Understanding the Tissues Representation in the Surface Interval Widget

I’m trying to understand what the “Tissues” screen in the Surface Interval Widget is actually saying. I THINK that the percentage it is showing is trying to represent what percentage of M Value you are at at a given moment. If that is correct, this number is incorrect.

I have dove the MK2s directly alongside a Shearwater Teric, Nerd and Perdix, all with the same GFs set. (I understand GFs so this isn’t a matter of not understanding GFs.) On the Shearwaters, let’s say I surface with a SurfGF of 60%. If I flip over to the Tissues screen on the MK2s Surface Interval Widget, the percentage there often reads 80-90%. While I could get being different by a few percentage points here or there, being off by 20-30% for the exact same dive just doesn’t make sense.

Can anyone advise exactly what the percentage number on the Tissues screen is actually saying? (If it is representing percentage of M Value, if Garmin is using the GF concept to calculate this, the number is wrong so there may be something wrong with the math.)

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0 harvalen over 3 years ago

In my experience the tissue percentage is a percentage of GF High and not the absolute M Value. So if you are using high conservatism 35/75 then tissue loading would show 80% when your SurfGF is 60, 0.75 * 0.80 = 0.60. This has at least matched up fairly well for me when I've analyzed my logs and compared to other computers.

0 CaptainTal over 3 years ago in reply to harvalen

Thank you for that input. Just so I'm clear, are you saying you're sure that's what they are doing or is that what you are guessing they are doing? (It seems that this feature is very poorly documented.)

0 harvalen over 3 years ago in reply to CaptainTal

It's my guess, but when I've checked it has matched that hypothesis fairly well. After 12.20 you can add Surf GF to a custom data screen and that seems to be a percentage of the absolute M value instead of a percentage of GF High.

0 CaptainTal over 3 years ago in reply to harvalen

Thanks. That is definitely interesting and confirms some of my observations. I like the Garmin but it just isn't ready for "prime time" in terms of technical diving. The layer of obfuscation from actual GF to a percentage of another number just disconnects the technical diver from the actual information too much for my comfort. I think I'll continue to use it as a backup but I can't see it being a primary computer for anything but the most basic recreational dive for me.

0 nradov over 3 years ago in reply to CaptainTal

As a fellow technical diver I'm curious why you want those numbers? Are you doing repetitive technical dives with surface intervals short enough that it would really make a difference? I'm highly skeptical that the algorithms used by either Garmin or Shearwater accurately model what's really happening in my body, so I don't place much faith in them.

0 CaptainTal over 3 years ago in reply to nradov

Especially in advanced technical diving, knowing your specific GF99 and SurfGF allows you to manage contingency plans and conservatism at various stages of a dive. While I absolutely know they do not accurately model what is really happening in the body, they are the best thing we currently have.

The deco stops are good but don't account for things like thermal stress, workload, etc. I like having that information so I can adjust my level of aggression or conservatism based on what I understand of those stresses.

In a contingency scenario, I find it helpful to know how much theoretical overpressurization I am experiencing at a given moment and how much I will experience if I surfaced at that moment. This gives me information for planning more aggressive deco profiles, if I needed to.