Shearwater Perdix Computer Users running Open Circuit...question?

[Neilwood]

While I agree with everything you wrote above @Boaty McBoatface, I worry that people are missing the primary reason for conservatism and indeed GF themselves, which are to all intents and purposes a conservatism setting with a lot more granularity.

The idea is that, for a given diving day (cold, workload, fatigue, hydration etc) and a given person (BMI, vascularity, cardio fitness, PFO etc etc) there is a given level of conservatism required to maintain the same DCS risk that a pure Buhlmann /DSAT/RGBM etc would produce in a member of that testing group (young, very fit Navy types usually)

Thus, if on a given day you should be using Med, then using Low to match someone’s NDL is a bad idea. If you’re ok with Low, then why not use it throughout?

There’s a lot more to setting conservatism than just maximising available dive time.

Besides, if your buddy goes beep then it’s time to get back on the boat, regardless of who has what set.

Agreed

Setting conservatism should be all about recognising risk factors and adjusting your computer to take account of them. So an overweight, low activity smoker should have a higher conservatism than a non smoking athlete with perfect weight (assuming no unforeseen medical conditions). I certainly shouldn't be trying to match someone healthier and 20 years younger just for the sake of it. I stick to MY chosen conservatism unless experience tells me that I can or should change it ie if it is a lot colder than usual, I might chose to go from medium to high.

Matching someone else's conservatism should really only be done if you are similar body types and risk tolerance

 

[DavidFL]

I started out using High; after about 10 dives on high, I switched to medium. I have a Suunto that is now my backup computer, and the medium setting on the Shearwater brings them closer to similar recommendations on the kinds of profiles that I dive. Plus 40/85 is likely to be where I would start out if I was explicitly setting GFs; all of that based on a long journey over the last couple of years to better understand decompression theory, approaches, and algorithms

 

[stuartv]

I can't see any reason anyone would EVER want or need to set their computer's conservatism to match someone else's computer.

Your computer's conservatism is to protect you. Set it to what you determine is right for you. All it's doing is adjusting what maximum the computer gives you. You don't have to stay down until it counts down to 0.

If you are diving with someone whose computer is more conservative than how yours is behaving, you don't need to change your computer. Just start your ascent when your buddy's computer says so. You don't have to make your computer tell you it's time to ascend before you actually ascend. It's fine to ascend with 5 or 10 (or ANY number) of minutes left on your computer.

And if it is your computer that is being more conservative, definitely do NOT feel like you have to adjust it to be less conservative just to match your buddy's computer! You do what's right for you. Never feel bad if what is right for you means that you are the one thumbing the dive! Every dive has to have someone thumb it. There's nothing wrong with it being you.

If you feel like it would be fine for you to use less conservatism, then cool. Go for it. Just don't do it because of pressure from a buddy or your buddy's computer.

 

[Premo83]

I have set mine to low conservatism mainly to align it with my Suunto back-up computer. On multiple dives over a couple days I build in my own conservatism. I don't push close to limits and make longer stops. Plus I'm relatively young and fit.

 

[w3dge]

I can't see any reason anyone would EVER want or need to set their computer's conservatism to match someone else's computer.

Your computer's conservatism is to protect you. Set it to what you determine is right for you. All it's doing is adjusting what maximum the computer gives you. You don't have to stay down until it counts down to 0.

If you are diving with someone whose computer is more conservative than how yours is behaving, you don't need to change your computer. Just start your ascent when your buddy's computer says so. You don't have to make your computer tell you it's time to ascend before you actually ascend. It's fine to ascend with 5 or 10 (or ANY number) of minutes left on your computer.

And if it is your computer that is being more conservative, definitely do NOT feel like you have to adjust it to be less conservative just to match your buddy's computer! You do what's right for you. Never feel bad if what is right for you means that you are the one thumbing the dive! Every dive has to have someone thumb it. There's nothing wrong with it being you.

If you feel like it would be fine for you to use less conservatism, then cool. Go for it. Just don't do it because of pressure from a buddy or your buddy's computer.

My feeling is that people have changed from a computer they have dived for a long while and are comfortable with, and would like their shiny new Shearwater to behave in a similar manner. So not necessarily setting it to match someone else, but to match another computer (either a backup or their previous that they were comfortable with (and maybe have a few years of empirical evidence that its safe for them)).

My $0.02

 

[Boaty McBoatface]

I can't see any reason anyone would EVER want or need to set their computer's conservatism to match someone else's computer.

I enjoy your posts, Stuart. I think the disconnect is that the responses to what I wrote immediately diverged from what I was actually trying to say.

No one is suggesting that people make different decisions about the actual, real-world risk they are willing to incur in order to accommodate someone else.

I am (and, to my reading of the latest posts, "some of us are") recognizing that there is a difference between actual risk incurred and the semi-arbitrary label of estimated conservatism displayed by a particular computer.

If you configure a computer which is observed not to bend divers to emulate the output of another computer running different math but is also observed not to bend divers, you haven't changed your actual risk in any measurable way.

A Suunto will tell you that diving repeat dives at Buhlmann 40/85 means certain death. A Shearwater set on Medium conservatism (40/85) will tell you that you're tempting fate if your first dive follows Suunto's recommendations. Neither computer seems to be killing people, so as each of us is trying to manage our own, personal risk, it is worth considering what another computer recommends with apparent success.

