Teric not Accepted as Primary Dive Computer?

[Esprise Me]

Can anyone explain what SurfGF is and how it's used in small words, or is it too complicated to sum up in a post? I feel like I'm almost following here...

Also, to the instructors who keep posting about how it's the instructors prerogative to set course requirements--again, I'm really interested to hear what yours are and your reasons. You obviously know a lot more than I do, and on a macro level I do believe that seemingly idiosyncratic practices by those with a lot of experience are not to be categorically dismissed if they can't yet be fully explained. Humans are intuitive animals, not computers, and we often guess at the right answer by going with our gut before we can show the mathematical proof for it. But no one in this thread has said anything that could reasonably be interpreted as saying you shouldn't be allowed to set those kinds of rules; people are just debating the wisdom of those rules individually. So the repeated insistence on your right to make the rules kind of comes across, to me at least, as if you're trying to shut down the discussion. Maybe that's not your intent, but I'm probably not the only one reading it that way.

If it is your intent, I'd ask you to consider how you'd feel if you happened to overhear either of the following conversations between me and another new diver.
Other diver: Hey, I notice you dive with [your preferred configuration--Perdix, AI + SPG, etc.] Have you ever tried [alternative]?
Me: Nope. This is how [your name] says to do it.
OD: Oh...but what about [reasonable, if flawed, counterargument]?
Me: (shrug) I'm not going to question [your name].
OD: Uh...OK

Vs.

Other diver: Hey, I notice you dive with [your preferred configuration--Perdix, AI + SPG, etc.] Have you ever tried [alternative]?
Me: I thought about it, but this is what [your name] teaches because [reasons].
OD: Oh, OK. I never thought of that. But what about [reasonable, if flawed, counterargument]?
Me: Yeah, I heard about that, but [well-thought-out response].
OD: Hmmm, interesting.

 

[jgttrey]

Mike, I think we're talking past one another.

First, I'm not challenging any instructor's right to set any requirement they want, but I do think it's fair game debate the substantive merits of the requirement. Especially when the implication for those reading along at home is that this is some deal-breaking flaw in any dive computer except for a Petrel or Perdix, even a Teric. Sorry, I just think that's wrong.

Second, my point is that if something is has happened on the a dive that requires you to get out of the water more quickly, or even makes that desirable, the theoretical advantage of resetting GFHi so you can maintain some slope other than some parallel offset to the M value line is, in practice, pretty well pointless and in some cases counterproductive relative to the more simple strategy of using GF99.

GF99 value, as you know, is the instantaneous current GF value of the leading compartment. In a true worst case emergency, ride it up right on the M value (i.e., ascend keeping it at GF99). If the urgency is less, use GF90 or GF80 or whatever. You're now ascending on a line parallel with, but offset from the M value and you've decided how much offset you think is justified. That's hardly a difficult thing to assess.

On the other hand, changing GFHi only, which you would only do in the case of some need, ranges from mostly pointless to possibly a bad idea. If you have a reasonably high GFLo, then the slope difference is not going to be very meaningful anyway (pointless).

On the other hand, if you have a low GFLo, and there is a meaningful difference in the slopes of the m-value line and your GF line, then changing only the GFHi will continue to compel deeper stops, extending deco, which is the last thing I want to do in an emergent situation (counterproductive).

For example, let's pick something with a reasonably healthy amount of deco: you have a 200' dive with 40 min of BT. DIL is 10/50 with 1.2 setpoint.

If you dive 30/70, you have a 152 minute run time. That's 112 minutes of ascent/deco. Boom. You have a problem right at the end of your bottom time.

Option A - you change your GFHi to 95 to get out of the water. Now you're at 30/95. Run time is 124 minutes, with 84 minutes of deco because you're still doing a bunch of deeper stops, incurring more shallow time and burning through more gas along the way.

Option B - you monitor GF99 picking a value that corresponds to the degree of the emergency and ride that value up. If it's pretty urgent, use 95. If not, use 80 or 85, or whatever. That doesn't involve a lot of math or consideration. Basically, at that point you'd be diving 95/95 or 85/85 or 80/80, depending on which you picked. You'd avoid deeper stops, burn less gas and be closer to the surface if the problem got worse. At 95/95, run time is 10 minutes less and you shallow up much faster. If it's less emergent, 85/85 gets you out of the water in 123 minutes, about the same as your 30/95 in option A, but with a lower surfacing gradient and less gas consumption. That is, in fact, a more conservative approach than simply setting GFHi to some higher number.

