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.

Authorized for Cold Water Service: What Divers Should Know About Extreme Cold

The following is reprinted from my article published in ECO Magazine, March 2015.  It was published in its current format as an ECO Editorial Focus by TSC Media. Thank-you Mr. Greg Leatherman for making it available for reprinting.ECO Magazine

It is the highpoint of your career as an environmentally minded marine biologist. The National Science Foundation has provided a generous grant for your photographic mission to the waters 100 ft below the Ross Ice Shelf, Antarctica. Now you’re on an important mission, searching for biological markers of climate change.

Under Antarctic Ice, photo by Dr. Martin Sayer.

Above you lies nothing but a seemingly endless ceiling of impenetrable ice, 10 ft thick. Having spent the last several minutes concentrating on your photography, you look up and notice you’ve strayed further from safety than you’d wanted. The strobe light marking the hole drilled in the ice where you’ll exit the freezing water is a long swim away. And, unfortunately, your fellow scientist “buddy” diver has slipped off somewhere behind you, intent on her own research needs.

You’re diving SCUBA with two independent SCUBA regulators, but in the frigid cold of the literally icy waters, you know that ice could be accumulating within the regulator in your mouth. At the same time, a small tornado of sub-zero air expands chaotically within the high-pressure regulator attached to the single SCUBA bottle on your back—and that icy torrent is increasingly sucking the safety margins right out of your regulator. You are powerless to realize this danger or to do anything about it.

At any moment, your regulator could suddenly and unexpectedly free flow, tumultuously dumping the precious and highly limited supply of gas contained in the aluminum pressure cylinder on your back. You’re equipped and trained in the emergency procedure of shutting off the offending regulator and switching to your backup regulator, but this could also fail. It’s happened before. 

As you try to determine your buddy’s position, you’re feeling very lonely. You realize the high point of your career could rapidly become the low point of your career—and an end to your very being. Picture046

The preceding is not merely a writer’s dramatization. It is real, and the situation could prove deadly—as it has in far less interesting and auspicious locations. Regulator free flow and limited gas supplies famously claimed three professional divers’ lives in one location within a span of one month.

There is a risk to diving in extreme environments. However, the U.S. Navy has found that the risk is poorly understood, even by themselves—the professionals. If you check the Internet SCUBA boards, you constantly come across divers asking for opinions about cold-watersafe regulators. Undoubtedly, recent fatalities have made amateur divers a little nervous—and for good reason.

Internet bulletin boards are not the place to get accurate information about life support safety in frigid water. Unfortunately, the Navy found that manufacturers are also an unreliable source. Of course, the manufacturers want to be fully informed and to protect their customers, but the fact remains that manufacturers test to a European cold-water standard, EN 250. By passing those tests, manufacturers receive a “CE” stamp that is pressed into the hard metal of the regulator. That stamp means the regulator has received European approval for coldwater service.

As a number of manufacturers have expensively learned, passing the EN 250 testing standard is not the same as passing the more rigorous U.S. Navy standard, which was recently revised, making it even more rigorous by using higher gas supply pressures and testing in fresh as well as salt water. Freshwater diving in the Navy is rare—but depending on the brand and model of regulator in use, it can prove lethal.

The unadorned truth is that the large majority of manufacturers do not know how to make a consistently good Performing cold-water regulator. Perhaps the reason is because the type of equipment required to test to the U.S. Navy standard is very expensive and has, not to date, been legislated. Simply, it is not a requirement.

Some manufacturers are their own worst enemy; they cannot resist tinkering with even their most successful and rugged products. This writer is speculating here, but the constant manufacturing changes appear to be driven by either market pressures (bringing out something “new” to the trade show floor) or due to manufacturing economy (i.e., cost savings). The situation is so bad that even regulators that once passed U.S. Navy scrutiny are in some cases being changed almost as soon as they reach the “Authorized for Military Use” list. The military is struggling to keep up with the constant flux in the market place, which puts the civilian diver in a very difficult position. How can they—or you—know what gear to take on an environmentally extreme dive?

My advice to my family, almost all of whom are divers, is to watch what the Navy is putting on their authorized for cold-water service list. The regulators that show up on that list (and they are small in number) have passed the most rigorous testing in the world.

Through hundreds of hours of testing, in the most extreme conditions possible, the Navy has learned what all SCUBA divers should know:

• Even the coldest water (28°F; -2°C) is warm compared to the temperature of expanding air coming from a first stage regulator to the diver. There is a law of physics that says when compressed air contained in a SCUBA bottle is expanded by reducing it to a lower pressure, air temperature drops considerably. It’s the thermal consequence of adiabatic (rapid) expansion.

