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by Peter Buzzacott
Most technical divers understand and know how to calculate their Surface Air Consumption (SAC) rate, which is typically used in gas planning. But did you know that SAC rates can also be used to measure various workloads—for example, the difference in workload when diving doubles vs. singles, using various techniques, or under different environmental conditions?
Let’s use an example to recap the theory; I’ll use metric throughout this blog for simplicity. A diver is at 10m depth, which is 2 ATA in the sea. The diver’s tank is a standard 12-liter tank filled with 200 bars (plus a safety margin); therefore the diver has 2,400 usable liters of gas at the surface. At 10m depth, that would be down to 1,200 breathable liters, and if the diver used the whole 1,200 L of usable gas over one hour, then the diver’s surface equivalent breathing rate would be 1,200/60, which is 20 liters per minute—a common, if slightly high, breathing rate. One might argue the actual breathing rate is 40 liters per minute, and that the equivalent of 40 liters per minute comes out of the tank, but it is actually compressed down to 20 liters because the diver is at 2 ATA. This is why we call it surface air consumption— because even though the breathing gas is twice as dense at 10 meters depth, the breathing rate is equivalent to 20 L/min at the surface (not 40 L/min at the surface). Of course, the air is twice as dense, so the work of breathing is greater, but SAC does not account for this.
Okay, that’s the math out of the way. In the real world, divers often use the average depth from their dive computers to estimate the average ambient pressure throughout the dive, then divide the volume of gas used by the number of minutes spent underwater and the average pressure. This gives a fairly accurate estimate of the surface equivalent breathing rate. For part of my PhD research, I clipped accurate depth loggers to the chest D-rings of divers and recorded over 1,000 recreational dives. I also used a calibrated pressure gauge to measure the tank pressure before and after each dive, and noted the tank volume stamped on each tank. Then, I asked every diver if they thought the workload during their dive was “resting/light,” “moderate,” or “severe/exhausting.”
Good in the SAC?
In the 1980s, a scientist named Dwyer had divers fin at a certain pace against a set resistance and measured both their SAC and how much oxygen remained in their exhaled gas. This was done at a few set depths, including a series of dives at 10m depth. An equation was generated for each depth to convert SAC into oxygen consumption. The mean depth of my 1,000 recorded dives was 10.7 m, so I used Dwyer’s 10m equation to convert everyone’s SAC into oxygen consumption. I then made one last conversion by dividing everyone’s oxygen consumption by their weight (in kilograms, of course). This standardized the oxygen consumption across the whole range of divers, from small to large and whether male or female.
Interested readers can read my paper, but to make the results meaningful here, I’ll compare the dives by how many metabolic equivalents (METS, i.e., the rate at which a person expends energy relative to the mass or weight of that person) each dive represented. When our oxygen consumption is at its lowest, for example, when we are asleep, we consume around 3.5 ml/kg.min of oxygen, which is by convention considered to be 1 MET. If I consume 35 ml/kg.min of oxygen while running on the treadmill, then I would be running at 10 METS.
In my study, the divers who said their dive was relaxing had an average workload of 5.3 METS. It is important to remember that is an average, because some people had a very low SAC and some people really chewed through their gas, but the average for a resting dive was 5.3 METS, which is about the same as a brisk walk. The dives with a moderate workload averaged 5.8 METS, which is about the same as riding a bicycle at a modest pace (~15 km/hr), and the severe/exhausting dives averaged 6.2 METS, which is about the same workload intensity as an aerobics class.
Dwyer also recorded his divers’ heart rates and tried to generate equations to link SAC with heart rate. Recently I tried something similar and took a research team to Oregon to measure SAC and heart rate in volunteer divers at the Oregon Coast Aquarium. We fitted waterproof EKGs to each diver, plus those sensitive depth/time loggers, and estimated their workload just as I’d described above. But, just like Dwyer, we couldn’t generate a reliable conversion from SAC to heart rate, or vice versa. Therefore, the most reliable estimate of workload that we divers have is, in my opinion, SAC/kg. To give some sort of guide, the “resting/light” divers in my PhD research had an average SAC/kg of 0.21 L/kg.min, the moderate divers 0.23 L/kg.min, and the exhausted divers 0.29 L/kg.min.
