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Chris, thanks for a very informative post. I wonder what would happen if the airflow dropped to 2 lpm, maybe due to an obstruction. I guess that intuitively I would assume that it would indicate a proper mix, since there is no increase at 5 lpm, at 2 lpm it would just take longer for the reading to stabilize. I think I've been convinced, which is a good thing, because I can't seem to get my hands on an analyzer with a flow meter, so now I can go out and get one with a simple flow restrictor. Thanks Chris, and sorry for thinking out loud, but maybe others are interested in this as well. BTW, the ones that you hold up to the valve are pretty scary, just crank the valve a little bit and watch the numbers dance. Chris Elmore wrote: ~ >Paul (and everyone else if Techdiver is working), > Although my statement below seemed to make intuitive sense I >did not have any data to support my statement that any pressure rise >at the oxygen sensor was minimal and wouldn't significantly impact >the reading. This weekend I did an experiment to find out just what >the pressure rise is. First I tapped a small opening to the Maxtec >barb adaptor and routed a line to a sensitive pressure gauge. I then >zeroed the pressure gauge and ran the airflow at 5 liters per minute >(what I use on my oxygen sensor), 10 lpm, and 15 lpm and looked at >the pressure change. > At 5 lpm there was no measurable change in pressure. > At 10 lpm the pressure increased by about .021 in Hg. (about .0103 psi) > At 15 lpm the pressure doubled to about .042 in Hg. (about .0206 psi) > >With this pressure rise a 35.0% mix would read 35.0245% at 10 lpm >and 35.049% at 15 lpm. >Four points: 1) At the "standard" flow rate of 5 lpm there was no >measurable pressure increase. 2) The error at higher flows (10 & 15 >lpm) is well within the calibrated accuracy of the sensors. 3) What >error there may be is on the side of safety (displaying a higher >than actual PO2) but is not physiologically significant. > >Bottom line: Analyzing your mix on a windy day is more likely to >influence your results than any pressure increase in the adaptor at >5 lpm. >On the other hand, I have seen widely fluctuating readings when the >sensor is held up to the flow coming from the valve instead of using >some method to stabilize the flow over the sensor. >C. -- Paul Braunbehrens mailto:Bakalite@ba*.co* http://www.daw-mac.com Mailing list for digital audio on the mac -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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