Diving Hawaiian Style

We just got back from a wonderful trip to Maui and Oahu. If you’ve never dove in the Hawaiian Islands it well worth the trip. One of the advantages of Hawaii is that it’s quick 5 hour flight from Seattle and there are no customs hassles once you get there.
The diving in Maui is some of the best on the islands. I chose to dive with the crew from Lahaina Divers out of Ka’anapali. The guys were knowledgeable and competent; plus they gave me as much latitude with my dive profiles as possible. We made several dives out off the island of La’nai to the Cathedrals I and II. The Cathedrals are left-over mega lava bubbles that filled with water and make spectacular cavern dives. Wildlife included shark, dolphin, and bunches of cool little critters. The visibility was a good 100’ and toasty warm; but you probably already guessed that. We also trekked out to the Island of Molokini. The island is a sunken crater which makes for fabulous dive and snorkel site. Dives inside the crater are ok; you see bunches of butterfly and trigger fish. However my wife Windy saw as much snorkeling as we did on our dives; octopus, moray and other neat fish. We ventured to the back wall the following day and were pleased to find white-tip reef shark, Galapagos shark and even a frog fish in over 200’ of underwater cliffs.
Often overlooked is the diving within Oahu. The island usually makes people think of Wakiki and surfing; but he diving on Oahu is good as well. It’s one of the few islands with diveable wrecks. With Ocean Concepts we dove the Mahi wreck; one of the best wreck dives on the island. The wreck sits in about 95 feet of water with much of the superstructure intact. It’s a former 176’ minesweeper/cable layer that was sunk in 1986 to become as an artificial reef. It has become one of the most popular wrecks in Hawaii but because the tower collapsed in one of the hurricanes divers are advised to remain on the outside of the wreck.
The second wreck I visited I chose to dive with Island Divers on what they call the LCU; an upside-down landing craft (as in beach landings) lies in 95'/29m of water on a sandy bottom with spotty reefs surrounding it. Not the most exciting wreck since it lies keel up; but it offers a nice swim-through underneath the wreck and is often home to shark. There is also a tighter swim-through through the pilot house, but no penetrations without being hazardous. One final wreck we visited on the trip was the Sea Tiger, definitely one of my favorite wrecks on Oahu. It’s large for the island; there are several swim-throughs and penetration possibilities. We spotted eagle rays circling the wreck, and several leaf scorpion fish. Sunk in 1996 by a submarine company the Sea Tiger doesn't have a whole lot of coral growth but even so schooling fish, moray eels, and nudibranch are very common. I went with Aaron’s dive shop for this dive, and we chartered the “Moe Mac” for the trip. The Sea Tiger rests upright on a sandy bottom at 130'/40m, but dive depth is generally between 80-100'. Apart from missing wood planks and some decay inside the wreck the Sea Tiger is very much intact and impressive site. All three shops did a good job. Ocean Concepts had the best dive boat, but were fairly restrictive in their dive policies. Island Divers had the best attitude with experienced divers, and Aaron’s had the best atmosphere of a dive shop community if you get to the North Shore. I would recommend any of the three; but a lot depends on the DM and captain.

Calculating your air consumption or SAC rate

Ok, the other day I had a discussion with a couple of dive friends about SAC or air consumption rates. SAC is the acronym for “Surface Air Consumption” and is a variable rate that helps technical divers compute how much air they will likely use at depth. It’s usually calculated in two forms; “working rate” and “resting or deco rate”. The formula for calculating your rate is actually fairly simple:

The calculation is performed by calculating the volume of gas used (PSI used * ( Tank volume rating / Tank pressure rating )), dividing this by the time at depth, then dividing this result by the pressure at the average depth ((D/33)+1). This result is the Surface Air Consumption (SAC).

The procedure for finding your SAC rate is to first identify your tank’s working pressure; that’s the pressure stamped on the tank eg. 2450 psi. Second is to find your tanks official volume at that pressure. This is a little harder, since many tanks don’t specify it on the tank so you may have to visit the manufacturer’s website and identify your tank by size and pressure. Fortunately some tanks are labeled. So, if my tank is rated at 2450 psi at 80 cubic feet I can easily calculate that at the surface each cubic foot is equivalent to 30.6 psi. (2450/80).
Now since the air is compressed at depth, but I still need the same volume for each breath; my consumption increases as I either breathe air that is more dense (depth) or faster (rate). What the SAC does is to determine a baseline at the surface which I can use to calculate at a specific depth my usage based on my prescribed rate. We isolate those variables by performing the following exercise. First to find your working rate you need to dive to a specific depth; most often we use 60’ saltwater or 60 fsw. Once at 60 feet we record or pressure as accurately as possible (helps to have a digital pressure gauge). We then swim at a steady depth (60 fsw) for 15 solid minutes, recording the pressure at each 5 minute interval When we’re done we now know how much air we consumed in psi for a 15 minute working swim. Let’s say we use 1000 psi out of the tank when we’re doe.

Since the air is denser at depth we need to calculate the affect in atmospheres (remember that 33 ft salt water per atmosphere from Open Water Class). So we have to figure in the equivalent by taking our 60 ft dividing by 33 for each atmosphere, and then adding our 1 atmosphere for the pressure exerted by the air at sea level.

I now have everything I need to finish the calculation. I would take my psi used times the volume rating for the tank divided by the tank’s pressure rating (1000*(80/2450)) = 32.65. I then divide by my time at depth 32.65/15 = 2.17 I need my pressure at 60 fsw which is (60/33)+1) or 2.81 atm. And 2.17/2.81 = .774! Or, it means I breathe at a working rate .774 cubic feet of air a minute on the surface. Now all I need to do to figure out how much air I would breathe at 99 fsw is to multiply my SAC rate by the atmospheres of that depth (4) which would be 3.09 cu ft each minute.

I can then do the same exercise for a resting rate, and figure how much air I would use during my safety or deco stop.

Ken