Bottom line, resetting GFHi only as an expedited ascent strategy puts most of the emphasis in the wrong place. To the extent your GFLo is such that this strategy is meaningfully different that just keeping a constant GF on the ascent, is different in the WRONG direction -- deeper stops, more gas, further from surface with fewer options if problems worsen.

 

[jgttrey]

Can anyone explain what SurfGF is and how it's used in small words, or is it too complicated to sum up in a post? I feel like I'm almost following here...

SurfGF simply shows the surfacing gradient of your leading tissue compartment if you were to surface at that moment. Whatever depth you're at, if SurfGF equals, say, 90, it means if you popped to the surface, your leading compartment would be at 90% of the M-value.

GF99 is different. It is the current GF of the leading compartment at your current depth. For example, at 100' on some dive, you're GF99 might show "80" meaning your leading compartment is at 80% of the m-value, but SurfGF might be 150%, meaning surfacing right then would be a very bad idea. SurfGF becomes more useful when you start getting shallow on deco and you want to pad stops. GF99 is useful on ascent because it's showing the GF% at that point. At the surface GF99=SurfGF.

 

[doctormike]

Mike, I think we're talking past one another.

First, I'm not challenging any instructor's right to set any requirement they want, but I do think it's fair game debate the substantive merits of the requirement. Especially when the implication for those reading along at home is that this is some deal-breaking flaw in any dive computer except for a Petrel or Perdix, even a Teric. Sorry, I just think that's wrong.

Second, my point is that if something is has happened on the a dive that requires you to get out of the water more quickly, or even makes that desirable, the theoretical advantage of resetting GFHi so you can maintain some slope other than some parallel offset to the M value line is, in practice, pretty well pointless and in some cases counterproductive relative to the more simple strategy of using GF99.

GF99 value, as you know, is the instantaneous current GF value of the leading compartment. In a true worst case emergency, ride it up right on the M value (i.e., ascend keeping it at GF99). If the urgency is less, use GF90 or GF80 or whatever. You're now ascending on a line parallel with, but offset from the M value and you've decided how much offset you think is justified. That's hardly a difficult thing to assess.

On the other hand, changing GFHi only, which you would only do in the case of some need, ranges from mostly pointless to possibly a bad idea. If you have a reasonably high GFLo, then the slope difference is not going to be very meaningful anyway (pointless).

On the other hand, if you have a low GFLo, and there is a meaningful difference in the slopes of the m-value line and your GF line, then changing only the GFHi will continue to compel deeper stops, extending deco, which is the last thing I want to do in an emergent situation (counterproductive).

For example, let's pick something with a reasonably healthy amount of deco: you have a 200' dive with 40 min of BT. DIL is 10/50 with 1.2 setpoint.

If you dive 30/70, you have a 152 minute run time. That's 112 minutes of ascent/deco. Boom. You have a problem right at the end of your bottom time.

Option A - you change your GFHi to 95 to get out of the water. Now you're at 30/95. Run time is 124 minutes, with 84 minutes of deco because you're still doing a bunch of deeper stops, incurring more shallow time and burning through more gas along the way.

Option B - you monitor GF99 picking a value that corresponds to the degree of the emergency and ride that value up. If it's pretty urgent, use 95. If not, use 80 or 85, or whatever. That doesn't involve a lot of math or consideration. Basically, at that point you'd be diving 95/95 or 85/85 or 80/80, depending on which you picked. You'd avoid deeper stops, burn less gas and be closer to the surface if the problem got worse. At 95/95, run time is 10 minutes less and you shallow up much faster. If it's less emergent, 85/85 gets you out of the water in 123 minutes, about the same as your 30/95 in option A, but with a lower surfacing gradient and less gas consumption. That is, in fact, a more conservative approach than simply setting GFHi to some higher number.

Bottom line, resetting GFHi only as an expedited ascent strategy puts most of the emphasis in the wrong place. To the extent your GFLo is such that this strategy is meaningfully different that just keeping a constant GF on the ascent, is different in the WRONG direction -- deeper stops, more gas, further from surface with fewer options if problems worsen.

Thanks for the explanation... I'm not saying you are wrong, I understand the man on wire emergency ascent strategy. And it's totally reasonable to discuss why you don't agree with JCs requirement. But I think that you are missing the point that I am making.