As we all know, good trim is essential for reducing workload while diving—as is correct weighting and good buoyancy control. Other factors include finning technique and drag. Using SAC/kg, divers can measure for themselves how much of an effect there is when going from a single tank to a set of doubles, or when adding a stage or multiple stages. Divers can also see just how much of an improvement they make when they perfect their technique over many dives—for example, their DPV technique. [ed: It’s OK to try this at home kids!]
Another advantage of SAC/kg is that it can be compared within a dive team between males and females, between small divers and big divers. In Oregon, the aquarium divers were in drysuits, the water was cold, and the divers were working (cleaning the viewing windows, vacuuming the floor, etc). Their average SAC/kg was 0.26 L/kg.min, which was equivalent to a moderate-to-difficult workload. The full paper was published in the latest issue of Occupational Medicine.
Buzzacott P, Pollock NW, Rosenberg M. Exercise intensity inferred from air consumption during recreational scuba diving. Diving and Hyperbaric Medicine. 2014;44:74-8.
Dwyer J. Estimation of oxygen uptake from heart rate response to undersea work. Undersea Biomedical Research. 1983;10:77-87.
Buzzacott P, Grier JW, Walker J, Bennett CM, Denoble PJ. Estimated workload intensity during volunteer aquarium dives. Occupational Medicine. 2019; https://doi.org/10.1093/occmed/kqz011
Peter Buzzacott, MPH, PhD, is a former recreational and technical diving instructor, now a university researcher with more than 60 published papers on diving injuries. He is a Guest Professor at the Second Military Medical University in Shanghai, in the hyperbaric research laboratory that, in 2017, discovered the first nutritional supplement shown to prevent decompression sickness. Before that, he conducted decompression experiments during a two-year post-doctoral fellowship at a university in northern France. He is an avid cave diver.
Teaching Again as the World Tries to Reopen
Conducting classes and supervising dive ops during the midst of a pandemic can be challenging. Here GUE instructor Francesco Cameli details his experience teaching his first “Fundies” class since lockdown! Welcome to the new normal.
By Francesco Cameli
Header Photo by Damon Loble.
The world has definitely become a little stranger as we all try to deal with the ongoing COVID-19 pandemic and are forced to adapt our way of life to safeguard ourselves from infection. Needless to say, it has been a challenging time for diving instructors around the world, and certainly here in Southern California.
Whether you like reef structure, macro photography, kelp forests, or deep wrecks, Southern California offers some of the best diving in the world. However, it is often overlooked in favor of warmer waters and more exotic locations. It’s true that conditions here are at times challenging, but as the saying goes, “if you can dive in SoCal, you can dive anywhere.” As such, it has been fertile ground for divers seeking out GUE training in an effort to perfect their underwater skills.
As we launched into 2020, diving conditions in SoCal were stunning. We were seeing 30 m/100 ft plus visibility days, and the new GUE community-inspired dive boat, Big Blue, had just been delivered and began conducting dive operations. Importantly, our four active local GUE instructors were busy with classes. Then the pandemic hit. In early March, the governor of California issued a stay-at-home order and all activity shut down. As a result, courses—and to a large extent local diving—were placed on hold.
As of early July, we have resumed limited diving operations carefully following the recommendations from Divers Alert Network to insure our divers’ safety. We have also begun modified classes. In fact, as luck would have it, I just finished teaching my first Fundamentals class in this new COVID-19 world, and wanted to share my experience of what it was like.
The short answer is that it was not unlike many Fundamentals classes before it. While there were a couple of small changes that I think overall actually made for a better learning experience for my students, there were a few minor drawbacks.
Taking The Plunge
Because of the required social distancing, I experimented with working with the students remotely for all of the lectures and some of the land drills. At first, I was unsure as to how this was going to pan out, but it actually turned out great. You see, typically here in Los Angeles, because of the logistics of going diving, we tend to pre-load the class with all the academics in one marathon day consisting of Modules 1-6 followed by the swim test. That was traditionally how I had been doing it. Frequently though, you could see the students start to struggle with concentration after about four modules. So I would take a break and do the swim test but then, they would be tired from the swim test. Not so anymore! This last class, we scheduled three Zoom meetings, each lasting about two to three hours, where we could go through the modules at a leisurely pace. This allowed each student to meet in the comfort of their home and at a time that suited them.
