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On Mon, 3 Aug 1998, Dan Volker wrote: > Will, > Are you trying to say that if one of us is exerting at 200 feet, there is no > difference in O2 consumption, versus if we are running at a heart rate of 65 > bpm??? > There is no possible way you can think this---but from your post it looks > like that was what you were saying. Please clarify this. No no - that's not what I'm saying at all. Of course you consume more O2 when working than at rest. But the trick is that the amount of O2 you consume PER BREATH is relatively constant (if you add a healthy safety margin). Since you are breathing faster under working conditions, you use more O2, and if you are counting breaths before purging, then you will purge more often than while at rest. That's why breath counting is a better method than time in determining when it's necessary to purge - and alot more efficient. For example, say at the surface you consume .1 PO2 a minute from your breathing loop while exercising heavily. You are breathing, say 50 breaths per minute. At 25 bpm, (moderate workload) you will only be consuming .05 PO2 per minute, so you will only have to purge half as often. In the water, at a PO2 of 1.2 and an acceptable minimum PO2 of .4 (a difference of .8 PO2), you would then get 50 breaths * 8 = 400 breaths before you needed to purge. That's *way* theoretical of course, because it doesn't account for loop dead space volume vs. lung capacity, and in truth there *is* some variance between breathing rate and O2 consumption per breath, so you need to add some conservatism there, as well. But it still ends up being whoppingly efficient. I've done some actual treadmill, at-rest, and in-water tests on the theoretical math (with applied modifications for loop-to-breath volume ratios and general conservatism), and it comes out right on the money. But you have to measure a few personal and system parameters very carefully, then double-check the results with a PO2 monitor in the loop. For instance, at low work levels, you can usually get about 1.2-1.5 hours off of an AL13 filled with EANx40, more-or-less independant of depth. I say "more-or-less", because among other things, counterlung sizing and minimum breathable volumes come into play. -Will > Dan > > > -----Original Message----- > From: William M. Smithers <will@tr*.co*> > To: Kevin Connell <kevin@nw*.co*> > Cc: techdiver@aquanaut.com <techdiver@aquanaut.com> > Date: Monday, August 03, 1998 5:31 PM > Subject: Semiclosed Bailout (was Re: Re: Death was a Bigot) > > > > > > >On Mon, 3 Aug 1998, Kevin Connell wrote: > > > >> Will, again, I have little knowledge of rebreathers, but I thought fully > >> closed units have one dilutent gas (helium or nitrogen) and oxygen. > >> > >> If so, how can you possibly use two seperate non-breathable gasses > without > >> a working PO2 sensor? > >> > >> If not, then the dilutent must be breathable, but it would have to be > like > >> a really low O2 heliox or nitrox, and I can't see that as "very > efficient" > > > >First of all, I'm no longer subscribed to techdiver - so > >I won't get any follow-up posts unless I'mm CC'd on them. > > > >OK, on the efficeint semi-closed bit, check back in the > >rebreather archives a month or two (subject was > >"depth modified SC" or something like that). I > >listed the math for how to calculate the optimal > >breathe-purge ratio for semi-closed operation. > > > >First of all, the method only uses diluent up > >to 20ft, where you switch to 100% O2. > >The basic concept is that a ratio of 5 breaths > >to 1 exhale-through-the-nose-and-add-gas method, > >which is the classic semi-closed ratio, is > >massively wasteful. Why? Well, it typically > >takes something like 10-20 minutes to breathe > >down a 1.2 loop at depth to .4 or so. That's > >alot of breaths - depending on depth and mix, > >you can actually get in excess of 200 breaths > >before dumping a breath. As you get shallower, > >and the PO2 of your gas drops, you get less > >breaths per purge, BUT, you are inhaling > >less gas per breath, so in terms of volume > >of gas used, it's about equal for both > >deep and shallow. Once you get shallow, > >that ratio may drop to 20:1 or so if you > >are breathing your bottom mix. > > > >One of the key elements here is that the > >amount of metabolized o2 is roughly the > >same per breath - regardless of how fast > >you are breathing. In other words, your > >PO2 per minute consumption rate stretches > >proportionately as your breathing rate lowers. > >For this method to work, you must know your > >maximum working breath rate and how fast you > >metabolize O2 at that rate. A table is constructed that > >shows the number of breaths you can > >take for a given depth, as you ascend. > > > >Here's a sample table where you have > >small quantites of heliox10 and > >EANx40 available. To keep it short, > >I'm only showing periodic samplings. Note > >that this is using MY personal values > >on MY rebreather, and will be different > >for each user. Also, this assumes > >you have planned a set of bailout > >tables that allows "sawtoothing" > >PO2's from ambient to .4 > > > >Depth Mix Breaths before dump, workload independant. > >----- --- ------------------------------------------ > >0 40% 30 > >30 40% 110 > >60 40 220 > >100 40 360 > >110 10 10 > >120 10 20 > >160 10 55 > >200 10 90 > >250 10 135 > >300 10 180 > >400 10 273 > > > >As you can see, you can get quite a bit more distance > >out of your SC gas than is commonly thought. The same > >basic idea was applied in designing the Halcyon, but > >since they (justifiably) don't want the Po2 varying > >very much while diving, they tend to inject gas > >much more often. But for bailout purposes, it's > >acceptable to calculate for varying PO2 in order > >to get maximum time out of your available gas. > > > >Hope this helps clarify it, > > > >Regards, > > > >-Will > > > > > >> At 01:05 PM 8/3/98 -0400, <will@tr*.co*> wrote: > >> >That's why the concept of redundancy is used in electronic rebreathers. > >> >And again, even if all the electronics fail, you can then > >> >fly it in a very efficient semi-closed mode. > >> > > >> > >> > >> ----------------------------- > >> Kevin Connell <kevin@nw*.co*> > >> > >> NW Labor Systems > >> http://www.nwls.com > >> ----------------------------- > >> > >> > >-- > >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > >Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > > > > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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