All decisions about ascent plans are arbitrary bright lines through a grey area, and the variables that we don't include in the model have big implications as well. And every Buhlmann based ascent plan is going to involve "deeper stops" than 99/99, but the goal isn't to surface with the least inert gas loading in the slow compartments, it's to minimize bubble formation.

There is clearly a difference in decompression stress AND in operational simplicity between raising your GFHi to 85 (having your computer give you new stops), and watching your dive computer constantly to keep your GF99 below 85 the whole way up.

Yes, man-on-wire is more aggressive, gets you out of the water faster, uses less gas, and incurs greater decompression stress. I wouldn't call that "more conservative" at all if you are comparing 30/85 to 85/85. If there was no decompression benefit of the sloped line, why would we even bother with GFLo?

So is raising GFHi the right choice? Or is it better to choose 85/85? Or 95/95? It obviously depends on WHY you are modifying the plan. All I'm saying is having the ability to call an audible and change your GFHi is a useful tool. And it sounds like that's a tool that JC wants his students to have.

 

[jgttrey]

I wouldn't call that "more conservative" at all if you are comparing 30/85 to 85/85.

That's not the comparison I made. 85/85 gets you out of the water in about the same time as 30/95, not 30/85 and I'd far rather get out with an 85 SurfGF rather than 95, even at the expense of deeper stops. That is particularly true as the science tends to be pointing us towards increasingly hi GFLo figures anyway, so yes I believe apples-to-apples, for the same amount of deco it is more conservative (though I recognize the ever-shrinking numbers of deeper stop advocates might disagree with me). It's also indisputably more conservative for the non-deco reasons I described, which are really important in situation like this - everything is easier when shallow and you preserve more options, including padding the last stop with your the gas you saved to add even more conservatism.

But beyond that quibble/clarification, it doesn't sound like we are disagreeing on the facts. I'm fine with having that feature - you don't have to use it. I sure wouldn't for the reasons I've explained, and I hope JC would teach the use of GF99/SurfGF as well, because that is after all, the primary purpose of having those values to begin with. But, IMHO, it is silly to make the ability to change GFHi on the fly a requirement. He can do as he wishes and that is his right, of course. But perhaps this debate will illuminate the issue for those thinking about which computer to buy.

 

[davehicks]

I have both the Petrel (ext) and Teric and use them both in my diving. I mostly dive a rebreather in cold water with a drysuit, heavy hood, and often limited visibility. The Petrel is my primary and hardwired to the RB and the Teric is backup. I really like the Teric and don't see any limitations in terms of the data it can display or collect for both PPO2 or OC modes. I love the SurfGF and display that on the main view. The two computers are functionally equivalent in most tangible ways.

However, as a 50+ year old person with 50+ eyeballs I find that the Teric is harder to read than the Petrel. It is just too small to absorb information at a glance. The same info is shown on it's bright and clear display, but I often can't read it well. I have not used the Teric in warm water or a wet suit yet, and in a dry suit you often have a little less flexibility due to your heavy suit/garments and hood. I can't get the Teric as close to my face, turn the head as easily for a good view, etc. With the Petrel full sized display I can easily see the info at a side glace from my peripheral vision. To see as well on the Teric I have to make a conscious and deliberate effort. I have found this to be less of a problem in warm water & wet suit conditions and look forward to using the Teric in those conditions.

Technical difference aside, I agree that the smaller Teric is less appropriate for technical diving where information awareness is critical. I would also suggest it be used as a backup only.

 

[doctormike]

That's not the comparison I made. 85/85 gets you out of the water in about the same time as 30/95, not 30/85 and I'd far rather get out with an 85 SurfGF rather than 95, even at the expense of deeper stops. That is particularly true as the science tends to be pointing us towards increasingly hi GFLo figures anyway, so yes I believe apples-to-apples, for the same amount of deco it is more conservative (though I recognize the ever-shrinking numbers of deeper stop advocates might disagree with me). It's also indisputably more conservative for the non-deco reasons I described, which are really important in situation like this - everything is easier when shallow and you preserve more options, including padding the last stop with your the gas you saved to add even more conservatism.

But beyond that quibble/clarification, it doesn't sound like we are disagreeing on the facts. I'm fine with having that feature - you don't have to use it. I sure wouldn't for the reasons I've explained, and I hope JC would teach the use of GF99/SurfGF as well, because that is after all, the primary purpose of having those values to begin with. But, IMHO, it is silly to make the ability to change GFHi on the fly a requirement. He can do as he wishes and that is his right, of course. But perhaps this debate will illuminate the issue for those thinking about which computer to buy.

