Here are a few emotional (read - not necessarily well thought out)
grousings, grumblings, and thoughts about a couple oxygen analyzers.  I
still have an old MiniOX I which I'm planning to replace before my next
dive trip.  It's a delicate piece of equipment from MSA Medical Products
that needs its own cushioned bag which also contains a 2 liter per minute
flow control from Dive Rite (don't get upset you DIR guys I'm not
advocating transpacs), tubing, an oxygen sensor (which needs to be replaced
at least once a year and is generally even more expensive at dive shops
than through medical supply houses (I once paid $125 for one at Brownie's
Third Lung)), and an electrical connecting cord that attaches the sensor to
the analyzer.  Without a flow restrictor for the inflator hose the analyzer
will not read the percentage accurately and tends to read too high an O2
reading for the nitrox at flows in excess of 2 liters per
minute.  Similarly, any calibration of air to 20.9% will be off if a
restrictor is not used unless the user is very, very good at estimating a
flow of two liters per minute.

        Hold on, I know what you're about to say.  "Doesn't the percentage of
oxygen in the air drop with increasing altitude, temperature, and
humidity."  Yes indeed it does, and ANALOX Sensor Technology has a card
that compensates for two of those three variables, temperature and
humidity; so instead of calibrating for O2 at 20.9% you could
conceivably  find yourself at a temperature and humidity extreme
calibrating for O2 at 18.5%.

        Having done the air calibration, in my opinion it is wise to check it a
couple of times.  If the analyzer doesn't go to and hold at the original
calibration (give or take a tenth of a percent or so) something is wrong;
either a battery is bad, the sensor is shot, or the flow is off from the
optimum flow for the analyzer.  Similarly when analyzing the O2 percentage
of the nitrox, I do it a couple more times to make sure the readings are
exactly reproduced or very close.  Also having gotten a reading, I find it
good to let the flow through the analyzer continue for a bit longer to make
sure the reading doesn't change.  (In real life, I get a little slacker
after the first bottle, but it's good to remember that while nitrogen
forgives, oxygen doesn't.  Unless you've got a full face mask or you're in
a chamber there's no second chance from an oxtox hit.)

        Since one risks the DCS hit if the oxygen percentage is too low and the
oxygen toxicity hit if it's too high, my feeling is that the high end is
the more dangerous end.  In my case, I usually dive nitrox like air and
just limit my MOD, so the only side I really worry about is the high O2
side.    Given the condition of diving nitrox like air where the nitrox is
really DNA made through a membrane system, I'm a little less anal about the
analysis if my deepest depth wouldn't give me a PO2 higher than I'm
comfortable with at 40% O2 since, unless the technology has changed, that's
as high an oxygen content as membrane systems can produce.  (Originally,
the technology was used to produce nitrogen, and the nitrox was the
byproduct that was thrown away.  That's an interesting perspective on
what's treasure and what's trash isn't it?)

        The UBS O2 stick doesn't have a restrictor and is used by pressing it
against the O ring at the tank's yoke with non-DIN tanks.  I've seen it
used and it seemed accurate, but I'm very uncomfortable with the
possibility of variability of flow rates, and currently, I don't feel
competent to use it to the standard I'd like to have.  It's available for a
little over $200 and the promoters say it's good for at least 100 dives and
can take a fair amount of abuse.  UBS also sells a PrO2 oxygen analyzer
which comes in a box like the MiniOX I, but seems to be a lot more
rugged.  It sells for about 50% more than the stick and is available with a
restrictor.

        I'm no nitrox instructor (don't take my nom de plongee too seriously) and
just have a layman's interest in self-preservation, but for me, unless at
some time in the future I can reasonably be persuaded otherwise, the way to
go is to get an O2 analyzer with a restrictor (flow control) that provides
a uniform flow of gas to the sensor.  [I''m sorry that these comments
weren't better thought out, but I just brought Brooxie back from her first
dinner with wine following a recent illness and while we enjoyed the wine
too much (or is that too much of the wine), I wanted to get this out quick
enough to be topical.  Feel free to flame, but keep in mind that these are
somewhat emotional observations, and I don't feel obligated to stand by
them or defend them under the rigorous scrutiny which is commonly brought
to bear here in Scuba-SE.]


DPTNST,


John




.