Connect with us


Rules of Thumb 2: Further Mysteries of Ratio Deco Revealed

Are you able to calculate your decompression for a 40-50m/130-165 ft dive using only your average depth and bottom time? Here British techmeister Rich Walker further divulges the enigmatic mysteries behind GUE’s ratio decompression protocols in this part two of the series.



by Rich Walker
Header image original photo by Derk Remmers, edit by Amanda White

For Rules of Thumb-Part One see: Rules of Thumb: The Mysteries of Ratio Deco Revealed

In my last article, I explored some simple strategies you could use to calculate your no-decompression limit (NDL) and how much decompression time you needed if you went beyond that limit. That information applies to dives shallower than 30 m/100 ft. In my own diving, the tools are hugely valuable. I don’t need an expensive dive computer, or a powerful tool for cross-checking a computer if Santa dropped one down my chimney at Christmas. 

But, I can hear all of you hardcore technical hipsters now: 

“Yeah, but…”

My favourite words! But, it’s probably a fair question. Is there a way to work out decompression requirements in the 30-50 m/100-170 ft range? It would be a pretty short article if the answer were no. 

Now, when we looked at the shallower strategies, we began by working out the NDL. To be honest, in the 30-50 m/100-170 ft depth range, there is no NDL that makes a dive worthwhile. If you’re happy with a 10 minute bottom time, then fill your boots. It takes me a good half an hour to even work out what day it is every morning, so I see little point in getting all dressed up for 10 minutes of working out where I am, realising that it might be a nice wreck, and then having to start the ascent. 

So let’s not bother with NDL in this range. There might be one, but it’s not useful. In my last piece, I shared some tables that were generated from Global Underwater Explorers’ (GUE) DecoPlanner, using gradient factors of 100/100. I’m adopting a similar tactic now, but I’ll be using GUE’s default values of 20/85 gradient factors

“Yeah, but…”

I know. Simon says. I’m not going to get into the merits and detriments of gradient factors. But, if you think that changing 20 GFLo to 30 or 40 is going to make a big difference, then run the tables and see the spectacular 3m/10 ft difference in the first stop depth and the whopping 1-2 minute change in decompression times. And, that all assumes you ascended at exactly 9 m/30 ft/min. 

Okay, back to the topic after being so rudely interrupted. 

  • Subscribe for free
  • Area 9
  • Halcyon Sidemount

Table It

Our depth range is past the point where nitrox has any significant benefit, and that air gas would likely blur my vision, so let’s use a trimix of 21% oxygen and 35% helium, or 21/35. We already know that we’re looking at decompression dives, so we’re also going to add a simple nitrox 50% for the shallower phases of the dive to help with off-gassing. We’ll switch to that gas at 21 m/70 ft. 


Description automatically generated

The table, generated by Deco Planner, shows the decompression time required for each bottom time at each depth. I’ve given you a minute at 21 m/70 ft to get your gas switch done, and I’ve also ignored any decompression deeper than the gas switch. Typically, this is 1 minute or so at 24 m/80 ft for longer dives. This stop is usually done automatically as you slow down your ascent rate (you are ascending at 9 m/30 ft/min right?) as you approach the gas switch. 

Anyway, let’s find some patterns in this pile of numbers. I’ve highlighted one interesting point—the 30 minute bottom time at 45 m/150 ft. Here, the bottom time is equal to the decompression time. If we look at the 25 minute dive, we can see that the decompression time is 23 minutes, so if we did the same decompression as the bottom time, then we’d be on the conservative side. The same is true of the 20 minute bottom time. If we did 35 minutes of decompression on the 35 minute dive, we’d be 2 minutes short. The first thing to note is that decompression time does not increase in a straight line—but I never said it did. I’m trying to force a simple straight line rule onto a curve, and that will have limitations. And a man’s got to know his limitations. 

“Yeah, but…”

Of course. We need to look at what happens at different depths as well. If we start at our original point, 30 minutes at 45 m/150 ft, and go 3 m/10 ft shallower, the decompression time is 4 minutes shorter. Go one step shallower, and we get another 4 minutes shorter. So, maybe a simple and conservative rule might be to say, “If we are 3 m/10 ft shallower than the 45 m/150 ft depth, then we get 5 minutes less decompression than the bottom time.” There are a whole 35 different boxes to evaluate in the above table, so it’s worth laying out that analysis now. 