• Gas expansion does not have to be adiabatic. Isothermal (no temperature change) expansion is a process where the expansion is slow enough and heat entry into the gas from an outside source is fast enough that the expanded gas temperature does not drop.

• The best regulators are designed to take advantage of the heat available in ice water. The most critical place for that to happen is in the first stage where the greatest pressure drop occurs (from say 3,000 psi or higher to 135 psi above ambient water pressure (i.e., depth). They do that by maximizing heat transfer into the internals of the regulator.

• First stage regulators fail in two ways. The most common is that the first stage (which controls the largest pressure drop) begins to lose control of the pressure being supplied to the second stage regulator, the part that goes into a diver’s mouth. As that pressure climbs, the second stage eventually can’t hold it back any longer and a free flow ensues.

• The second failure mode is rare, but extremely problematic. Gas flow may stop suddenly and completely, so that backup regulator had better be handy.

• Second stage regulators are the most likely SCUBA components to fail in cold water due to internal ice accumulation.

• Free flows may start with a trickle, slowly accelerating to a torrent, or the regulator may instantly and unexpectedly erupt like a geyser of air. Once the uncontrolled, and often unstoppable free flow starts, it is self-perpetuating and can dump an entire cylinder of air within a few minutes through the second stage regulator.

• A warm-water regulator free flow is typically breathable; getting the air you need to ascend or to correct the problem is not difficult. In a cold-water-induced free flow, the geyser may be so cold as to make you feel like you’re breathing liquid nitrogen and so forceful as to be a safety concern. Staying relaxed under those conditions is difficult, but necessary.

• Water in non-polar regions can easily range between and 34°F to 38°F; at those temperatures, gas entering the second stage regulator can be at sub-freezing temperatures. European standard organizations classify ~10°C (50°F) as the cold/non-cold boundary. The Navy has found in the modern, high-flow regulators tested to date that 42°F is the water temperature where second stage inlet temperature is unlikely to dip below freezing.

• The small heat exchangers most manufacturers place just upstream of the second stage is ineffective In extreme conditions. They quickly ice over, insulating that portion of the regulator from the relative warmth of the surrounding water. Heat Ex Regulator

• Regulator “bells and whistles” are an unknown and can be problematic. Second stage regulators with multiple adjustments can do unpredictable things to heat transfer as the diver manipulates his controls. The last thing a cold-water diver should want is to make it easier to get more gas. High gas flows mean higher temperature drops and greater risk of free flow.

• Only manufacturer-certified technicians should touch your regulator if you’re going into risky waters. The technician at your local dive shop may or may not have current and valid technician training on your particular life support system. Don’t bet your life on it— ask to see the paperwork.

• Follow Navy and Smithsonian* guidance on handling and rinsing procedures for regulators in frigid waters. A single breath taken above the surface could freeze a regulator before you get your first breath underwater.

U. S. Navy reports on tested regulators are restricted. However, the list of those regulators passing all phases of Navy testing is available online. If your regulator, in the exact model as tested, is not on that list, do yourself a favor and don’t dive in frigid waters.


Separator small

The original Editorial Focus article is found in the digital version of the March ECO magazine here, on pages 20-25.


Redundancy – a Life Saver in Diving and Aviation

Photo taken from the author’s aircraft one stormy Florida Panhandle morning. (click to enlarge)

I was recently flying a private aircraft down the Florida Peninsula to Ft. Lauderdale to give a presentation on diving safety. As I continually checked the cockpit instruments, radios and navigation devices, it occurred to me that the redundancy that I insist upon in my aircraft could benefit divers as well.

In technical and saturation diving, making a free ascent to the surface is just as dangerous as making a free descent to the ground in an airplane, at night, in the clouds. In both aviation and diving, adequate redundancy in equipment and procedures just might make life-threatening emergencies a thing of the past.


As I took inventory of the redundancy in my simple single engine, retractable gear Piper, I found the following power plant redundancies: dual ignitions systems, including dual magnetos each feeding their own set of spark plug wires and redundant spark plugs (two per cylinder). There are two sources of air for the fuel-injected 200 hp engine.

There are two ways to lower the landing gear, and both alarms and automatic systems for minimizing the odds of pilot error — landing with wheels up instead of down. (I’ve already posted about how concerning that prospect can be.)

I also counted three independent sources of weather information, including lightning detection, and two powerful communication  radios and one handheld backup radio. For navigation there is a compass and four electronic navigation devices: one instrument approach (in the clouds) approved panel mount GPS with separate panel-mounted indicator, an independent panel mounted approach certified navigation radio, plus two portable GPS with moving map displays and superimposed weather. Even the portable radio has the ability to perform simple navigation.

photo (17) - no Exif
There’s two of just about everything in this Arrow panel.