Right, I think that we are saying similar things. But I do think that this thread has been useful in bringing up a discussion of exactly this point - the difference between fixed GF99 ascents and the traditional GF model of the Lo to Hi slope. Of course, by picking your GFHi, GFLo and GF99 values, you can come up with comparisons that are more or less conservative.

One last thought. Moving your GFHi up during a dive isn't necessarily meant to be an emergency escape protocol, which is why I wouldn't throw out the benefit of a ceiling line slope greater > 1. It's meant to be a way of simply getting you out of the water faster. Maybe you can pick a constant GF99 that works out to be as safe or safer than any given increase in GFHi. But I prefer to just let the computer do it's thing and recalculate my stops, which is why Shearwater includes this functionality (even before the upgrade, when they didn't have SurfGF but they did have GF99). As they mention on their website:

"Fully adjustable gradient factors, which includes the ability to change GFhigh during a dive, provide a great deal of control. If an unexpected amount of work or excessive warmth was experienced during the descent or bottom phase of a dive, GFhigh could be reduced to prolong the ascent. If part of a critical gas supply was lost during the dive, GFhigh could be increased to expedite the return to the surface.

One practical note is that adding extra shallow stop time is not the same as reducing GFhigh. Extra shallow stop time will reduce the effective GF at the point of surfacing, but higher effective GF values could have been reached earlier during ascent. The impact of reducing GFhigh is greatest near the surface, but it will moderate the ascent profile throughout. Extra shallow stop time provides additional protection and is well worthwhile if time, gas, and conditions allow."

 

[ScubaWithTurk]

If it is your intent, I'd ask you to consider how you'd feel if you happened to overhear either of the following conversations between me and another new diver.
Other diver: Hey, I notice you dive with [your preferred configuration--Perdix, AI + SPG, etc.] Have you ever tried [alternative]?
Me: Nope. This is how [your name] says to do it.
OD: Oh...but what about [reasonable, if flawed, counterargument]?
Me: (shrug) I'm not going to question [your name].
OD: Uh...OK

Vs.

Other diver: Hey, I notice you dive with [your preferred configuration--Perdix, AI + SPG, etc.] Have you ever tried [alternative]?
Me: I thought about it, but this is what [your name] teaches because [reasons].
OD: Oh, OK. I never thought of that. But what about [reasonable, if flawed, counterargument]?
Me: Yeah, I heard about that, but [well-thought-out response].
OD: Hmmm, interesting.

It was not my intent to shut down the discussion but as instructors, we all have our way of diving and teaching. We teach what we feel is best practice. Hopefully this is developed through experience and study.

As for the two conversations, The first conversation would NEVER be a student of mine. I teach people to be thinking divers. Yes, I teach certain configs because I believe them to be best but we will discuss many different ones and the pros and cons of each. Blindly following anyone leads to trust me dives. It also leads to a diver who can't think on their own and who wants them as a buddy?

The second conversation is exactly what I would hope to hear from my students and from people in the water with me on my team. The second is part of how we determine what we feel is best practice. Discussions like that are the ones divers should be having with one another.

 

[ChuckP]

That Teric wording on his website must be new - I had a class back in March/April and I could of swore one guy had two Terics.

I see where he is coming from on the requirements - having one brand of computer makes dive reviews in the classroom uniform. He also spends a lot of time reviewing the hows and whats of the computer - that would be impossible using different manufactors.

The Teric is great, the display just can't show as many things at once as the Perdix/Petrel - he likes surf gf, gf99, @ +5 - heck he wanted so many things at your finger tips, it took two computers to display it all.

The changing the GF Hi was one method he taught, GF99 was another - I still don't get GF99 as good as I should but by changing Hi up or down, I could make it work....

He teaches a lot of different methods, very few are engraved in stone this is the only way type teaching, take it or leave it - the sun will still come up tomorrow.

 

[Jay]

Mike, I think we're talking past one another.

First, I'm not challenging any instructor's right to set any requirement they want, but I do think it's fair game debate the substantive merits of the requirement. Especially when the implication for those reading along at home is that this is some deal-breaking flaw in any dive computer except for a Petrel or Perdix, even a Teric. Sorry, I just think that's wrong.

