Eating Crow – Safe Water Temperatures for Scuba Regulators

CrowScientists and engineers love to argue, and unlike the case with politicians, compromise is not an option. Technologists speak for nature, for the truth of a universe which does not speak for itself. But when a technologist is wrong, they usually have to eat some crow, so to speak.

Stephen Hawkings, the famous cosmologist, freely admits his brilliant doctoral dissertation was wrong. Crow was eaten, and Hawkings moved on to a better, arguably more correct view of the universe.

Now, on a much less grand scale, this is my time for eating crow.

There has been quiet disagreement over the water temperature above which a scuba regulator is safe from free-flowing or icing up. Those untoward icing events either give the diver too much gas, or not enough. Neither event is good.

Based upon an apocryphal Canadian government study that I can’t seem to put my hands on anymore (government studies are rarely openly available), it has long been believed by the Canadians and Americans that in water temperatures of 38°F or above, regulator icing problems are unlikely. That temperature was selected because when testing older, low flow Canadian regulators, temperatures inside the regulator rarely dropped below 32°F when water temperature was 38°F.

Regulator ice
U.S. Navy photo.

As shown in an earlier blog post, in 42°F water and at high scuba bottle pressures (2500 psi) in instrumented second stage regulators (Sherwood Maximus) second stage internal temperature dropped below zero Celsius (32°F) during inspiration. During exhalation the temperature rose much higher, and the average measured temperature was above freezing. Nevertheless, that regulator free flowed at 40 minutes due to ice accumulation.

Presumably, a completely “safe” water temperature would have to be warmer than 42°F. But how much warmer?

My European colleagues have stated for a while that cold water regulator problems were possible at any temperature below 10°C, or 50°F. However, as far as I can tell that assertion was not based on experimental data. So as we began to search for the dividing line between safe and unsafe water temperatures in another brand of regulator, I assumed we’d find a safe temperature cooler than 50°F. For that analysis, we used a generic Brand X regulator.

To make a long story short, I was wrong.

To understand our analysis, you must first realize that scuba regulator freeze-up is a probabilistic event.  It cannot be predicted with certainty. Risk factors for an icing event are diving depth, scuba bottle pressure, ventilation (flow) rate, regulator design, and time. In engineering terms, mass and heat transfer flow rates, time and chance determine the outcome of a dive in cold water.

At NEDU, a regulator is tested at maximum anticipated depth and ventilated at a high flow rate (62.5 L/min) for a total period of 30 min. If the regulator free flows or stops flowing, an event is recorded and the time of the event is noted. Admittedly, the NEDU test is extremely rigorous, but it’s been used to select safe regulators for U.S. military use for years.

Tests were conducted at 38, 42, 45 and 50°F.

Next, an ordinal ranking of the performance for each regulator configuration and temperature combination was possible using an NEDU-defined probability-of-failure test statistic (Pf). This test statistic combines the number of tests of a specific configuration and temperature conducted and the elapsed time before freezing events occurred. Ordinal ranks were calculated using equation 1, Eqnwhere n is the number of dives conducted, E is a binary event defined as 0 if there is no freezing event and 1 if a freezing event occurs, t is the elapsed time to the freezing event from the start of the test (minutes), and k is an empirically determined constant equal to 0.3 and determined to provide reasonable probabilities, i is the index of summation.

Conshelf XIV pic 2
Click for a larger image.

Each data point in the graph to the left represents the average result from 5 regulators, with each test of 30-min or more duration. For conditions where no freezing events were observed at 30 min, additional dives were made for a 60-min duration.

As depicted, 40-regulator tests were completed, using 20 tests of the five primary second stages and 20 octopus or “secondary” second stages. Regression lines were computed for each data set. Interestingly, those lines proved to be parallel.

A second stage of a typical scuba regulator. The bite block is in the diver’s mouth.

The “octopus” second stage regulator (the part going in a scuba diver’s mouth) differed from the primary only by the spring tension holding the regulator’s poppet valve shut. More negative mouth pressure is required to pull the valve open to get air than in the primary regulator.

The test statistic does not provide the probability that a given test article or regulator configuration will experience a freezing event at a given temperature. However, it does provide the ability to rank the freezing event performance of regulator configurations at various temperatures.

Our testing reveals that in spite of my predictions to the contrary, for the Brand X regulator our best estimate of a “safe” water temperature, defined as Pf = 0, is roughly 53°F for the standard or “primary” second stage regulator and 49° F for the octopus or secondary regulator.

For all practical purposes, the European convention of 50°F (10°C) is close enough.

Eating crow is not so bad. Some think it tastes a little like chicken.

Separator smallEquation 1 came from J.R. Clarke and M. Rainone, Evaluation of Sherwood Scuba Regulators for use in Cold Water, NEDU Technical Report 9-95, July 1995.

On the Odds of Being Struck by Falling Satellites

UARS satellite before deployment. Photo credit: NASA Johnson Space Center.

NASA says the odds that someone will be struck by falling space debris when the bus-sized NASA Upper Atmosphere Research Satellite comes down this week is 1 in 3200. Which got me to thinking … if I was struck while out walking Friday night, would I be unusually lucky because I beat the odds, or unlucky because I beat the odds?

Would my life insurance company pay off? Arguably it would not be an act of God, or an act of war, so I think the insurance company should pay. But I really don’t know if they would; admittedly, I don’t have a falling space debris clause in my policy. (As the space around our planet becomes increasingly crowded, perhaps space debris insurance would be a good investment.)

Now if the odds were 1 in 3200 for each of us, can you imagine the chaos? That would be a mass casualty event in the making. Those odds would be much higher than the odds of being killed by almost anything else I can think of.

From Dr. Strangelove. Click to activate the video.

I suspect there would be anti-NASA marches on the capitols of all the nations affected, which would be most of the world’s nations, by people demanding we nuke the satellite before it poses a hazard. Or maybe they’d demand we send space cowboys up to guide the careening space bus to a safer impact. (I’m not sure how those heroic bronco busters would get back; maybe they’d ride it down a la Dr. Strangelove.)

Fortunately, the odds are mighty small (1 in 21 trillion) that you or I would be hit by this particular satellite. There are much greater chances of winning a state lottery.

But assuming a piece did actually hit me without putting a hole through my head or chest, maybe simply winging me, could I profit from it? Would I become an instant celebrity? Would there be book deals? Can you imagine the television talk show questions, like “How did you feel about your impending death when you saw the fire ball heading your way?”

Let’s face it, with burning metal hurtling to Earth at 18,000 miles per hour I likely wouldn’t see it in time to react, and if I did see it, I undoubtedly wouldn’t have time to mentally compute its trajectory. Should I stand still or run? In fact, I think that calculation would be impossible. An incoming missile simply gets larger and larger in your field of view, giving you perhaps just enough time to say “Oh…” but not enough time to finish the four letter expletive you had intended.

But frankly, I’m not at all concerned. If it happens at all, it wouldn’t happen to me. It always happens to the other guy. Which I’m sure is what the insurance companies are hoping – it will be the other guy, and the other guy will be uninsured.

If pressed, I suppose I could see the insurance company’s point; If I did get squashed by supersonic satellite debris it probably would be an act of God.

Now, I’m trying to think, have I done anything to tick Him off lately?