The basic rule we’re trying to test is this: At 45 m/150 ft, total decompression time is equal to the bottom time. For each 3 m/10 ft shallower, we can say that the decompression time is 5 minutes shorter than the bottom time. The table below shows the real decompression requirement, our estimate as well as an error figure to see where the limitations lie. 

Chart, scatter chart

Description automatically generated

Each of the boxes corresponds to an individual depth and time, and in each box there are three numbers. The real decompression in the center, the prediction of the simple rule in the bottom right, and  the error in the top left. A negative number indicates missing decompression using the rule. A positive number indicates a conservative estimate. I also stopped calculating when the error became more than 5 minutes, hence the bottom right of the table is missing. The limitations are starting to appear.

Photo by Sean Romanowski.

More Detailed Analysis

Now, the sharper amongst you will notice that in the top left areas of the table, there seems to be a lot of red. This is particularly apparent when the predicted decompression is zero. But, let’s face it; any dive involving a gas switch to 50% is going to involve a slow-down and some sort of “safety stop.” Indeed, if you were to slow the ascent from 21 m/70 ft to a comfortable 3 m/10 ft/min, and then make a 5 minute safety stop at 6 m/20 ft—as recommended by your friendly local dive instructor—then the smallest practical decompression you could make would be 10 minutes. A minute at each stop from 21 m/70 ft to 9 m/30 ft, and then 5 minutes at 6 m/20 ft.

The rest of the table follows a more predictable pattern, but any estimated decompression longer than 30 minutes tends to get a little inaccurate. If the estimate is 30 minutes or less, then you are within a few minutes of the true figure. I’ve redrawn the table to reflect the “minimum decompression” and removed all dives with longer than 35 minutes of estimated decompression.

Chart, scatter chart

Description automatically generated

Now there are lots more green numbers, and the red numbers are smaller. We’ll get to the deeper depth shortly, but let’s finish up this job first. We’ve developed a simple rule, with some limitations, that allows us to estimate the amount of decompression we need to do for a given depth and bottom time. 

What we haven’t done is examine how that decompression is organised in the water column. My method is driven by practicality, as well as a little religion. The way I do it is like this.

  • Subscribe for free
  • Area 9
  • Halcyon Sidemount

I know that the longest stop will need to be done at 6 m/20 ft, and that the rest of the time should be spent distributed across the 21-9 m/70-30 ft stops. I find that the easiest way is to simply divide my estimate of the total decompression by two, and spend that amount of time on the 6 m/20 ft stop. 

I then divide the remainder equally across the intermediate stops. So, a 30 minute decompression would be 3 minutes at 21, 18, 15, 12 and 9 meters (/70, 60, 50, 40, and 30 ft) and then 15 minutes at 6 m/20 ft. A 20 minute decompression would be 2 minute stops and then 10 minutes at the final stop, and a 10 minute decompression would be 1 minute stops and a 5 minute final stop. 

“Yeah, but…”

You’re right, 25 minutes of decompression and just about every other number gets clunky on the divisions, particularly for the intermediate stops. So, I standardise my decompressions to be one of three simple possibilities: 1’s and 5, 2’s and 10, or 3’s and 15. I calculate my decompression estimate based on the bottom time and average depth of the dive and pick the decompression schedule that fits, and round up to the next schedule if I end up in-between. I’m all for an easy life. 

Now there’s no wetnotes full of tables. Just three different decompression schedules—pick 10, 20, or 30 minutes of decompression. 

I mentioned religion a few lines back, and there is a school of thought that says that dividing the decompression by two to work out the final stop is a bit aggressive, and that it should be more like two thirds on the final stop. Technically, it’s right: and, indeed, on longer and deeper dives, this will work better. But for dives in this range, the simple split works very well. 

Extending The Tool A Little Deeper

Now, I promised you a tool that would work to a depth of 51 m/170 ft, and so far we’ve only got to 45 m/150 ft. So, let’s finish off the tool. First, we’re going to switch to a different gas. Trimix 21/35 is a little light on the helium for dives past 45 m/150 ft (END >3.6 ATA), so an 18/45 is a better choice. The nitrox 50% is still a good choice for a decompression gas though. 

We can follow a very similar approach to what we did above:

A picture containing text, clock

Description automatically generated

If we try our estimate strategy—but, instead of subtracting 5 minutes of decompression from the bottom time, we add 5 minutes to our estimate for every 3 m/10 ft deeper than 45 m/150 ft—we see that things are actually quite conservative. Here’s the full error table. 