Different manufacturers are using different mathematical approximations derived from observations of of young, fit navy divers and exploding goats. We're then taking those average results and trying to apply them back to an individual's physiology in a specific dive profile on a particular day (a statistical fallacy) and then arbitrarily adjusting the computer's math further by way of a bunch of guesswork as to the personal implications of some of the additional known risk factors which we cannot in the least quantify.

And we call this mismatched mishmash of math and muse "conservatism." Computers are a wonderful tool and a boon to safety. But, conservatism isn't what our computers tell us it is, it is what we tell ourselves.

-BMcB

 

[stuartv]

I enjoy your posts, Stuart. I think the disconnect is that the responses to what I wrote immediately diverged from what I was actually trying to say.

No one is suggesting that people make different decisions about the actual, real-world risk they are willing to incur in order to accommodate someone else.

I am (and, to my reading of the latest posts, "some of us are") recognizing that there is a difference between actual risk incurred and the semi-arbitrary label of estimated conservatism displayed by a particular computer.

If you configure a computer which is observed not to bend divers to emulate the output of another computer running different math but is also observed not to bend divers, you haven't changed your actual risk in any measurable way.

A Suunto will tell you that diving repeat dives at Buhlmann 40/85 means certain death. A Shearwater set on Medium conservatism (40/85) will tell you that you're tempting fate if your first dive follows Suunto's recommendations. Neither computer seems to be killing people, so as each of us is trying to manage our own, personal risk, it is worth considering what another computer recommends with apparent success.

Different manufacturers are using different mathematical approximations derived from observations of of young, fit navy divers and exploding goats. We're then taking those average results and trying to apply them back to an individual's physiology in a specific dive profile on a particular day (a statistical fallacy) and then arbitrarily adjusting the computer's math further by way of a bunch of guesswork as to the personal implications of some of the additional known risk factors which we cannot in the least quantify.

And we call this mismatched mishmash of math and muse "conservatism." Computers are a wonderful tool and a boon to safety. But, conservatism isn't what our computers tell us it is, it is what we tell ourselves.

-BMcB

if you're diving deeper with people using other computers, and NDLs are becoming a limiting factor, you can match
up with them by setting Shearwater conservatism on low for the first dive and switch to medium conservatism thereafter.

The disconnect (for me) is coming from both of the posts above.

RainPilot says it well:

using Low to match someone’s NDL is a bad idea. If you’re ok with Low, then why not use it throughout?

There’s a lot more to setting conservatism than just maximising available dive time.

The bold part of the first quote is really the issue, I think. Configuring one algorithm to emulate another isn't really a valid way to think of it. Unless you know that one algorithm is actually a logical superset of the other and you CAN legitimately use settings to make it only use the subset that matches the other. For example: You can configure Buhlmann w/GF to "emulate" straight up Buhlmann by using GF100/100. Then, by definition, they are the same.

When you compare a Buhlmann/GF computer to one running some proprietary RGBM-based algorithm, it is simply not valid to conclude that because they have the same NDL at some point, they are equally safe to use.

So, like RainPilot said, if you are "safe" with a Perdix set to Conservatism=Low on the first dive, then why would you think you're any less safe to use the same setting on a 2nd or 3rd or 4th dive?

And if your buddy's Suunto becomes the limiting factor on repetitive dives, then seriously why NOT leave the Perdix on Low? You'll be getting out when your buddy's computer says to, regardless, right (if it's the limiting factor)? This goes back to what I said earlier. Setting your computer to be MORE conservative, just to match a buddy's really makes no sense. If his counts down to 0 and yours still says 5 minutes left, you get out anyway. No need to change your computer's setting to make it say 0 at the same time as your buddy's.

 

[admikar]

In a buddy team, you go up when one of you hit minimum gas pressure (whether it is 700 psi, 1000, half tank, rule of thirds is irrelevant). Why do you refuse to go up when one of you hit NDL limit ( or any other number)?
It is akin to changing to smaller tank to match your buddy gas consumption?! I am sure you're not going to do that?

 

[PfcAJ]

I can't see any reason anyone would EVER want or need to set their computer's conservatism to match someone else's computer.

Match them so you both end up with the same ascent schedule. Part of the dive planning process is agreeing on these types of things. Min gas, who leads the dive, who does what (who leads deco, who shoots SMB, whatever else you might have going on during the dive), deco strategy, max dive time. All these things are parameters you discuss and agree on, then do it.

 

[admikar]

Match them so you both end up with the same ascent schedule. Part of the dive planning process is agreeing on these types of things. Min gas, who leads the dive, who does what (who leads deco, who shoots SMB, whatever else you might have going on during the dive), deco strategy, max dive time. All these things are parameters you discuss and agree on, then do it.

What is ascent schedule in NDL diving? Deep stops? Safety stop at 5 or 3 meters? I can do all that without changing GF on my computer. OP was conservatism settings in rec mode, so I guess no deco. Max dive time can be agreed beforehand by comparing your plans, but I still don't need to change my computer settings to comply with dive plan, just remember when to go up.
Besides, ascent schedule is run by most conservative computer in a dive team. I will not change my GF to 100/100 to match you, but I will expect you to follow when I start up. I will do the same with a buddy that dives more conservative computer than I do.