As these were going so well, I then tried Valve drills, Basic 5, and SMB deployment with us all sitting in our rigs. I have to say, that seemed to be well received, too. I was able to focus easily on each student, and the picture quality meant it was really no different than if they had been standing before me in person. So far, so good!
Now, because these meetings only took up the equivalent of one day of time, we were left with three days of diving (versus two, normally). The cool part is that we had ample time, thanks to our swift local boat Big Blue to still get three dives in per day. So instead of doing six dives, we did nine. We filled in the missing land drills on the boat, which—as it was travelling with COVID-19 load restrictions—still gave us plenty of room to social distance and work on the drills.
Everyone of course was wearing a mask at all times and kept a bit more to themselves. The students benefited greatly from these extra dives as we could really focus and spend time on the subject matter that required more attention. I feel, after all, that my students benefit most from the time they get with me in the water, so the more the merrier.
We were even able to conduct the swim test in the open ocean. The boat leant itself well to the task as 10 times around the boat rather neatly worked out to be 300 yards. As for the breath hold, we deployed 15 m/50 ft of current line with a diver holding the line taught while the students swam from the boat to the buoy.
So, what about the diving itself? Well, managing the students on the surface was easy enough. Keeping them 2 m/6 ft or so apart worked while we discussed dives and debriefed. Then once the regulators were in, we would get closer and descend as a team. Honestly, this was no great chore.
The one slight hiccup I had not anticipated, however, came in the form of the S-Drill COVID-19 style. The donation itself went without an issue, and the switching to a necklace instead of the donated regulator was no problem at all. The full long hose deployment and swimming part were also somewhat unspectacular with the exception of the fact that you could see the cogs turning as the out of gas (OOG) student tried to decide on hose routing once a direction was picked. Because they were simply holding the donated regulator and were in fact breathing from their own necklace, they did not always have the regulator correctly orientated and therefore had to be reminded that the hose would normally be on the right side of their face. This of course is something we never actually think of when the regulator is in our mouths. The biggest confusion, in my opinion, came at the moment when it was time to clean up.
What I noticed was that frequently the donating student would try to clean up too soon before the OOG student was back on their primary regulator. I can only assume that they were confused by seeing their team mate holding a regulator in their hand and not breathing from it, which would normally signify it was time for the donating diver to clean up. This small confusion was easy enough to clear up in the subsequent debrief, and the students managed just fine after that. This was a small price to pay, I feel, to ensure that our students remained safe and that we reduced as much as possible the risk of passing COVID-19 through regulator sharing. Just in case, we had disinfectant ready on the boat to re-sanitize a regulator that was unwittingly used not by its rightful owner, but the need never presented.
In conclusion, I think the differences are not so great that the class suffered in any way. As it went, in this case, both students earned a well-deserved tech pass and were rather chuffed (those Brits!) with how the class proceeded. I myself greatly enjoyed the online lecturing and the extra dives. I may well try to find a way to keep conducting fundamentals classes in this manner in the future to maximize water time and split up the modules for better assimilation and retention.
Happy Diving everyone!
Born in France but hailing from Italy via England, Francesco’s passion for the ocean was ignited early on by the work of Jaques Cousteau, and Luc Besson’s film “The Big Blue.” Growing up in the seaside village of Portofino, Italy, Francesco spent just about every daylight hour of his summers freediving. In his 20s and 30s, he found himself locked in a recording studio in London or Los Angeles making records for the likes of Queen and Duran Duran as well as Korn, Stone Sour, Avatar, and others. Francesco rediscovered the ocean on a trip to Kona, which is where his scuba journey began in earnest. Since then, he has averaged over 200 dives a year cultivating his own skills. Once he found GUE, he worked his way through the curriculum and became a GUE instructor in 2019. That year, the passionate and exacting polymath was one of the busiest GUE instructors in Los Angeles, and is now working to become a Tech One instructor in 2020. Some say you can occasionally hear him singing to the fish.
Big Blue was built by a GUE instructor with the GUE and tech diving community in mind. She’s got a big wide ladder, wide diver spaces to accommodate doubles, CCRs, comfy seating for long rides to far targets and speed. Big Blue is one of the fastest dive boats in the Los Angeles area. What’s more, it offers a tech savvy crew. Visit us at: www.bigbluediveboat.com or on FB!
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