Difference between MB levels and Gradient Factors

[boulderjohn]

The issue for me is that there is zero information provided by Scubapro on what values result in dive profile changes. What skin temperature is the point at which the profile is changed? What pulse rate or respiration rate triggers the change?

That's my point. The old PADI rules on cold water were as vague as vague could be, and they really amounted to "be a bit more conservative when you are cold. Use your head about it."

Years ago, when I dived in Puget Sound (one day total), I had not at that point dived in water that cold (around 46° F) before, and I was not sure what to wear. I wore the White's MK3 underwear and 7mm hood. At the end of a one-hour a night dive, I was sweating. I was way overheated. So how do I plan for temperature on that dive?

On long dives, particularly long deco dives, the temperature problem has long been noted. During the bottom part of the dive, when on-gassing is at its peak, we are usually active and warm, with blood flowing nicely to encourage that unwanted on-gassing. When we are doing decompression stops, the standard procedure is to stay still as a rock, getting colder and colder and slowing blood flow during the desired off-gassing. Richard Pyle (mentioned in earlier posts) eschews those motionless stops and has his divers doing mild exercise instead, and I taught the same thing--keep moving to stay warm and keep the blood flowing.

Obviously, we have to make intelligent choices about the human factors, but we cannot make intelligent choices if we don't know what choices are being made for us by our computer.

 

[tursiops]

Obviously, we have to make intelligent choices about the human factors, but we cannot make intelligent choices if we don't know what choices are being made for us by our computer.

Exactly.

 

[Sevenrider860]

What agency teaches this?

As for what I taught, I taught my students to research all materials to the greatest degree possible, after which we would discuss what they had found and then arrive at a conclusion. I don't recall anyone finding anything at all suggesting that we do anything other than 30 FPM.

I don't know if other agencies specify specific ascent rates. PADI does not specify a specific rate and in fact does discuss that some ascent rates can be variable.

"You’ll probably find that typical ascent for deco diving is 10 metres/30 feet per minute. Some desk top deco software and some dive computers have variable ascent rates, with as fast as 18 metres/60 feet per minute during the deeper part of the dive, then slowing to 10 metres/30 feet per minute, and sometimes slowing even more near the surface. Whatever the prescribed ascent rate, that’s the rate you should follow."

I my first technical diving course it was hammered into me that in the initial part of the ascent from depth, you should be getting out of the on-gassing zone as quickly as possible. At the time we were using either a 45ft or 60ft/min ascent rate. I remember is that I was consistently counseled on being too slow on my initial ascents during the debriefs. That was remembered as "No slower that 30 feet per minute." Any slower and you have additional on-gassing. I now use a 30ft per minute ascent rate from depth and it still fits into that rule.

Once above the on-gassing depth, We used a 3m/10 feet per minute ascent rate...but up to a 10m/30ft minute could be used. That was remembered as "No faster than 30 feet per minute".

The point is (this did start about how I use the Scubapro PDIS depth to show me the depth that my leading compartments will start off-gassing) if you need to stop on your ascent to adjust something that may take additional time, it is nice to know what your off-gassing depth will be versus to stop and make the adjustments below that depth.

 

[Wibble]

It's fair to say that this thread's definitely moved into technical diving territory; basic it is not.

Wasn't the ascent rate of 9m/30ft per minute used by most agencies and planners to work out a dive profile? Was important doing manual Ratio Deco -- maybe still is for some?

The fact is that it doesn't matter that much any longer as your computer(S) measure your ascent rate and updates its calculations accordingly. You follow the computers' calculations. The main thing is slowing down and stopping below the first stop. There's a lot to do on the ascent especially if it's a free ascent with an SMB or two with other divers, so sometimes slower's good.

 

[dmaziuk]

The fact is that it doesn't matter that much any longer as your computer(S) measure your ascent rate and updates its calculations accordingly.

That isn't how that works: if your computer's fastest TC is 5 minutes, the meaningful recalculation frequency, from the algorithm's POV, is measured in minutes. Say you use Nyquist's law as a guide and set your recalculation rate to 2 minutes and 29 seconds: in that time you could ascend 20 m @ 9m/min, dropping your Pamb by 2 atm, or you could ascend 40 m @ 18m/min and drop your Pamb by 4 atm.

IRL your computer's fast enough to support pressure sampling frequency & recalc frequency on the order of seconds, so what you're saying is probably likely true, but I'd want to read the fine print to be sure.

