Surface GF and Other Teric Musings

Powerful yet Small

Imagine having a tool that would give you a numerical percentage value of the decompression stress you would encounter if you were to instantaneously ascend to the surface. Not all dives are created equal. You might be keeping your dives within the NDL, doing your safety stops, but the fact is that different NDL dives will give you different degrees of DCS stress depending on  your dive profile.

When we launched the Teric we included in it a new feature we called Surface GF (SurfGF). In this blog post we'll discuss SurfGF and how it can help you manage your decompression stress.

Traditionally some divers consider watch style dive computers to be somewhat less capable than full sized units. It stands to reason that you can’t really fit all the functionality into such a compact format. Add to that the fact that you will probably not be able to fit in comprehensive relevant information into a small round screen and still make it easily readable to farsighted eyes.

That is why it's understandable to find divers surprised when I tell them that the Teric is our most capable stand-alone dive computer to date.

Begin with the end in mind

Image by Julia Bahlsen

This is the second habit Stephen Covey attributes to highly effective people. The first habit is to be proactive. He relates the first two habits by saying that flexing your proactive muscles will make things happen if you begin each project with a clear vision of your desired direction.

A few weeks ago, I found myself at a dinner hosted by Living Oceans in Singapore. I ended sitting by Mark Messersmith COO for Halcyon Dive Systems. We briefly talked about GUE’s concept of beginning with the end in mind when it comes to scuba training. Once you are committed to this sport, you will be better served by building up a framework where you can start fleshing in the skills and proficiency that will take you to your desired direction.

So, research topics about the kind of diving you want to be doing. Build up to and regularly practice the skills that you will need for that prized bucket list dive. Kit yourself up with equipment that can grow up with you and will be able to take you all the way there.

Scalable Equipment that Can Grow Up with You – Dive Computer example

The Teric has 5 different modes that can cover many different underwater activities ranging from practicing breath holds in your local community center pool to doing deep sea research on rebreathers. There is an open circuit recreational mode that simplifies the information displayed for the diver and condenses it to only include data relevant for this type of diving.

Setting your conservatism levels in this mode can be as simple as selecting from 3 settings: Low, Medium or High. There is a fourth setting called Custom. We recommend that divers refrain from tweaking the custom settings until the diver is familiarized with Gradient Factors (GF). Once you have grown to be knowledgeable about GF, then you can access conservatism levels that range from 10 to 99 in GF Low (90 settings) and from 30 to 99 in GF High (70 settings).

What is GF?

Erik Baker developed gradient factors as a conservatism strategy. It is often used in conjunction with the Buhlmann ZHL-16 algorithm. This conservatism strategy provides divers with a good amount of flexibility that allows us to fine tune our dive profiles and how we address DCS risk during a dive. One of its virtues is that it allows users to implement varying degrees of deep stops – how deep do you want to start your stops – through the GF Low parameter. At the same time, by adjusting the GF High parameter you decide how long your shallower stops will be.

Both the Buhlmann ZHL- 16 and the Gradient Factors conservatism strategy are open source and readily available to divers for in depth study in relation to their individual responses to decompression stress. This is one of the reasons that has made it so popular with divers, which in turn has yielded a good amount of empirical validation for divers and earned their trust.

For more information read Erik Baker’s papers:
https://www.shearwater.com/wp-content/uploads/2019/05/understanding_m-values.pdf
https://www.shearwater.com/wp-content/uploads/2012/08/Deep-Stops.pdf

 

If You Can't Measure it, You Can't Manage it! -Peter Drucker

No dive computer or algorithm can see what is really going on in the body of a diver. Algorithms are just limited mathematical approximations that we use as an attempt to come up with a methodology for managing DCS risk. So, while we are not actually measuring ppN2 in your real tissues we do have numerical values to reference in relation to DCS risk.

A common discussion topic among divers is what GF settings are you using and why. The discussion might include elements about the validity of deep stops, considering the latest research, but the use of GF figures is tacitly accepted as a numeric scale associated with DCS risk.

Dynamic, Real-Time Risk Management

Due to its acceptance throughout the diving community, it is very likely that divers will learn about deco diving by using Bulhmann ZHL-16 with GF. You don’t even need a dive computer to use it. There are several desktop planners that can produce Buhlmann GF tables. Once you generated the tables you can execute the dive with a simple bottom timer, or a depth gauge and watch. If you were to use a dive computer with the same Buhlmann GF settings, you can dive the profile and have it match the table produced by the desktop planner.

