[Warning: Long reply, explores tangents]

Earlier, Al McDonald wrote:

My O2 system I set to not even give me a
puff of O2 till over 12,000ft I use the
M+H EDS set to D12 setting.

Riiiiiiight. But this knob goes to "eleven"...

You must be really infirm to get hypoxic
at just over 11k do you smoke or are you
very old?

I think Al is just being a little bit silly again. Many otherwise
normal-seeming people show symptoms of hypoxia at altitudes as low as
8000 feet msl. And that doesn't make them bad people.

Besides, what's the problem with using oxygen? The gas itself is
probably one of the cheapest elements of a soaring flight.

Say that a 22-cubic foot bottle fill costs $20, and you dispense it at
the conservatively recommended rate of 1 liter per minute per 10,000
feet of altitude. Say that your flight averages 10,000 feet.

Lets's math out:

1 ft^3 = 28.3 liters

22 ft^3 = 22*28.3 liters = 623 liters

$20/623 liters = $.032 liter

The cost to you is about 3.2 pennies per minute, or:

$.032/liter * 1 liter/minute * 60 minutes = $1.92/hour

And that's with a relatively wasteful constant-flow system. An EDS
system like Al uses dispenses oxygen pulses synchronized with your
breathing. In the N mode, that system is typically about 5 times more
effective at delivering oxygen into the blood as conventional constant
flow systems. So the cost is more like:

$1.92 /5 = $0.38/hour

The D modes that Al uses set altitude thresholds of 5000 feet, 10,000
feet,
or 12,000 feet. The system starts despensing as you go up through the
threshold altitude, and stops dispensing as you go down through it.
Actually,
there's about 1000 feet of buffer to the threshold, and about 1000
feet of hysteresis, but those are trivia points. Anyhow, the D modes
can double the effectiveness of the EDS system when you spend a lot of
time at or near the chosen mode threshold. So under those
circumstances the cost might be like:

$1.92 /10 = $0.19/hour

That means that Al is probably saving about $0.19 per hour on weak
days by using the D modes instead of N mode.

The thing that makes me a bit leery of the D modes is an observation
that Dr. Steele Lipe has made in private correspondence:

Quote:

There is some indication that removing the oxygen while descending through 12,500 feet to a landing may result in abnormally low oxygen levels, lower than expected! It is my recommendation that once starting oxygen it should be continued until landing and roll out. The rational is simple; if there is, as I have seen, a secondary reduction in oxygen levels after removal of your oxygen supply during descent and the landing phase is the most stressful part of the entire flight lower than expected oxygen levels and fatigue from the flight could result in inattention to ones flying, thus contributing to a PROBLEM.

Based on that observation, it might be worth considering switching to
N mode before landing. Personally, if I used the EDS system, I'd
rather spend the extra two bits per hour and use N mode for the whole
flight.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com