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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'.
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