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Date: Tue, 26 Oct 1993 14:07 EDT
From: <tab@go*.at*.co*>
Richard Chapski <Chapski@RI*.SC*.Sy*.CO*> wrote:
> With the above in mind I've found to get the most out of your meter.
> First let the probe come to ambient temperature. Never hold the probe by
> the body. When you measure the gas keep the flow a low as possible. You
> don't want to change the temp. of the probe or increase the partial
> pressure in the flow tube. I know some people who have even put an
> Oxygen welding reg with a flow control adjustment on there systems in
> order to keep the flow and temp. as low as possible.
This topic arose at the N.J. Mini-Tech Symposium last month; Glenn Butler
(Life Support Technologies) discussed analyzers in his presentation on O2
handling and safety, and John Crea (Submariner Research) had some input
as well.
The thing to remember is that these analyzers ("fuel-cell" type) sense
the partial pressure of oxygen (PO2), rather than the actual fraction
of oxygen in the mix (FO2). Unless calibration and analysis are done
under exactly the same flow conditions, you will get false readings
(higher flow rate == more total pressure against the sensor == higher
PO2 for a given FO2). You cannot accurately control flow by just
opening your tank valve "a little bit," or listening for when the gas
flow sounds the same.
The suggested method of analyzing was to use a flowmeter on your tank (a
medical-type oxygen flow-control regulator adapted to fit your tank valve
works fine), run plastic tubing to the sensor, then use another couple of
feet of tubing "after" the sensor as a reservoir. Calibrate the sensor to
air (just unplug it from the "tee" fitting into the tubing), flow your mix
*slowly* (~4 liters/minute) through the tubing for a minute, then turn
it off and wait for the meter to settle down. The reservoir will retain
the gas for at least several minutes -- plenty of time to get a good
reading.
In addition to temperature, humidity may also affect the sensor, so it's
really best (although not entirely practical) to calibrate to "dry" air
from a scuba tank (using the same method for calibration as for analysis)
rather than the atmosphere.
Yes, I agree this is sounds exactly like what I have observed. I believe
this method results in the highest accuracy from ones meter. One thing I
left out of my original message was the information on the flow control.
The ones I have seen where originally intended for argon. I would be
interested in knowing which of the two system is more inexpensive. If
it's significant there is no reason to use the more expensive.
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