My computer, OTOH, has its fastest TC at 2.5 minutes and is thus much more sensitive to ascent rate, which translates into annoying beeping every time I turn head-up and raise my arm to vent from the corrugated hose.

 

[JMarc]

The fact is that it doesn't matter that much any longer as your computer(S) measure your ascent rate and updates its calculations accordingly. You follow the computers' calculations. The main thing is slowing down and stopping below the first stop. There's a lot to do on the ascent especially if it's a free ascent with an SMB or two with other divers, so sometimes slower's good.

Computers can adjust for too slow ascent rate but not too quick. Not respecting the recommended ascend rate is just like not respecting a mandatory stop, you have to apply an emergency procedure and hope for the best as you are outside of what has been validated (as everything about decompression, there is a lot of variation between people and for a given person between days, nothing is certain, what you do increase or decrease the risk of DCS)

What is the recommended ascend rate decreased in my diver live-time (was 18 m/min when I started, is now 10m/min).

 

[J-Vo]

What agency teaches this?

The only place I have seen it is in Steve Lewis' book. It always seemed like an oversimplification to me. Witch tissue compartment are we talking about? Even on a fairly big TRIMIX dive I've seen my TTS going up on the 30' stop. And it's almost always still going up on the deeper stops below 80' or so. This tells me there are compartments still on-gassing well into deco off-gassing for other compartments.

 

[Wibble]

The only place I have seen it is in Steve Lewis' book. It always seemed like an oversimplification to me. Witch tissue compartment are we talking about? Even on a fairly big TRIMIX dive I've seen my TTS going up on the 30' stop. And it's almost always still going up on the deeper stops below 80' or so. This tells me there are compartments still on-gassing well into deco off-gassing for other compartments.

Wonder if that was your breathing gas not being optimal for deco?

Assuming Open Circuit and thinking if you've got only 50% as a deco mix, that's got a PPO2 at 9m/30ft of (ATA x Mix = 1.9 x 0.5 = 0.95) That's not very efficient as half of that mix is nitrogen.

If you used, for example, 80%, then that has a PPO2 at 9,/30ft of (1.9 x 0.8 = 1.52). That's very efficient as a deco gas compared with 50%.

The main stop at 6m / 20ft is the longest, roughly half of your entire decompression time (e.g. if you've an hour of deco, 30 mins will be spent at 6m). Using an efficient decompression gas will reduce that time. Some people use pure oxygen at 6m / 20ft which has a PPO2 of 1.6 at this depth (1.6 x 1). That's quite a hot gas and will drive up your CNS and Pulmonary oxygen toxicity, hence requiring "Air breaks" for long doses. It's also critical that you don't drop below 6m/20ft otherwise you could Ox Tox.

On the other hand, many people use 80% for the 6m stop because it's more gentle and less sensitive to toxicity issues. It's also more or less the same decompression schedule as you can get on it at 9m/30ft. The other Billy Bonus is you can get a higher pressure fill by topping off with air.

Will now stand by my bed for the customary SB beating for uttering this heresy.

 

[inquis]

Even on a fairly big TRIMIX dive I've seen my TTS going up on the 30' stop. And it's almost always still going up on the deeper stops below 80' or so. This tells me there are compartments still on-gassing well into deco off-gassing for other compartments.

A more likely possibility is you're deeper than the stop depth. It's expecting you to be nuts on 80 ft, but you're not. I get a noticeably later "cleared" signal from the computer next to my wrist compared to the one closer to my elbow. I can see being a foot or two deeper than the computer's expectations (i.e., the basis of TTS) easily tacking on another minute (or more) at various stops. Similarly, I can see increases in TTS due to ascents that are slower than it expects, which I suspect is VERY common, though that's probably a smaller effect.

Not to discount the possibility that there could be another compartment stealing the controlling roll, but that would be implicit in the ascent calculations the computer's already doing and therefore already included in TTS... but only if you were following the expected profile, of course.

 

[inquis]

Wonder if that was your breathing gas not being optimal for deco?

The particular gases you choose to carry are already implicit in the TTS calculation. Carrying 50% instead of 80% won't cause TTS to increase while sitting at a stop.

(No argument 80% results in less deco time for many single deco gas profiles, but there's more to that decision than just deco time in my view. YMMV, of course.)