The Physics of Manslaughter

Today’s post comes from the UK, and I try not to talk about legal issues in other countries because I just don’t understand the laws in other nations, and don’t want to stick my nose in them. The difference here is that the case involves some technical issues of SCUBA diving and of dive medicine, areas where I feel like I have some level of mastery. This is a technical post, so for those of you who are not interested in physiology and physics of SCUBA, this may or may not be interesting.

A diving instructor in the UK was teaching an experienced recreational diver a course on deep diving. The dive that they did was to 115 feet.

On this dive, they were diving at around 4.5 atmospheres, and this requires some level of care. I don’t see in this account where the instructor messed up, with the exception that I wouldn’t have had a student doing a check dive like this with an 80 cuFt cylinder (which is what the Europeans call 12 liter).

In this case, however, the government brought in a diver from the UK Navy as their expert witness. He testified that the instructor was wrong in three ways- the dive violated the rule of thirds, they were down longer than the dive tables dictated for that depth, and he held his struggling student underwater when the student was attempting to get to the surface, causing his death by drowning.

Let’s start by addressing each of these in turn. The rule of thirds. The rule of thirds is a rule that says you use one third of your air supply to get into the dive, one third coming out, and hold one third in reserve. This rule is generally only used when you are “diving in the overhead,” meaning that there is either a physical or physiological barrier that would prevent you from surfacing. A physical barrier would be diving in a cave, a shipwreck, or diving so deep for so long that you cannot surface because you have a decompression obligation to work through before you can surface. Neither of those was the case here. In that case, the rule is to ensure that you surface with at least 500 psi of gas left in your tank.

The second argument, that the dive tables’ “no decompression” limits for that depth had been exceeded is ridiculous. When you are a new open water diver, you are taught to use tables, but no one, and I mean no one, follows them. The invention of dive computers has rendered them obsolete. The reason for this is that the dive tables assume that you descend at the maximum safe rate from the surface to the maximum depth, then ascend at the maximum safe rate to the safety stop. This is called a “square profile” and no one dives like this in real life. A dive computer monitors your depth every 30 seconds or so, and gives you “credit” for time spent at shallower depths. This has the effect of more than doubling your permissible dive time. Everyone today “dives their computer.”

A great example of this is the standard dive on Florida’s coral reefs. Off the coast of West Palm Beach, there are several reef lines. The most interesting one from a SCUBA perspective is about a mile or so offshore, in 60-100 feet of water. If you were to dive the top of that reef, the tables say that you can spend a maximum of 40 minutes at 70 feet of depth before exceeding the no decompression limit. Most divers will spend a minute or two at that 70 feet, maybe 5 or 10 minutes at 65 feet, then more time at 55 or 60 feet, etc. The result is that divers with computers might well spend 55 to 65 minutes and still not exceed decompression limits. The Commander would have known this, himself being a certified PADI divemaster.

Instead, he contends that the “out of air” situation was so dire that the diver should have been permitted to make an unrestricted surfacing, despite the fact that the student was breathing on the instructor’s plentiful air source. Ridiculous.

I actually did this exact dive here in the states when I got my own extended diving certification some years ago. It is standard practice at the end of any dive that is deeper than 40 feet to stop at a depth between 15 and 20 feet for three minutes. This is called a “safety stop” and is intended to give any gases that have been absorbed in the blood time to diffuse out of the blood and prevent hyperbaric injuries. It’s recommended by each of the three big certification agencies. (NAUI, PADI, and SSI)

Another protocol that some divers follow is to stop for one minute at half of your current depth. So if you had been at 120 feet, a one minute stop at 60 feet is followed by a one minute stop at 30 feet, followed by a one minute stop at 15 feet. No matter how you do it, coming up as slowly as you can is how you avoid hyperbaric injury.

In fact, three of the dive accidents that resulted in injury, and the only diving fatality I have ever been present for was related to a diver ascending too quickly. The physics and physiology of breathing pressurized gases is technically demanding, especially so when diving to depths below 99 feet. Safety stops are VERY important, especially when you are diving at pressures higher than 4 atmospheres of pressure (99 feet).

I myself have had four diving emergencies that required either emergency surfacing or my buddy’s intervention. Three of them were due to equipment failure, and one because I was a moron. One of them required sharing air. We still had time to do our safety stop.

Even so, it’s obvious that the prosecution wanted to railroad this guy. The student in question had a history of high blood pressure, and the autopsy showed that he had alcohol and cocaine in his system. None of this was known to the instructor at the time of the dive.

In this case, the signs of immersive pulmonary edema were there. For those of you who may dive, or who may work in the medical field, pay attention. Immersive pulmonary edema is very similar to the flash pulmonary edema seen with heart failure patients who are suddenly taken off of CPAP. It’s complicated by the changes in pressure caused by depth changes messing with the Renin-Angiotensin-Aldosterone System (RAAS), which regulates blood pressure. Also adding to the complications is the creation of nitric oxide that occurs with sudden pressure changes in SCUBA diving. In patients with hypertension, heart problems, or kidney problems, this combination can be life threatening.

The signs were there: The student was easily winded with mild exertion, he couldn’t perform underwater navigation while at depth (indicating possible mental status changes from hypoxia), and was complaining that he wasn’t getting any air, even though everything was working perfectly ( a sign of shortness of breath). If he was taking an ACE inhibitor for his high blood pressure, this could even make this condition worse.

So how do you treat this? While diving, adopt the rules that I have always followed:

  • Any diver on any given dive can terminate the dive for any reason. This is done by giving any diver in your group a “thumbs up” sign, and is called “thumbing a dive.”
  • Any diver having apparent confusion, disorientation, or an equipment problem should cause the thumbing of the dive.
  • Any diver having shortness of breath should be placed on oxygen as soon as they are on the surface.
  • On the way down, take a few seconds at 65 feet or so to get organized. Look each other in the eye and make sure everyone gives you the “OK” sign.
  • At any dive below 60 feet, make sure that you do your safety stops.
  • Follow other safe practices like ascent rate, NDL limits, and make sure that everyone is diving within the limits of their training and experience.

My Qualifications

My Internet handle has been Divemedic for more than two decades for good reason. I am a certified Master diver, deep diver, mixed gas diver, public safety diver, and Rescue diver. I am certified by all three of the big US recreational SCUBA training agencies at one level or another: NAUI, PADI, and SSI. I have been SCUBA diving for about 30 years. I used to be on a professional dive rescue team. I have been employed at various times as a rescue and salvage diver and had more than 2,000 dives in my logbook, representing more than 900 hours underwater before I stopped bothering to log them, 16 years ago. Enough dives that I have literally worn out a few sets of equipment. I have been present for half a dozen dive casualties, one a fatality. So I understand many of the issues. With that being said, let’s get into the post.