Mailing List Archive Mailing List Archive
Note: My email's been screwy for the last week or so... apologies if this =
is a re-post.
Paul,
Although my comment 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.=20
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.=20
Three points:=20
1) At the "standard" flow rate of 5 lpm there was no measurable =
pressure increase.=20
2) The error at higher flows (10 & 15 lpm) is well within the =
calibrated accuracy of the sensors.=20
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 just held up to the flow coming from the valve instead of using some =
method to stabilize the flow over the sensor.
C.
Chris Elmore
College of Liberal Arts
Univ. of SC
(803) 777-1534 office
(803) 348-3055 mobile
Please use this address for all email: chris@sc*.ed*
>>> Paul Braunbehrens <Bakalite@ba*.co*> 08/17/00 10:48PM >>>
Chris Elmore wrote:
~
>Paul,
> As long as the area for exhausting the gas is larger than the=20
>area feeding the gas you should maintain an ambient pressure around=20
>the sensor. This would not be the case if the flow was great enough=20
>that resistance around the exhaust ports caused a back pressure.=20
>Certainly there may be micro-changes in pressure as you would get=20
>with any compressible fluid flow but these are not great enough to=20
>be significant. The sensors aren't designed for use above 1atm and=20
>by keeping the flow reasonable and using a vented adapter the area=20
>around the sensor stays at ambient pressure. Unless you're blasting=20
>gas through the line the error from pressure is negligible.
>C.
Chris, I'd love to believe you, because it means I can use a cheaper=20
and smaller setup. However, I hope you don't feel offended by my=20
asking for more data. This could be serious issue if a mistake is=20
made. I don't have a variety of sensors and flow meters and=20
restrictors at my disposal to check this stuff out.
Cheers.
--=20
Paul Braunbehrens mailto:Bakalite@ba*.co*=20
http://www.daw-mac.com Mailing list for digital audio on the mac
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