 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 DCS - 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. US 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. US 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 ft 20 min

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

  31. Repetitive Dive • As a “C” diver, you want to plan a no-stop repetitive dive back to 80 ft 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 ft (Refer to the No-decompression Limit column of Table 3)

  32. Determine the RNT • Trace down the column below C and across from 80 ft. 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 ft 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 ft 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 ft 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 ft 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 ft 80 ft 20 min RNT 13 min + ABT 20 min ESDT 33 min

  39. Exceptions to Normal Repetitive Dive Planning • Computing a dive to 60 ft for 30 minutes with a 30 minute SIT followed a dive to 60 ft indicates an RNT of 36 • How can this be? The NDL for 60 ft 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. US 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. US Navy Table 5 • A dive to 60 ft for 65 minutes results in a required decompression stop at 10 ft 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 ft for 65 minutes results in a required decompression stop at 10 ft for 2 minutes and a Repetitive Group of K K 10 ft 2 min 60 ft 65 min

  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

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