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Hi Paul, You are correct in that if gas is flowing out to the ambient air, the pressure must be somewhat above ambient. The trick is to make it as little above ambient as possible and yet still have gas flowing. Many divers choose to use some form of flow meter or flow control device while a bunch of us older guys simply turn the tank valve gradually closed until we can just barely hear the gas flowing. Chris Elmore has mentioned that it is the partial pressure of oxygen that the sensor is responding to. And we know from Dalton's law that the PP(O2) is equal to the fraction of oxygen in a gas multiplied by the total pressure of the gas. So if the gas pressure at the sensor is above ambient and ambient was the pressure at which the sensor was calibrated, the sensor will measure a higher partial pressure of oxygen and will therefore display an artificially high fraction of oxygen for the gas under analysis. If these terms are not familiar to you or this explanation is difficult for you to understand, then I recommend strongly that you take a course in Nitrox diving. It really is the best way to start learning about the use of gasses other than air for diving. You can get some good information from these lists, but occasionally you can get some bum dope also (of course never on "techdiver.") Take care and dive safe, Scott In a message dated 8/18/00 1:10:30 PM, Bakalite@ba*.co* writes: << I guess this doesn't make sense to me. If gas is flowing, how can the pressure be ambient? What is the error induced by having a higher PP due to a flow over the element versus a resing pressure. It seems to me that if the flow is off by 10% you'd get a 10% reading error, although in practice this doesn't seem to bear out. Sorry if these are stupid questions, but I'd rather ask some of those than tox on o2. Thanks. Chris Elmore wrote: ~ >Paul, > For flow rate, get a needle valve from the brass fittings >section at Lowes and use a flow meter to calibrate it. Once it's >calibrated epoxy the adjustment shaft and you're set to go- total >cost: about $5. You can even oxygen clean it if that makes you >happy. Flow rate doesn't really matter as long as you have enough to >purge the lines and move the gas over the sensor but not enough to >produce back pressure. Sensors measure partial pressure so you have >to keep the pressure at ambient. If you can hear gas flowing that's >enough. >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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