Chart, scatter chart

Description automatically generated

Note: The middle number represents predicted total decompression time from Deco Planner; the lower right the estimated decompression using the rule, and the upper left, the difference between the tool (green is greater, red is less)

Green figures all round! I’ve not calculated past 30 minutes of estimated decompression since we’d already established that as a limit.

So, we can finish off our complete rule for estimating decompression requirements for dives in the 33-51 m/110-170ft range. 

  • For a dive at 45 m/150 ft, the total decompression time is equal to the bottom time. 
  • For each 3 m/10 ft shallower than 45 m/150 ft, subtract 5 minutes from the bottom time to get the decompression time.
  • For each 3 m/10 ft deeper than 45 m/150 ft, add 5 minutes to the bottom time to get the decompression time. 

When you’ve worked out your total decompression time, pick one of the three schedules:


Description automatically generated

These tools are based on 21/35 trimix or 18/45 trimix for the bottom phase and nitrox 50% for the decompression. You can use other gases for sure, but it’s best to run the numbers through decompression planning software and make sure the rules work. And if they don’t, with a bit of work, you’ll find a strategy that does. 

“Yeah, but…?”

Subscribe for the InDepth Newsletter

For Rules of Thumb-Part One see: Rules of Thumb: The Mysteries of Ratio Deco Revealed

Dive Deeper:

InDepth: Standard Gases: The Simplicity of Everyone Singing the Same Song by Richard Walker

InDepth: Maintaining Unit Cohesion by Richard Walker

InDepth: Decompression, Deep Stops and the Pursuit of Precision in a Complex World by Jarrod Jablonski

InDepth: Part Two: Tech Divers, Deep Stops, and the Coming Apocalypse by Jarrod Jablonski

Rich Walker learned to dive in 1991 in the English Channel, quickly developing a love for wreck diving. The UK coastline has tens of thousands of wrecks to explore, from shallow waters to deep technical dives. He became aware of GUE in the late 1990s as his diving progressed more into the technical realm, and he eventually took cave training with GUE in 2003. His path was then set, and he began teaching for GUE in 2004. 

He is an active project diver, and is currently involved with the Mars project (Sweden) and the cave exploration team in Izvor Licanke, Croatia. He is the Chairman and founder of Ghost Fishing UK. He is also a full time technical instructor and instructor evaluator with GUE, providing these services via his company, Wreck and Cave Ltd. He sits on GUE’s Board of Advisors and serves several other industry organizations. 


Hyperbaric Chambers Are Turning Away Divers. Will There Be One Nearby When You Need It?

Unfortunately, it’s hard to make a business case for treating divers versus wound and burn care victims. As a result, many hyperbaric chambers no longer treat divers, leaving fewer facilities available for divers in need and increasing their post-dive time to treatment. InDEPTH editor Ashley Stewart reports on this growing crisis in the US and what can be done!




By Ashley Stewart

The hyperbaric chamber at the University of California San Diego. Photo courtesy of Sherri Ferguson

Steven Wells was diving on the scuttled wreck of the USS Oriskany off the coast of Florida in 2016 when a problem with his buoyancy compensator caused a rapid ascent to the surface.

Wells’ dive buddies followed the emergency action plan for the Oriskany listed on the Florida Fish and Wildlife Conservation Commission’s website at the time and brought Wells straight to Naval Air Station Pensacola, the nearest facility with a hyperbaric chamber. The facility turned him away because there was no one there to run it.

Wells was taken 30 minutes away to Baptist Hospital, which also has a chamber capable of treating his injuries, but the hospital had years earlier decided only to use it for wound care. Doctors there decided Wells would be taken by ambulance more than an hour away to Mobile, Alabama, the nearest facility that accepts divers.

By the time Wells arrived at the only chamber that would help him, it was too late.

Steven Wells

“I got a call from the hospital saying, ‘Your husband is on life support. You need to get here now,’” Rachel Wells said of her late-husband of more than 23 years. 

Julio Garcia — the program director of Springhill Medical Center’s wound care and hyperbaric facility where Steven Wells was to be treated — told InDEPTH that while no one can be certain how sooner treatment would have affected the outcome of Wells’ case, it would have given him the best chance for a full recovery.

Each year in the US, there are about 400 serious cases of decompression illness (DCI) — a category including both arterial gas embolism and decompression sickness — in divers, according to one 2020 paper. The Divers Alert Network (DAN) hotline dealt with 587 cases annually over the past five years.