The primary aircraft control gyro, the artificial horizon or attitude indicator, also has a fully independent backup. One gyro operates off the engine-powered vacuum pump, and the second gyro horizon is electrically driven. Although by no means ideal, the portable GPS devices also provide attitude indicators based upon GPS signals. In a pinch in the clouds, it’s far better than nothing. Of course, even if all else fails, the plane can still be flown by primary instruments like rate of climb, altimeter, and compass.

There is only one sensitive altimeter, but two GPS devices also provide approximate altitude based on GPS satellite information.


But what about divers? How are we set for redundancy?

Starting with scuba (self-contained underwater breathing apparatus), gas supplies are like the fuel tanks in an aircraft. I typically dive with one gas bottle, but diving with two or more bottles is common, especially in technical diving. In a similar fashion, most small general aviation aircraft have at least two independent fuel tanks, one in each wing.

The scuba’s engine is the first stage regulator, the machine that converts high pressure air into lower pressure air. Most scuba operations depend on one of those “engines”, but in extreme diving, such as low temperature diving, redundant engines can be a life saver. While most divers carry dual second stage regulators attached to a single first stage, for better redundancy polar divers carry two independent first stages and second stages. Two first stage regulators can be placed on a single tank.

An H-valve for a single scuba bottle. Two independent regulators can be attached.
Two Regs
A Y-valve for Antarctic diving with two independent scuba regulators attached.













Even then, I’ve witnessed dual regulator failures under thick Antarctic ice. The only thing saving that very experienced diver was a nearby buddy diver with his own redundant system.

There is a lot to be gained by protecting the face in cold water by using a full face mask. But should the primary first or second stage regulator freeze or free flow, the diver would normally have to remove the full face mask to place the second regulator in his mouth.

Two regulators, one full face mask. Photo courtesy of Michael Lang and Scuba Pro.

Reportedly, sudden exposure of the face to cold water can cause abnormal heart rhythms, an exceedingly rare but potentially dangerous event in diving. If the backup or bail out regulator could be incorporated into the full face mask, that problem would be eliminated. The photo on the right shows one such implementation of that idea.


Inner Space 2014_Divetech _Nikki Smith_Rosemary E Lunn__Roz Lunn_The Underwater Marketing Company_Nancy Easterbrook_rebreather diving_2014-05-27 22.30.47
Nikki Smith, rebreather diver with open circuit bailout in her right hand. Photo courtesy of Rosemary E Lunn (Roz), The Underwater Marketing Company.

Rebreathers are a different matter. Most rebreather divers carry a bailout system in case their primary rebreather fails or floods. For most technical divers, that redundancy is an open circuit regulator and bailout bottle. However, there are options for the bail-out to be an independent, and perhaps small rebreather. (One option for a bail-out semiclosed rebreather is found here.) Such a bail-out plan should provide greater duration than open-circuit bailout, especially if the divers are deep when they go “off the loop”.

U.S. Navy photo by Bernie Campoli.

For some military rebreather divers, there is at least one complete closed-circuit rebreather available where a diver can reach it in case of a rebreather flood-out.

A commercial saturation diver with semi-closed rebreather backpack as emergency bail-out gas.

For deep sea helmet diving, the bail-out rebreather is on their back and a simple valve twist will remove the diver from umbilical-supplied helmet gas to fresh rebreather gas.

The most common worry for electronically controlled rebreather divers is failure of the rig’s oxygen sensors. For that reason it is common for rebreathers to carry three oxygen sensors. Unfortunately, as the Navy and others have noted, triple redundancy really isn’t. Electronic rebreathers are largely computer controlled, and computer algorithms can allow the oxygen controller to become confused, resulting in oxygen control using bad sensors, and ignoring a correctly functioning oxygen sensor.

The U.S. Navy has performed more than one diving accident investigation where that occurred. Safety in this case can be improved by adding an independent, redundant sensor, by improving sensor voting algorithms, by better maintenance, or by methods for testing all oxygen sensors throughout a dive.

In summary, safe divers and safe pilots are always asking themselves, “What would I do if something bad happens right now?” Unfortunately, private pilots and divers quickly discover that redundancy is not cheap. However, long ago I decided that if something unexpected happened during a flight or a dive, I wouldn’t want my last thoughts to be, “If only I’d spent a little more money on redundant systems, I wouldn’t be running out of time.”

Time, like fuel and breathing air, is a commodity you can only buy before you run out of it.

Separator smallDisclaimer: This blog post is not an endorsement of any diving product. Diving products shown or mentioned merely serve as examples of redundancy, and are mentioned only to further diver safety. A search of the internet by interested readers will reveal a panoply of alternative and equally capable products to enhance diver safety.

