A Logical Application of Gradient Factors Settings for Open-Circuit Tech Divers

by Andy Davis

As an active technical diving instructor for many years, I see many tech divers struggling to use Gradient Factors to their full advantage.

This article explains my own systematic and easily applicable approach to setting Gradient Factors for optimal off-gassing effect.


Disclaimer:  Any opinions provided in this article only reflect my own successful approach to decompression using gradient factors. Dives requiring staged decompression are substantially more risky than dives that stay well within no-stop limits. No strategy can guarantee to protect every diver from DCS injury. Consequently, I reserve liability for any decisions you may make. Never risk your life on only one source of information. If you choose to make riskier dives, obtain the proper training and work up to them slowly to gain experience. 


What Gradient Factors to Use?

The 2011 NEDU ‘Deep Stop’ study cast free-gas models in a less favourable light. Those results, coupled with follow-on commentary from subject-matter authorities, like Simon Mitchell, reversed the trend towards deep stop profiles.

The popularity of Buhlmann ZHL-16C with Gradient Factors ascended back into supremacy amongst both the tech community and technical computer manufacturers.

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As of 2019, nearly every single legitimate tech computer comes loaded with an iteration of the ZHL-16B/C algorithm with gradient factor control.

The Barrier of Gradient Factor Theory

From my observations, the biggest actual difficulty that divers encounter when using gradient factor controlled algorithms is that setting the high/low variables demands some explicit knowledge of dissolved-gas decompression theory.

I guarantee that some readers probably just took a mental leap towards the door just from having heard those words mentioned.  Don’t run away yet folks – I have promised a simple and logical system, and that isn’t dependant on reading PhD-level papers.

gradient-factors-use-settings-decompression

For those who didn’t know there was ample background theory explaining how gradient factors work, do take a look at the links below.  The rest can just read on with the article…

or buy a copy of Mark Powell’s excellent ‘Deco for Divers‘.

To be fair, dissolved-gas decompression modelling is not a particularly lightweight subject at face value; and that in itself can be sufficiently daunting to deter many divers from sinking their teeth into it.  The subject can even intimidate some technical diving instructors from attempting to teach it (but they should…whatever happened to subject-matter expertise?).

The consequence of this understandable reluctance is that many divers allow themselves to be content with only using gradient factors on constant, arbitrary, default settings.

Or they will seek information about ‘the best gradient factor settings‘ from online forums and groups – where they’ll invariably absorb misleading or inappropriate direction that is completely out-of-context to their actual needs.

One of the biggest context misunderstandings arising from gradient factors debates is whether the participants are discussing open-circuit (OC) or closed-circuit (CCR) diving.

Gradient Factors in CCR Diving

Rebreather divers use constant set-points that continuously deliver an optimal breathing gas mix throughout the bottom and deco phases of a dive. As they are unconstrained by carrying only a few pre-set mixes, their choice of gradient factors doesn’t need to consider off-gassing efficiency or slow-tissue on-gassing as major controlling factors.

For the CCR diver, gradient factors can be chosen merely through personal physiological preferences (i.e. what makes them feel best after a dive), or in line with a prevailing agency or team philosophy. However they chose to shape their ascent curve, they will always be breathing an optimal mix whenever and wherever they stop. Higher off-gassing efficiency is assured.

An open-circuit technical diver who ‘mimics’ gradient factor preferences gleaned from a CCR diver’s perspective will inevitably fail to extract such an optimal outcome – especially where deeper stops may be an attraction.

Gradient Factors for Open-Circuit Tech

As mentioned, open-circuit technical divers aren’t fed a constant stream of optimally blended gas throughout each phase of their dive. Instead, they’ll have pre-filled cylinders of gas that’ll be used at varying phases; a bottom mix, perhaps travel-mix, and one or more deco mixtures (typically 50% and 100%). These gasses are breathed in distinct depth ranges, throughout which the off-gassing efficiency is variable (except O2).

As a result, the open-circuit technical diver must consider how their gradient factor settings promote off-gassing efficiency when bearing in mind the gas mixture actually being breathed.

We use two gradient factor setting to determine our ascent profile: low and high. It’s generally written as GF low/high – i.e. GF 30/70 would indicate that GF low is 30 and GF high is 70.

Setting Gradient Factor Low

In a nutshell, the Gradient Factor Low (GFlo) setting determines at what depth the first decompression stop will be mandated. The lower the number, the deeper the first stop will arise.

