Modified ratio deco

[crofrog]

If you can't be bothered to add a computer check, including which gasses are configured for the dive, you are diving beyond your ability.

A thinking diver should be thinking about everything, including their computer. Blind trust in ratios is as asinine as blind trust in your computers.

Interestingly enough, I saw a recent conversation involving local GUE divers, primarily trained and using ratio deco, where they didn't understand what their computer was telling them and why there was a discrepancy. There are plenty of letters in GUEEDGE where a computer check would fit. To this groups credit, they did catch it.

RTFM

How many people but together their breathers wrong every year. There needs to be defenses to trap and correct errors

 

[CG43]

There seem to be some misunderstandings about ratio deco as a general concept. At the core it's nothing more than a recognition that a 3-D surface (total deco time as a function of depth and bottom time) can be approximated by a tangent plane within a limited area, and a 2-D curve (the ascent profile) and be approximated by a series of straight line segments. That's it. It's trivially true and just basic high school math. We can then calculate ratios to align to any underlying deco model for a narrow range of dive profiles.

This is a brilliant summary of what ratio deco is and what it is not.

Where is the risk coming from? If someone isn't able to make those kind of repetitive calculations without screwing them up then they're likely to make other more serious errors in related areas like gas analysis and planning. I'm not trying to be a gatekeeper here but if someone is struggling with this then tech diving isn't the hobby for them. Dive buddies should also do the planning independently and then cross check each other to verify that they have similar numbers.

Aren't you being a bit too strict here?
Sure , it would be ideal if divers could calculate well , but what human being calculates like a computer .
Even if it is not your intention to play the gate keeper , this requirement excludes divers from tec diving . And that although there are computers that do a large part of the necessary calculations.

 

[crofrog]

Even if it is not your intention to play the gate keeper , this requirement excludes divers from tec diving . And that although there are computers that do a large part of the necessary calculations.

Technical diving isn’t for everyone. Based on some of the people I’ve seen in caves and on breathers it should be “gate kept”

I’m not sure if I’d do it with ratio deco but it’s a pretty good example. If you’re so far “behind the plane” that you can’t add / subtract and divide by 2 you probably shouldn’t be there.

 

[lostsheep]

How many people but together their breathers wrong every year. There needs to be defenses to trap and correct errors

Too many?

They too should be doing gear checks and thinking about erroneous readings or behavior.

 

[mr_v]

Technical diving isn’t for everyone. Based on some of the people I’ve seen in caves and on breathers it should be “gate kept”

I’m not sure if I’d do it with ratio deco but it’s a pretty good example. If you’re so far “behind the plane” that you can’t add / subtract and divide by 2 you probably shouldn’t be there.

And that's exactly why ratio deco is so popular these days. I can't speak for everyone, but my usual dive buddies and I are more than excited to put their computers aside so that we can do some underwater math. Also, dividing by two is for unwashed plebs. What happened to integrals?

Ratio deco folk have a tool looking for a problem.

 

[LFMarm]

The most likely dive computer failure mode is user error, like accidentally configuring the wrong gasses before the dive or selecting the wrong gas during the dive.

Very true. It’s much less likely to have a hardware failure.

My approach is to have computer checks (incl. programmed gases, active gases, GFs) as part of my pre-dive checklist. The (small) risk of hardware failure is mitigated by always diving with 2 computer (2 controllers when in CC that I can swap between monitor and controller circuit underwater if needed).

 

[Nick_Radov]

If you can't be bothered to add a computer check, including which gasses are configured for the dive, you are diving beyond your ability.

Sure, that's part of our equipment check. But sometimes people screw up, and it's easy to accidentally select the wrong gas on the computer during an open circuit dive. That's why you need to be able to validate what the computer is telling you. You don't ever want to put yourself in a situation where a single error can cause a serious problem.

 

[Nick_Radov]

Sure , it would be ideal if divers could calculate well , but what human being calculates like a computer .
Even if it is not your intention to play the gate keeper , this requirement excludes divers from tec diving . And that although there are computers that do a large part of the necessary calculations.

I think you're still partially missing the point. No one is expected to calculate like a computer. We're not running the Bühlmann ZHL-16C algorithm in our heads. A ratio deco approximation using whichever protocol you like is basic arithmetic which anyone of average intelligence can learn to do with a little practice. Like if you graduated high school then surely you can do this. All you really need to remember is a reference dive profile that relates bottom time to total deco time for a particular depth, plus the adjustment factors for how that deco time changes with minor variations in depth or bottom time (within a limited range where the relationships are roughly linear).

There are numerous factors which exclude divers from tech diving. It's not for everyone.

 

[lostsheep]

Sure, that's part of our equipment check. But sometimes people screw up, and it's easy to accidentally select the wrong gas on the computer during an open circuit dive. That's why you need to be able to validate what the computer is telling you. You don't ever want to put yourself in a situation where a single error can cause a serious problem.

So people screw up during their checks or changing gasses on the computer but not during their arithmetic, got it.

That makes perfect sense if you don’t really think about it.

I’m out of here, last word is yours.

 

[Nick_Radov]

So people screw up during their checks or changing gasses on the computer but not during their arithmetic, got it.

That makes perfect sense if you don’t really think about it.

I think you're still missing the point or arguing against a strawman but I'll make one final attempt to explain. Obviously we might screw up anything whether it's deco calculations or dive computer configuration or whatever. That's why within the "Swiss cheese" model of accident analysis we try to maximize the number of "holes" that have to line up before we have a serious problem. Since we have a cross check and contingency for everything (within reason), no single diver error or equipment failure will cause a serious problem. Being able to do a mental sanity check on your dive computer's output is just one (minor) piece of DIR, another tool in the toolbox. The divers who are dismissive of this approach generally haven't even tried it or never learned it from a competent instructor, so they have misconceptions about it not working or being too hard to do.