Cold Water Scuba Regulator Testing — U.S. Navy vs. EN 250

Under thick ice in the Ross Sea, near McMurdo, Antarctica.

When scuba diving under 3-m thick polar ice with no easy access to the surface, the last thing you want to worry about is a failure of your scuba regulator, the system that provides air on demand from the aluminum or steel bottle on your back.

However, cold water regulators do fail occasionally by free-flowing, uncontrollably releasing massive amounts of the diver’s precious air supply. When they fail, the second stage regulators, the part held in a scuba diver’s mouth, is often found to be full of ice.

The U.S. Navy uses scuba in polar regions where water temperature is typically -2° C (28° F).  That water temperature is beyond cold; it is frigid. Accordingly, the Navy Experimental Diving Unit developed in 1995 a machine-based regulator testing protocol that most would consider extreme. However, that protocol has reliably reflected field diving experience in both Arctic and Antarctic diving regions, for example, in Ny-Ålesund, Svalbard, or under the Ross Sea ice near McMurdo Station.

There are currently both philosophical and quantitative differences between European standards and the U.S. Navy standard for cold water regulator testing. Regulators submitted for a European CE mark for cold water diving must pass the testing requirements specified in European Normative Standard EN 250 January 2000 and EN 250 Annex A1 of May 2006. In EN 250 the water temperature requirement for cold water testing ranges from 2° C to 4° C. Oftentimes, regulators that pass the EN 250 standard do not even come close to passing U.S. Navy testing.

An iced up, highly modified Sherwood SRB3600 Maximus second stage regulator

The Navy’s primary interest is in avoiding regulator free-flow under polar ice. The breathing effort, which is a focal point of the EN 250 standard, is of lesser importance. For instance, the 1991 Sherwood SRB3600 Maximus regulators long used by the U.S. Antarctic program have been highly modified and “detuned” to prevent free-flows. You cannot buy them off-the-shelf. Detuning means they are not as easy to breathe as stock regulators, but they also don’t lose control of air flow to the diver; at least not very often. Here is a photo of one that did lose control.

NEDU performs a survival test on regulators, and any that pass the harshest test are then tested for ease of breathing. The so-called “freeze-up” evaluation breathes the regulator on a breathing machine with warmed  (74 ±10°F; 23.3 ±5.6°C) and humidified air (simulating a diver’s exhaled breath) at 198 feet sea water (~6 bar) in 29 ± 1°F (-1.7 ± 0.6°C) water. Testing is at a moderately high ventilation rate of 62.5 L/min maintained for 30 minutes. (In my experience a typical dive duration for a dry-suit equipped diver in Antarctica is 30-40 min.)

To represent polar sea water, the test water is salted to a salinity of 35-40 parts per thousand.  The possible development of a “freeze up” of the regulator 2nd stage, indicated by a sustained flow of bubbles from the exhaust port, is determined visually.

In contrast, the European standards call for slightly, but critically, warmer temperatures, and do not specify a duration for testing at an elevated respiratory flow rate. I have watched regulators performing normally under EN 250 test conditions (4° C), but free-flowing in water temperatures approaching 0° C. Those tests were run entirely by a non-U.S. Navy test facility, by non-U.S. personnel, using a U.K. produced breathing machine, with all testing being conducted in a European country. The differences in testing temperatures made a remarkable difference.

Haakon Hop of the Norwegian Polar Institute in Ny-Ålesund, Svalbard.

The NEDU testing results have been validated during field testing by scientific diving professionals under Arctic and Antarctic ice. The same regulators that excel in the NEDU protocol, also excel in the field. Conversely, those that fail NEDU testing fare poorly under the polar ice. For instance, a Norwegian biologist and his team exclusively use Poseidon regulators for their studies of sea life inhabiting the bottom of Arctic ice.  (The hard hat in the photo is to protect cold skulls from jagged ice under the ice-pack.) Poseidon produces some of the few U.S. Navy approved cold-water regulators.

As is usual for a science diver in the U.S. Antarctic Program, a friend of mine had fully redundant regulators for his dive deep under Antarctic ice. He was fully prepared for one to fail. As he experienced both those regulator systems failing within seconds of each other, with massive free-flow, he might have been thinking of the words of Roberto “Bob” Palozzi spoken during an Arctic Diving Workshop run by the Smithsonian Scientific Diving program. Those words were: “It’s better to finish your dive before you finish your gas…”

In both NEDU’s and the Smithsonian’s experience, any regulator can fail under polar ice. However, those which have successfully passed U.S. Navy testing are very unlikely to do so.


A previous blog posting on the subject of Antarctic diving may also be of interest.