Second, my point is that if something is has happened on the a dive that requires you to get out of the water more quickly, or even makes that desirable, the theoretical advantage of resetting GFHi so you can maintain some slope other than some parallel offset to the M value line is, in practice, pretty well pointless and in some cases counterproductive relative to the more simple strategy of using GF99.

GF99 value, as you know, is the instantaneous current GF value of the leading compartment. In a true worst case emergency, ride it up right on the M value (i.e., ascend keeping it at GF99). If the urgency is less, use GF90 or GF80 or whatever. You're now ascending on a line parallel with, but offset from the M value and you've decided how much offset you think is justified. That's hardly a difficult thing to assess.

On the other hand, changing GFHi only, which you would only do in the case of some need, ranges from mostly pointless to possibly a bad idea. If you have a reasonably high GFLo, then the slope difference is not going to be very meaningful anyway (pointless).

On the other hand, if you have a low GFLo, and there is a meaningful difference in the slopes of the m-value line and your GF line, then changing only the GFHi will continue to compel deeper stops, extending deco, which is the last thing I want to do in an emergent situation (counterproductive).

For example, let's pick something with a reasonably healthy amount of deco: you have a 200' dive with 40 min of BT. DIL is 10/50 with 1.2 setpoint.

If you dive 30/70, you have a 152 minute run time. That's 112 minutes of ascent/deco. Boom. You have a problem right at the end of your bottom time.

Option A - you change your GFHi to 95 to get out of the water. Now you're at 30/95. Run time is 124 minutes, with 84 minutes of deco because you're still doing a bunch of deeper stops, incurring more shallow time and burning through more gas along the way.

Option B - you monitor GF99 picking a value that corresponds to the degree of the emergency and ride that value up. If it's pretty urgent, use 95. If not, use 80 or 85, or whatever. That doesn't involve a lot of math or consideration. Basically, at that point you'd be diving 95/95 or 85/85 or 80/80, depending on which you picked. You'd avoid deeper stops, burn less gas and be closer to the surface if the problem got worse. At 95/95, run time is 10 minutes less and you shallow up much faster. If it's less emergent, 85/85 gets you out of the water in 123 minutes, about the same as your 30/95 in option A, but with a lower surfacing gradient and less gas consumption. That is, in fact, a more conservative approach than simply setting GFHi to some higher number.

Bottom line, resetting GFHi only as an expedited ascent strategy puts most of the emphasis in the wrong place. To the extent your GFLo is such that this strategy is meaningfully different that just keeping a constant GF on the ascent, is different in the WRONG direction -- deeper stops, more gas, further from surface with fewer options if problems worsen.

Right, I think that we are saying similar things. But I do think that this thread has been useful in bringing up a discussion of exactly this point - the difference between fixed GF99 ascents and the traditional GF model of the Lo to Hi slope. Of course, by picking your GFHi, GFLo and GF99 values, you can come up with comparisons that are more or less conservative.

One last thought. Moving your GFHi up during a dive isn't necessarily meant to be an emergency escape protocol, which is why I wouldn't throw out the benefit of a ceiling line slope greater > 1. It's meant to be a way of simply getting you out of the water faster. Maybe you can pick a constant GF99 that works out to be as safe or safer than any given increase in GFHi. But I prefer to just let the computer do it's thing and recalculate my stops, which is why Shearwater includes this functionality (even before the upgrade, when they didn't have SurfGF but they did have GF99). As they mention on their website:

"Fully adjustable gradient factors, which includes the ability to change GFhigh during a dive, provide a great deal of control. If an unexpected amount of work or excessive warmth was experienced during the descent or bottom phase of a dive, GFhigh could be reduced to prolong the ascent. If part of a critical gas supply was lost during the dive, GFhigh could be increased to expedite the return to the surface.

One practical note is that adding extra shallow stop time is not the same as reducing GFhigh. Extra shallow stop time will reduce the effective GF at the point of surfacing, but higher effective GF values could have been reached earlier during ascent. The impact of reducing GFhigh is greatest near the surface, but it will moderate the ascent profile throughout. Extra shallow stop time provides additional protection and is well worthwhile if time, gas, and conditions allow."

Thank you both for the informative discussion.

Given the text quoted immediately above from SW's website is older than the Teric, and the Teric doesn't have the ability to change GFs on the fly but also introduced SurfGF, then I'm curious about @Shearwater's thinking on this (using GF99, SurfGF versus modifying GFs on the fly) and if they'd like to chime in post returning from DEMA?