Using dive planner software, the diver can experiment with variable GFlo settings. For any given dive profile, they will notice that they have complete control over their initial stop depth.

So, where is a logical place to first stop?

I’ll suggest that an ideal first stop depth should coincide with the initial gas switch depth, i.e. the MOD of the first mixture switched to during ascent – whether that is an appropriately calculated trimix travel gas or just the 50% deco gas on shallower tech dives.

This assumes that the diver will (1) opt for a 50% deco mix as their first-choice initial deco gas mix, and (2) that the diver will use an appropriate travel gas when using hypoxic trimix.

In general, I am not a big fan of back-gas deco. The sole purported benefit from deep stops (on back-gas) is micro-bubble management. That may be a philosophical or holistic attraction to some, but it inevitably costs the diver with respect to greater slow-tissue on-gassing and, thus, longer overall deco obligations.

Forgive me for perhaps being obtuse, but I thought the driving aim of decompression was to actually get rid of inert gas, not to load more of it deeper into our tissues?

Back-gas decompression stops only benefit slightly through the diffusion value of leading (faster) tissues. Fast tissues, and fast-tissue micro-bubbles, that will be more than adequately resolved in the early phases of accelerated deco anyway.

Note: Of course, there may exist certain very extreme hypoxic dives whereby the diver simply can’t carry enough cylinders to enable gas switches at the significant depths where helium fast-tissue micro-bubble control might be advantageous. That said, who even does very extreme hypoxic dives on open-circuit nowadays?

I am also not a fan of ascending on back-gas when I have a much more optimal mix to breath. I want to utilise a deco gas immediately from its MOD for the maximal off-gassing I can achieve.

If you’ve got it, use it.

And if you’re going to use it, why not slow your ascent down with a schedule of shorter stops up until your longer decompression depths occur? All that you’re really doing is slowing a 9-10m per minute ascent down to a 3m per minute ascent. Your gas switch is providing you adequate differential for off-gassing, so there’s no need to rush once switched.

There’s another big benefit behind this simple GFlo system – it consistently determines your first stop at the same depth – that being the gas MOD/switch depth.  Consistency and simplicity are great things in technical diving – they help prevent human factors errors (i.e. screw-ups) from occurring.

If you rely upon a constant, or default, gradient factor low – then your first stop depth will vary from one dive profile to another. That increases the risk of confusion leading to omitted stops.  You’re much less likely to make a critical error if you always begin your staged decompression from the MOD of your first gas switch, especially if you consistently opt for the same standard mixes.

Gradient Factor Low – Example Use

So, for example: on a typical extended-range or normoxic trimix dive, I will usually be carrying a cylinder of 50% deco gas. As the MOD for that gas is 21m/70ft, I will use my planning software to adjust my GFlo setting until it calculates my first deco stop at that same depth. Easy.

At that time/depth, and once switched onto my 50%, I can be confident that I am neither loading my slow tissues nor missing out on a more optimal off-gassing differential.

Example 1:

In this example (planned on Multi-Deco using ZHL-16C) the diver uses the default Shearwater setting of GF30/70 for a 20-minute air dive to 40m with 50% deco mix.   You can see that even on this lightweight tech dive, the algorithm mandates the first stop at 18m – above the optimal gas-switch depth for the 50% mix carried.

gradient-factors-use-40m-Default-GF-30-70

A simple downwards tweak of GFlo to a setting of 25/70 drops our first mandated stop down to 21m. This corresponds with our mix MOD and gas switch. The runtime increases by 1 minute to reflect the length of that extra stop. However, we have simplified our ascent plan and added some extra conservatism with respect to the extra 1 minute of off-gassing.  For those concerned over micro-bubbles, the deeper stop on a richer mix will have some extra effect on faster-tissue bubble resolution.

gradient-factors-use-40m-Default-GF-30-70

Example 2:

Let’s look at the opposite case – in this instance a 50m dive, also for 20-minutes using air and 50%.  Again, we start with the Shearwater default settings of GF 30/70.  We can see that the model now mandates the first stop at 24m.  This is below our deco mix MOD of 21m. During this stop, we can assume that our slow-tissue will further on-gas.

gradient-factors-use-50m-Default-GF-30-70

In this instance, a small upwards refinement of GFlo to a 40/70 setting neatly coincides our first designated stop with our deco mix MOD. In doing so, we have cut 2-minutes from the overall run-time; a result of eliminating stop time that served no benefit, along with avoiding the consequent slow-tissue penalty that had to be paid in the shallow deco (6m and 3m stops) phase of the dive.

gradient-factors-use-50m-Adjusted-GF-40-70

Even in these small-scale examples, the principles start to demonstrate where systematic adjustment of GFlo promotes both decompression efficiency and conservatism benefits.