The availability of hyperbaric chambers to treat decompression illness is something many divers take for granted. We try to avoid dive-related injuries through training, but expect treatment to be available when we need it. 

The reality — as Steven and Rachel Wells tragically learned — is that only a minority of divers are close to care for diving-related injuries, according to medical professionals in the field. The estimates vary, but it’s generally believed there are about 1,500 hyperbaric medicine facilities in the US and only 67 are currently treating diving accidents, according to DAN.

  • Halcyon Sidemount

The estimates vary, but it’s generally believed there are about 1,500 hyperbaric medicine facilities in the US and only 67 are currently treating diving accidents, according to DAN.

“The problem is only getting worse, not better,” Garcia, the Springhill Medical Center program director, said. Garcia has been sounding the alarm about this problem for more than a decade. His hospital takes patients from as far away as Florida cave country and treated 20 DCI cases in 2022. Those patients had an average transportation time of 11.5 hours, according to an InDEPTH analysis of Garcia’s records.

Florida stands out because it’s a popular diving destination, DAN Research Director Frauke Tillmans said, but the situation is not much better across the US. Many of the 1,500 hyperbaric medicine facilities, like Pensacola’s Baptist Hospital, have transitioned to treating wound care only for economic reasons. Emergency hyperbaric services are expensive to train and staff, and come with increased liability.

Patient briefing before treatment at the Environmental Medicine and Physiology Unit at Simon Fraser University. 

Time to treatment can be important in DCI cases

Time is of the essence when treating DCI. Divers Alert Network Director of Medical Services Camilo Saraiva told InDEPTH time to treatment is a “pivotal determinant” when it comes to outcomes for DCI patients. “Swift intervention significantly influences the effectiveness of therapeutic recompression,” Saraiva said.

Decompression sickness, for example, results from rapid changes in pressure and can form gas bubbles in body tissues. Initiating recompression therapy minimizes bubble size and number, Saraiva said, enhancing their elimination and reducing the risk of further vascular obstruction and tissue damage.

“The timely provision of hyperbaric oxygen therapy not only aids in bubble resolution but also mitigates the potential for neurological deficits and other severe complications, highlighting the critical role of early treatment in optimizing outcomes for DCI patients,” Saraiva said.

The 2018 paper “In water-recompression” stated delays to recompression in military and experimental diving are typically less than two hours and more than 90% of cases are completely resolved during the first treatment.

Frank K. Butler and Richard E. Moon, hyperbaric medicine experts, wrote in a 2020 letter to the Undersea and Hyperbaric Medicine journal editors suggesting a minority of patients who need life-saving hyperbaric oxygen treatment (HBO2) are close to a major hospital with a 24-hour emergency hyperbaric facility.

Julio Garcia’s log on patient time to treatment at Springhill Medical Center. Click to enlarge

“Despite the urgent need for treatment, most hyperbaric chambers will decline to accept emergent patients at present,” Butler and Moon wrote. “Patients may eventually receive HBO2 but after a significant delay and a transfer of several hundred miles. Many never receive indicated HBO2, often resulting in poorer patient outcomes.”

Patients who are delayed treatment, they wrote, face the possibility in some cases of “death, permanent neurological damage, permanent loss of vision, or loss of an extremity, most of which would have been readily preventable had emergent HBO2 been administered.”

Why fewer chambers treat dive injuries

As recently as two decades ago, according to Butler and Moon, the majority of hyperbaric treatment facilities were available 24/7 to treat emergency patients. The percentage of those facilities now treating emergency patients is unclear, but it’s universally agreed the number has fallen significantly.

The reasons for the loss of emergency HBO2 facilities, Butler and Moon suggest, include “a better economic return when chambers focus on wound care patients as opposed to emergencies; the greater legal liability involved with treating high-acuity emergency patients; and the increased training and staffing requirements that would be required to manage critically ill patients — especially diving injuries and iatrogenic gas embolism patients.”

A letter from an administrator at Baptist Hospital — which sent Steve Wells to Springhill Medical Center — viewed by InDEPTH shows the hospital discontinued hyperbaric emergency services in December 2010, citing lack of staffing for specialty trained hyperbaric physicians who can provide 24-hour patient care. Baptist has yet to respond to InDEPTH’s request for comment.

