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On Thu, 17 Aug 2000, Paul Braunbehrens wrote: > I guess this doesn't make sense to me. If gas is flowing, how can > the pressure be ambient? It isn't, but how much is a function of the resistance to flow. A very low flow rate, like 2-3 liters/minute, is enough for measurement and will not raise the pressure on the sensor excessively. > 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. You're correct, the effect is not exactly linear. But it does not matter, assuming your calibration and measurement protocols take that fact into account. You should always calibrate with a known FO2 gas at the same flow rate you will use for measurement. This calibration should be done immediately before making your measurement. If you are completely anal and want the best possible accuracy, use a calibration gas close in FO2 to the gas being measured, and recalibrate between measurements. Be sure to allow gas to flow long enough for readings to stablilize in both the calibration and the measurement phases. An alternative measurement method is to measure at 1ATA. To do this, you need a reservoir to hold the gas being measured, and expose the sensor to that gas. This would allow you to calibrate to air at ambient. I used to make my measurements this way, and it worked well. But I presently use the flow meter method, and my no-longer-otherwise-used pony bottle is my calibration gas container, filled with plain old air. -- Art Greenberg artg@ec*.ne* -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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