Setting Gradient Factor High

The Gradient Factor High (GFhi) setting determines the overall length of decompression, with a ratio-bias towards shallow/O2-range decompression. The lower the number, the longer the decompression mandated.

This boils down to a rather simple means of controlling the overall conservatism of your deco dive.

If GFhi 100 (%) is zero conservatism (raw m-value), then any deduction from that number is a % conservatism value.

So, a GFhi setting of 70 equates to 30% conservatism below that absolute safety threshold.  A GFhi of 85 gives just 15% conservatism. Simple.

gradient factors use setting

Obviously, I won’t try to suggest how much conservatism YOU need in YOUR decompression schedules. It’s a personal factor – and should take account of both your specific physiology, long-term and day-to-day DCS predisposition factors.

A nominal setting for one diver may be hyper-aggressive for another. And vice-versa.  There’s no magic formula to arrive at an ideal setting.

Experienced technical divers pay close attention to their post-dive vitality and learn to tailor their conservatism based on subtle (or not so subtle) feedbacks from inside their body.  Less experienced divers have to build up their experience and gain reliable confidence in how much DCS tolerance they have.

DCS susceptibility is a bell-curve and you have to accrue lots of experience to learn where you float on that demographic spread.  Some people won’t symptomatically bend after insane ascents or huge amounts of missed deco. Others can bend on the most benign of shallow no-stop dives. Everyone else lies somewhere in-between.

You may call me a prude, but I’d suggest that it’s best to learn your tolerances without having to visit a recompression chamber.

You’ll never be aware of the occasions when you WOULD have gotten bent but were otherwise saved by a sufficiently conservative schedule.  You’ll only ever learn such a definitive lesson if you do get bent.  Try to avoid that lesson – it sucks. Just ask anyone who’s learned it.

Likewise, don’t think that a few hundred dives are anywhere near sufficient to have determined your general DCS susceptibility.  You have to dive enough times, inside a given range, to have given the ill-omened stars ample chance to align and try to kick your arse.

So in that respect, start with a low GFhi and slowly throttle back your conservatism over many, many repeated dives. Patience and self-discipline help with the application of that strategy. If/when you feel physically sub-optimal post-dive, it’s time to back off and drop your GFhi down a notch, or two.

Summary – Gradient Factor Use

  1. For a given dive, identify the MOD of the first gas you’ll switch onto during ascent (appropriate travel gas or 50%).
  2. Adjust the GFlo to coincide the first stop with your first gas switch at MOD.
  3. If that happens to provoke a significant variation in stop depth, take some time to apply common sense and also reconsider what gasses you might use.
  4. Adjust the GFhi to provide an overall level of prudent conservatism for the deco.

That’s it.  Super easy-peasy and you no longer need to dive using default gradient factor settings or lurk on techie internet forums listening to irrelevant debates by CCR divers…


Case-Study: Gradient Factors In Use (A 60m Trimix Dive)

For those wishing to read more, I have written a case-study example of three Gradient Factor profiles on a 60m trimix dive:

A Gradient Factor Comparison Using A Simple 60m Trimix Dive Case-Study


About the Author

andy davis technical diving philippines

Andy Davis is a RAID, PADI TecRec, ANDI, BSAC and SSI qualified independent technical diving instructor who specializes in teaching advanced sidemount, trimix and wreck exploration diving courses across South East Asia.  Currently residing in ‘wreck diving heaven’ at Subic Bay, Philippines, he has amassed more than 9000 open circuit and CCR dives over 28 years of diving across the globe.

Andy has published many magazine articles on technical diving, has written course materials for dive training agency syllabus, tests and reviews diving gear for major manufacturers and consults with the Philippines Underwater Archaeology Society.

He is currently writing a series of books to be published on advanced diving topics.  Prior to becoming a professional technical diving educator in 2006, Andy was a commissioned officer in the Royal Air Force and has served in Iraq, Afghanistan, Belize and Cyprus.


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