NOAA Teacher at Sea
Heidi Wigman
Aboard NOAA Ship Pisces
May 27 – June 10, 2015
Mission: Reef Fish Survey
Geographical area of cruise: Gulf of Mexico (25°32.388’N 083°38.787’W)
Date: June 5, 2015
Weather: 82° @ surface, NE winds @ 5-10 knots, seas 0-2 ft, chance of showers and Tstorms, average depth 77m
Science and Technology Log:
I first got my Open Water SCUBA certification in Santa Monica, CA at about 14 years old. My dad and I would explore the waters off of Avalon on Catalina Island, and some offshore sites in Southern California. As a freshman in college, I made my way up through PADI’s advanced course and into my certification as a PADI Rescue Diver. Along the way, I had the chance to do several deep dives (depths below 100′), wreck dives, night dives, drift dives – it was an amazing experience. Later in life, I had the chance to actually work underwater, and would spend 3-4 hours below the surface each day. No matter where I was sent, and what the visibility was (sometimes nil) I felt like I was in my element. During the Reef Fish Survey cruise, we are not doing any dive operations, but I thought that it would be a good opportunity to look at some of NOAA’s dive missions and some of the science and math behind SCUBA.
The use of scientific and research diving has been performed since 1952, with the invention of the Self-Contained, Underwater Breathing Apparatus (SCUBA). Underwater operations have led to significant discoveries in marine science and beyond. Some of the specific types of dives performed by NOAA are biological surveys and sampling, shellfish studies, botanical sampling, geological mapping, deployment, inspection, maintenance, and recovery of instruments, and archaeological site documentation and excavation.
Understanding the basic physics behind diving can help to guide a diver to make smart choices, and stay safe. Anyone that has plunged below the surface a few feet, has felt the pressure that is exerted on them; generally with a discomfort in the middle ear. With diving, you not only have to deal with the atmospheric pressure, but hydrostatic pressure (pressure due to the weight of water), water density, temperature, buoyancy, and gas laws. During SCUBA diving, the body’s tissues absorb additional Nitrogen from air breathed under pressure. Excess Nitrogen will remain in these tissues for a period of time depending on depth and duration of dive. Lucky for divers, the U.S. Navy developed a set of dive tables that can be used to determine safety limits with dive times, in order to account for decreasing the amount of Nitrogen in the body, and avoiding dive related traumas. With our ever-increasing reliance on technology, it is important to have a working knowledge of dive table usage as well.
*This lesson is to give a general and basic understanding of dive tables, NOT to instruct on the usage for purposes of diving
The idea behind dive tables is to use the maximum depth of a dive (even it is only momentarily), and determine the ABT (Actual Bottom Time); and in the case of a repetitive dive, the amount of surface time in order to release the residual Nitrogen from the body. Keep in mind, ABT refers to the start time of the diver’s descent and ends when the diver begins a direct, uninterrupted ascent to the surface.
Let’s look at a scenarios for determining ABT for a diver that is only planning one dive in a 12-hour period:
Your dive team must recover a sampling device located in a bay, whose depth does not exceed 53ft. At this depth, how long do you have to search for and recover the device without exceeding the U.S. Navy No-Decompression Limits?
[Answer: {Fig.B} if you round up to the depth of 55 ft, the team can stay under for 74min., any longer would require a decompression stop on ascent]
What if you want to go on a second dive later in the day? This is where you will look at the Letter Group Designation for a surface interval, based on the ABT at a certain depth.
At 10:13 am, dive team Alpha descends to conduct an inspection of their research vessel. During the inspection, they accidentally drop a dive light. The vessel is at anchor and the hard, sandy bottom is only 47ft deep, and the water is relatively clear. The divers recover the light, and complete their inspection. They begin their ascent at 10:52. What is the Letter Group at the end of the dive? [Answer: {Fig.C} – Repetitive Group Designation is F].
By Following column F {Fig. D}, you can see the times (represented in hours:minutes) to help plan surface interval for your following dive. So if you rested for 2hrs 45min after your first dive, and were planning a 60 depth on your second dive, you would have to plan for 41 minutes of ABT.
Math Question of the Day: By using {Fig. A} the No-Compression Dive Table, try to figure out the following scenario:
Dive team Bravo plans to make an afternoon dive to complete the aquatic life census they started in the morning. The diver’s surfaced from their first dive at 9:57 am, and determined that, following this dive, their Repetitive Group Designation (Letter Group) was E. They anticipate re-entering the water at approximately 3:00pm. What will their Letter Group be at the beginning of this next dive?
Previous Answers:
Trigonometry of Navigation post: 18 m/s @ 34°SE
Bandit Reels post: about 14.6 nautical miles
The STEM of Mapping post: layback = 218m, layback w/ catenary = 207m
Underwater Acoustics: about 163 sq. meters
Coming Soon . . . Fisheries Science