I’ve been diving a Teric for CCR deco dives for while now. Surface GF (SurfGF) is the reason why. There was a point in time where only the Teric had this feature. SurfGF is the surfacing gradient factor expected if the diver instantaneously surfaced. I setup the Teric in CC/BO mode and enter the fixed ppO2 that I intend to maintain during the dive. The Teric in CC/BO mode (fixed ppO2) is usually within a minute or two of the wired Petrel 2 controller reading the O2 sensors directly. With SurfGF and at any point in the dive, I can see what my GF would be if I were to surface. This information elevates the situational awareness in my diving.

When I trained to become a private pilot, I was taught to always be on the lookout for an emergency landing surface, just in case. Similarly, while diving, if I were forced to go up to the surface at any point of my dive, SurfGF would give me a numerical value associated with DCS risk. It’s one thing to be flying over clean, flat, wide open fields and a whole other thing to fly over heavily wooded steep mountain sides. In the same vein, facing a forced ascent when your SurfGF says 73% is a much different scenario than facing a SurfGF of 187%. The higher the percentage, the more likely you are to getting bent. Any percentage above 100% carries a risk higher than surfacing at the Bulhmann M-value itself (GF 100% = M-Value).

Making a mental note of your SurfGF at the time you arrive at the surface can also provide valuable information. Sometimes after a dive you can feel more tired than usual. Is it DCS stress? It would help if you have a numerical value to associate that feeling with. A few years ago, I thumbed out of a dive planned to 80msw (265 fsw) before descent because my secondary regulator started to free flow. I waited at the surface while the rest of the team completed the dive. When I got home at the end of the day, I felt the same kind of tiredness I often felt during aggressive dives. The problem was that I didn’t dive that day. It is sound advice to listen to your body and what it is telling you about DCS stress. Unfortunately, interpreting feedback from your body can be very subjective. If we have a numerical SurfGF value to associate to this feedback, we can then reduce some of the uncertainty.

SurfGF can give you valuable information when facing difficult choices. It is better to be bent than drowned. Sometimes breaking a deco stop is the safer choice. Shearwater won’t lock you out for doing that. SurfGF will give you a number that can influence a tough decision on whether to head to surface during some urgency or do some more deco. When used along with GF99 you can numerically manage your deco stress all throughout the entire dive. GF99 tells you the gradient factor as a percentage for your current depth and time.

SurfGF is also useful for dives within the NDL. You can objectively see the effects of doing a 3 minute safety stop and compare it to a 5 minute safety stop or even to having no safety stop at all. If you have a strenuous dive doing heavy kicking while battling a current, you can consciously choose to surface at a SurfGF of 70% instead of your usual 85% just to pad your chances. Now you can apply numerical values to your padding. Please bear in mind that while SurfGF is a powerful tool it is not a replacement for proper dive planning.

You can program your Teric to have the SurfGF value to appear in your home screen and always be present during the dive without needing to touch a button. After divers discovered SurfGF on the Teric and the word spread around, we got many requests to add this feature to our other computers. So we did. Even though SurfGF is now available in our other computers, I still dive the rebreather with a Teric on my right wrist. The Teric is a watch made by divers for diving. There is a lot of effort dedicated to make it excel in any environment divers might find themselves.

5 Responses

  1. Avatar Adrian

    Very interesting!!!! Thanks!!

    Reply
  2. Avatar Nick Butcher

    I love this feature on my Petrel 2 JJ Controller but please can you stop it showing in red once I’m above my high GF. Almost all of my dives involve staged deco and a GF above 85% (in my case) is not an alarm situation.

    Reply
    • Shearwater Research Shearwater Research

      Duly noted and added to our list of change requests. We cannot promise implementation or a timeline, but we will bring it to discussion. Thanks for taking the time to give us feedback on this matter.

      Reply
  3. Avatar Mark

    Is this SurfGF displayed on a Perdix? If not, will you offer it on an update?

    Reply
    • Shearwater Research Shearwater Research

      Hi, Mark. Yes, SurfGF is available on the Perdix. Just make sure you install the latest Perdix firmware, v65 or later. Cheers

      Reply

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