Julio R. Garcia at Springhill Medical Center Hyperbaric Center

There’s also the issue of pay. Garcia, the Springhill program director, said the current rate of pay for doctors who administer hyperbaric treatments regardless of length is around $150. A typical hyperbaric treatment for other conditions is about two hours. Diving treatments are usually six or seven, he said. “What doctor wants to get paid $150 to be up all night for seven hours, at that point making less than the technician?” Garcia said. “The fix is that healthcare payers need to pay more for the supervision of the treatment for diving injuries. Make it something that’s worth a doctor’s time besides the goodness of their hearts.”

Silence from lawmakers

Medical and diving organizations in 2020 sent a letter to the House and Senate, federal government agencies, governors of Florida and California, and the American Hospital Association expressing concerns about the lack of availability of chambers to treat diving injuries.

“There are approximately three million recreational scuba divers in the US,” the letter stated. “In the unlikely event that they suffer a diving-related injury, they trust that the US medical system will provide state-of-the-art care for their injuries, but the steadily- decreasing number of hyperbaric treatment facilities in the US willing to treat them emergently for decompression sickness or arterial gas embolism often places them at much greater risk than they realize.”

Garcia has on his own contacted lawmakers, reporters, medical systems — even private space companies like SpaceX because his facility is also the only one nearby treating altitude decompression sickness from space and air travel.

Little has changed, Garcia said.

Garcia showed InDEPTH a 2014 letter from a Defense Health Agency director who said, while there are three Undersea and Hyperbaric Medicine Society-accredited clinic hyperbaric medicine facilities and two additional facilities that can treat civilian emergencies, they are not staffed 24/7 and not designed for patients with other medical illnesses. Garcia at the time requested the creation of a federal grant to support the expansion of 24/7 hyperbaric services, but the director said that was outside of the agencies’ purview. 

The hyperbaric chamber at the University of California San Diego. Photo courtesy of Sherri Ferguson

Two years after this exchange, Steven Wells was taken to and turned away from one of these facilities — the NAS Pensacola, listed on the Florida Fish and Wildlife Conservation Commission’s (FWC) emergency action plan at the time. 

The Florida Fish and Wildlife Conservation Commission website now shows a map of the nearly 4,000 artificial reefs across Florida’s 1,350 miles of coastline. Two chambers, one in Mobile, Alabama, and one is Orlando, cover 500 of those miles densely packed with dive locations, according to Garcia.

The FWC website now shows a map of the nearly 4,000 artificial reefs across Florida’s 1,350 miles of coastline. Two chambers, one in Mobile, Alabama, and one is Orlando, cover 500 of those miles densely packed with dive locations, according to Garcia. A report from the University of West Florida estimated the sinking of the Oriskany, scuttled in 2006, generated nearly $4 million for Pensacola and Escambia County in the next year alone.

So many reefs, so few chambers! FWC map screenshot

An FWC spokesperson said the agency provides diver safety reminders and recommended actions on its website “as a courtesy” and is not intended for emergency response. FWC and Visit Florida did not respond to inquiries about how much Florida’s government spends on advertising the artificial reefs and other diving activities, or whether any effort to expand the availability of hyperbaric facilities to treat the divers who show up as a result.

“My question is what is my husband’s life worth compared to your chambers,” Rachel Wells, Steven Wells’ widow said. “Why did he have to die?”


DIVER: A Crisis in Emergency Chamber Availability by Dan Orr (April 2022)

Divenewswire: A Crisis Lurking Below the Surface Emergency Hyperbaric Treatment Availability by Dan Orr (August 2021)

Undersea and Hyperbaric Medicine (2020): Emergency hyperbaric oxygen therapy: A service in need of resuscitation – an open letter by Frank K. Butler, MD, and Richard E. Moon, MD

White paper: Access to emergent hyperbaric oxygen (HBO2) therapy: an urgent problem in health care delivery in the United States (2020)

InDEPTH: A New Look at In-Water Recompression (IWR) (2019) by Reilly Fogarty

Diving and Hyperbaric medicine (2018): In-water Recompression, Doolette DJ and Mitchell SJ 

aquaCORPS (1993): In-Water recompression As An Emergency Field Treatment for Decompression Illness by Richard L. Pyle and David A. Youngblood

InDepth Managing Editor Ashley Stewart is a Seattle-based journalist and tech diver. Ashley started diving with Global Underwater Explorers and writing for InDepth in 2021. She is a GUE Tech 2 and CCR1 diver and on her way to becoming an instructor. In her day job, Ashley is an investigative journalist reporting on technology companies. She can be reached at:

  • Area 9
Continue Reading


WordPress PopUp Plugin