Professional Diving, Truth, and the Uncertainty Principle

Is anything known for sure?

Heisenberg’s Uncertainty Principle applied to quantum events avows that there is no certainty until you look. Well, this morning, I looked, and I’m just as confused as ever.

It was a chilly morning in late November. As we warmed up with coffee, I wondered how cold it was outside. So, in the modern style, my wife and I checked the Weather Channel on our phones. One indicated it was 47°F, but the other showed it was 48°F.

That can’t be, I said. So, with identical phones side by side, both tuned into Panama City Beach, Florida weather on the Weather Channel, one phone said it felt like 45°F, and the other said it felt like 43°F.

As Charlie Brown would say, “Good grief.”

Wanting to find some agreement among our devices, I checked a nested set of humidity and temperatures sensors grouped together in our kitchen. Humidity indicators are notoriously inaccurate, yet amazingly, the measured humidity was in reasonable agreement. But inside temperature varied from 70.3°F to 72.8°F.

According to Segal’s Law, “A man with a watch knows what time it is. A man with two watches is never sure.”

This aphorism is falsely attributed to Lee Segall of KIXL, now KGGR in Dallas. Regardless of the source, it is often repeated because it makes such good sense. If you multiply the number of devices three times, as above, the situation is no more precise. (But that’s where statistics comes in, I suppose.)

Giving up on simple things like local environmental parameters, I turned to the latest news on the VAERs update for the vaccines.

I wish I hadn’t. Yes, there is a chance you’ll be fine, but there’s also a small chance you’ll have heart problems and even a small chance you’ll die.

Frankly, my one-time shot at slot machines and the roulette table in Vegas did not end well. So, is there anything we know that can be guaranteed accurate?


I’ve spent a long Navy career in diving science, so I know there are serious certainties there. If you consume more air than is in your scuba tank, you’ll drown. If you stay down too long and surface too quickly, you’ll get the bends, aka decompression sickness.

But what if I use a decompression computer to plan my dive and follow its guidance to the letter? Unfortunately, there’s still a chance you’ll end up in a treatment chamber. Both people’s health and the water environment change constantly, and no decompression algorithm is perfect, or omniscient.

Undersea Oxygen Clinic, Tampa, Florida

From an engineer’s perspective, the tensile strength of a bolt is known within strict limits. If the force applied to that bolt exceeds its limits, then bad things might happen. Buildings might fall, or planes might crash. Or your muffler might fall off.

It’s hard to know what the effect of a broken bolt will be unless you understand precisely the function of that bolt. There is uncertainty in the outcome of a bolt breaking.

Uncertainty vexes some engineers to no end. I’ve watched them squirm as I reveal the role of statistics and probability in acceptance decisions about diving equipment. People are not bolts whose tensile and shear strength can be measured. As Heisenberg predicted (out of context), a dive outcome cannot be predicted with certainty.

Equipment Testing

The same thing applies to diving equipment. The Navy Experimental Diving Unit is entrusted with determining the safety and suitability of underwater breathing apparatus. Both physiologists and engineers envision a line in the sand for a given water depth and diver breathing rate.

If a UBA exceeds that line during testing, it should be rejected for military use. Right? After all, a limit is a limit.

Test and Evaluation Laboratory, Navy Experimental Diving Unit, Panama City, Florida. Photo by Stephen Frink.

Well, not exactly. When translating engineering limits into human terms, things get messy.  If a published limit is exceeded, just like taking the COVID vaccine, some people will fare well, while others may pass out. In other words, failure is classified as the probability of an untoward event where untoward translates to anything that threatens a diver or a diving mission.

For any given dive, and any given diver, the probability of a dive failure cannot be known precisely. Dive failure, like decompression sickness, is probabilistic.

One type of untoward event: Ice inside a second-stage scuba regulator.

To illustrate, the following table and text are from NEDU Technical Report 16-04, Physiological Event Prediction in Evaluations of Underwater Breathing Apparatus, October 2016.

Usually, a UBA evaluated at NEDU is suitable for most diving depths and any foreseeable work/ventilation rate, as shown in Table 1.

The only time that limits were exceeded was at the greatest depth and ventilation rate.

But what if the data had revealed a slightly larger “out of limits” region, as in the next table? What decision regarding safety would then be made?

Hypothetical Test Data

In this hypothetical case, human judgment is required. It is not sufficient to declare the diving equipment unsafe for use. It simply means divers need to pace themselves when working and breathing hard near a depth of 200 feet. Reducing their workload enough to slow their breathing to 62 liters per minute or less (still a high ventilation rate) is a safe way to keep the UBA within limits.  

This is nothing new. Every salvage diver knows to occasionally interrupt hard work periods with periods of rest. Catching your breath is kind of important.

Limits are not absolute

As a person with too many watches, or thermometers can attest, you can’t be sure what all the various goal numbers and limit numbers mean. Instead, collectively they should be used as a guide to safe diving.

Hanging on the umbilical in a KM 37.

Whether you’re a sport diver or professional, if an underwater breathing apparatus is functioning normally but doesn’t meet all of the EU (EN250) or U.S. Navy engineering limits under all possible testing conditions, that doesn’t mean it’s not a useful piece of diving gear. You just have to use it judiciously. After all, good human judgment is always required for safely operating life support equipment.

It is a wise diver who remains mindful of their life support system’s limitations and plans their dive to stay within those limitations. That way, the probability of experiencing an untoward event is minimized.